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Proposal to the National Science Foundation
for a Conceptual Design Study:
X-ray Laser User Facility
at
Bates Laboratory
Massachusetts Institute of Technology
Principal Investigator
David E. Moncton
Co-Principal Investigators Science Collaborators
William S. Graves Simon Mochrie Keith A. Nelson
Franz X. Kaertner Gregory Petsko Dagmar Ringe
Richard Milner Henry I. Smith Andrei Tokmakoff
Bates Senior Staff Contributors
Manouchehr Farkhondeh William M. Fawley James FujimotoJan van der Laan Hermann Haus Erich Ippen
Christoph Tschalaer Ian McNulty Denis B. McWhanFuhua Wang Jianwei Miao Michael PellinAbbi Zolfaghari Mark Schattenburg Gopal K. Shenoy
Townsend Zwart
April 2, 2003
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SUMMARY
Recent advances in accelerator, laser, and undulator technology have created the possibility of
constructing a national user facility based on an intense free-electron laser at extreme ultraviolet and x-ray
wavelengths. MIT is exploring the construction of such a facility at its Bates Laboratory site. The facility
would produce x-ray beams with peak brilliance some ten orders of magnitude greater than is presently
available from todays synchrotron sources, and pulse durations from 100 femtoseconds to less than 1femtosecond. The wavelengths produced will range from 0.3 to 100 nm in the fundamental, with
substantial power in the x-ray 3rd harmonic at 0.1 nm. The possibility of future upgrades to even shorter
wavelengths will be preserved in the design. Based on a 4 GeV superconducting linac incorporating a
number of extraction points, the complex will include the potential for twenty or more undulators and
x-ray beamlines.
In order to produce beams of the highest quality, various methods of seeding the electron beam
with high harmonics of laboratory lasers are currently under investigation, as is lasing by self-
amplification of spontaneous emission. A number of these methods will be exploited to produce radiation
sources matched to experimental needs. Advanced laser technology, used to seed the electron beam for
short pulse production, will also be used to produce the electron beam and for use in pump-probeexperiments. Thus, lasers will be an integral part of the facility.
As described in the body of the proposal, the scope of science possible with such a facility is both
broader and in some sense deeper than that pursued at todays synchrotron facilities or laser sources,
because it combines high power and coherence for the first time in the 100-0.1 nm range. The science that
is foreseen spans many disciplines including atomic and fundamental physics, condensed matter physics
and materials sciences, femtochemistry, structural biology, and various fields of engineering. The source
we propose, and the experimental methods it will spawn, will generally be qualitatively new and have
high impact in many fields of science and technology. The strength of the science and technology base in
the northeast region, and in particular at MIT, make this a superb location for such a facility.
But MIT is ideally suited to be the host institution for an equally important reason: the remarkableeducational opportunities that such an endeavor would create and sustain. Beyond the enormous impact
on graduate and post-graduate education of a facility with such a broad user science program, is the
opportunity to provide education in the underlying accelerator sciences and technologies. Such
technologies are becoming increasingly critical to society, but they are difficult, if not impossible to teach,
without the existence of working accelerators. A program at MIT would have immense impact in
developing a future generation with skills in this area. The presence of such a laboratory will also allow
enhancements of novel teaching methods for K-12 students and teachers. Many of these educational
opportunities will be exploited early in the proposed study by using infrastructure that currently exists at
Bates Laboratory.
The scale of the facility, with its technical infrastructure, is ideally matched to the 80-acre, 1.2 km
long Bates site. The scope of such a project has been under study at Bates by a small group for nearly ayear. The three-year study proposed here will, in the first 18 months, produce a conceptual design, an
R&D plan, a cost estimate and schedule, and a detailed scientific case, as integral parts of a proposal for
construction beginning in FY2007. During the balance of the proposal period, R&D will be undertaken, a
management plan developed, and the facility design advanced to the point necessary to begin
construction. It appears that the capital cost for the facility will be around $300M, assuming construction
beginning in FY 2007, and depending on the number of beamlines implemented with construction funds.
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TABLE OF CONTENTS
SUMMARY........................................................................................................................................... iii
INTRODUCTION ................................................................................................................................. 1
1 SCIENCE AND EDUCATION................................................................................................... 2
1.1 Overview of Source Properties........................................................................................... 2
1.2 Science ............................................................................................................................... 4
1.2.1 Nanoscale Dynamics with X-ray Transient Grating Spectroscopy....................... 5
1.2.1.1 Introduction........................................................................................... 5
1.2.1.2 The MIT/Bates X-ray Laser.................................................................. 6
1.2.1.3 X-ray Time-Resolved Transient Grating Spectroscopy........................ 6
1.2.1.4 Beamline Concept ................................................................................. 9
1.2.1.5 Proposed Work...................................................................................... 9
1.2.2 Nanoscale Dynamics with X-ray Photon Correlation Spectroscopy..................... 11
1.2.2.1 Introduction........................................................................................... 111.2.2.2 The MIT/Bates X-ray Laser.................................................................. 11
1.2.2.3 Beamline Concept ................................................................................. 11
1.2.2.4 Experiments with Photon Correlation Spectroscopy ............................ 12
1.2.2.5 Proposed Work...................................................................................... 14
1.2.3 Femtosecond Spectroscopy in Solution-Phase Chemical
and Biophysical Systems....................................................................................... 15
1.2.3.1 Introduction........................................................................................... 15
1.2.3.2 The MIT/Bates X-ray Laser .................................................................. 16
1.2.3.3 Time-Resolved X-ray Spectroscopy of Molecular
Dynamics in Solution............................................................................ 16
1.2.3.4 Beamline Concepts and Proposed Work............................................... 17
1.2.4 Trace Chemical Analysis in the Particle Counting Limit...................................... 20
1.2.4.1 Introduction........................................................................................... 20
1.2.4.2 The MIT/Bates X-ray Laser.................................................................. 21
1.2.4.3 Experimental Methods .......................................................................... 22
1.2.4.4 Proposed Work...................................................................................... 23
1.2.5 Structural Biology ................................................................................................. 24
1.2.5.1 Introduction........................................................................................... 25
1.2.5.2 The MIT/Bates X-ray Laser.................................................................. 26
1.2.5.3 Experimental Concepts ......................................................................... 27
1.2.5.4 Proposed Work...................................................................................... 32
1.2.6 Electron Dynamics with Attosecond Resolution................................................... 34
1.2.6.1 Introduction........................................................................................... 34
1.2.6.2 The MIT/Bates X-ray Laser.................................................................. 34
1.2.6.3 Experimental Concepts ......................................................................... 35
1.2.6.4 Proposed Work...................................................................................... 36
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TABLE OF CONTENTS (CONT.)
1.2.7 X-ray Microscopy With Atomic Resolution ......................................................... 37
1.2.7.1 Introduction........................................................................................... 37
1.2.7.2 X-ray Microscopy: Source Considerations........................................... 381.2.7.3 X-ray Microscopy: Real-Space Methods.............................................. 39
1.2.7.4 X-ray Microscopy: Reciprocal Space Methods .................................... 42
1.2.7.5 Conclusions........................................................................................... 45
1.2.8 Nanometer Lithography ........................................................................................ 46
1.2.8.1 Introduction........................................................................................... 46
1.2.8.2 Achromatic-Interferometric Lithography.............................................. 46
1.2.8.3 Zone-Plate Array Lithography.............................................................. 47
1.2.9 Status of Scientific Programs at Operating UV FELs ........................................... 48
1.2.9.1 Low Energy Undulator Test Line (LEUTL) at ANL ............................ 49
1.2.9.2 Deep Ultraviolet FEL at BNL............................................................... 49
1.2.9.3 Tesla Test Facility at DESY.................................................................. 511.3 User Program...................................................................................................................... 52
1.4 Education and Outreach Program....................................................................................... 53
1.4.1 Programs During Design Study............................................................................. 54
1.4.2 Planning for Programs During Facility Construction and Operation.................... 55
2 TECHNICAL CONCEPT SUMMARY ...................................................................................... 56
2.1 Facility Description............................................................................................................ 57
2.2 Injector ............................................................................................................................... 57
2.3 Linac................................................................................................................................... 59
2.4 Conventional Lasers and Seed Generation......................................................................... 59
2.5 Undulators .......................................................................................................................... 602.6 FEL Properties.................................................................................................................... 60
2.7 Photon Beamlines............................................................................................................... 63
2.8 Comparison to Other Sources............................................................................................. 64
2.9 Research and Development Program ................................................................................. 67
3 THE BATES LINEAR ACCELERATOR CENTER.................................................................. 69
4 INTERAGENCY AND INTERNATIONAL COOPERATION................................................. 70
4.1 Interagency Cooperation .................................................................................................... 70
4.2 International Cooperation................................................................................................... 71
5 DELIVERABLES AND PROPOSED SCOPE OF WORK ........................................................ 72
REFERENCES ...................................................................................................................................... 76
APPENDIX A TECHNICAL CONCEPT ........................................................................................... A-1
APPENDIX B PROJECT PLANNING.............................................................................................. B-1
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INTRODUCTION
The fields of laser and accelerator technology stand now at a point of remarkable opportunity: the
creation of highly coherent, powerful and ultra-short pulses of x-ray radiation ranging in wavelength from
100 to 0.1 nanometers. The x-ray laser has been the Holy Grail of both the laser and the x-ray research
communities since the invention of the laser. Now, not only can such a device be built, but it can also be
part of a very cost-effective national user facility providing many independent photon beamlines fordifferent areas of research. When contemplating the impacts of an x-ray laser on human knowledge and
technology, think of the independent impact of the x-ray and the laser as the starting point. All of that
scientific and educational benefit has been produced without any laser reaching the x-ray regime and
without any x-ray source producing coherent radiation. That is now possible for the first time, and it is so
compelling that in comparing the x-ray laser with current sources, a Global Science Forum sponsored by
the Organization for Economic Cooperation and Development (OECD) held in September 2001 [1]
concluded that, because of the vast increment in performance, it is very likely that entire new types of
scientific measurement and applications will be enabled. Now, eighteen months after that statement was
written, most experts would call it an understatement.
Three key elements of the facility we envision would make it unique. First, a 4-GeV linearaccelerator with superconducting radio frequency cavities would produce such high electron pulse rates
that twenty or more beamlines could be extracted to serve a large user community. Second, integrated
high-harmonic generation laser technology would seed the electron beam and generate photon beams with
high longitudinal coherence and pulse lengths significantly below 100 femtoseconds, perhaps below 1
femtosecond. Third, taking advantage of the ability of linear accelerators to extract beams at different
energies, we envision a facility spanning both the traditional extreme ultraviolet and x-ray wavelength
range. This approach provides for integration and synergy between the UV and x-ray communities and
the laser community, where scientists are anxious to move to wavelengths shorter than conventional
table-top technology can provide with high pulse power. Here, we propose a three-year study to develop a
construction proposal, optimizing the machine for the remarkable science and education opportunities it
will enable, and proceeding with the necessary preparations to be ready for construction in 2007.
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1 SCIENCE AND EDUCATION
The potential of x-ray laser radiation for science and education is extraordinary. One might
determine the structure of a single molecule with one x-ray pulse without the need for crystals; probe the
dynamics of atoms, molecules or of condensed matter on fundamental time scales and length scales
simultaneously; study the properties of matter at very high energy densities; improve technologies for
fabricating, inducing, and observing structure at the smallest length scales; and probe and exploit non-linear phenomena in the x-ray regime. It can be safely said that the x-ray laser promises to be a
comprehensive probe of spatial and temporal structure on all scales and at all resolutions relevant to all
forms of condensed matter. And it will be an exquisite tool to manipulate matter as well.
Educational opportunities are also exceptional for many reasons. The first is the scope of science
that the facility would support, with almost every experiment involving students as is common at todays
synchrotron radiation facilities. The success of the User Program discussed in Section 1.3 is essential in
this regard. Second, the technology associated with the facility spans many fields of physics and
engineering, thus providing a context for development of a curriculum in accelerator science and
technology, based on academic excellence at MIT and surrounding universities. Accelerators are
becoming increasingly important to society, but few schools offer substantial programs. Third, theexistence of a large-scale high-technology construction project would open many one-of-a-kind
experiences for students and teachers in a rich array of subjects from environmental planning and
architecture to project management. Finally, MIT has been a leader in educational innovation, including
the TEAL (Technology Enabled Active Learning) classroom [2], iLAB [3], the CMSE RET (Center for
Materials Science and Engineering Research Experience for Teachers) program [4], and the Cambridge
University collaboration [5], to name a few. These activities have reached well beyond traditional
graduate and undergraduate populations to include K-12 students and teachers, with a strong emphasis on
diversity. All of these experiences will be enhanced and new ones will be generated by the proposed
study, the eventual construction, and ultimately, the scientific operation of the x-ray laser facility.
1.1 OVERVIEW OF SOURCE PROPERTIES
Photon Beam Parameters. The extraordinary science potential of an x-ray laser source is a
direct consequence of the photon beam parameters and how they compare to other sources, as discussed
in detail in Section 2.8. The beams will have a tunable wavelength range from 100 nm to 0.1 nm
(wavelengths shorter than 0.3 nm will be obtained from higher harmonics); they will be fully coherent in
the transverse direction; they will be pulsed at a rate of 1 kHz, and each unseeded pulse will have a
duration up to 100-200 femtoseconds, contain up to 1 mJ of energy, and have a bandwidth of about 0.1%.
Of course it is the peak brilliance that is the strong suit of these machinessome ten orders of magnitude
greater than current sources and having pulse lengths 1000 times shorter. As we will see, a great deal of
the proposed science is driven by techniques exploiting these characteristics to access new regimes of
temporal behavior. However, the time-average brilliance properties also exceed third-generation
synchrotron sources by several orders of magnitude, and the flux is comparable at similar bandwidths.
Photon Degeneracy. High peak brilliance is a deeper concept than simply having a large number
of photons at once. For the first time, we can begin to think about an x-ray source having a large number
of photons occupying a single quantum state at one time. This parameter, called photon degeneracy, is
less than one for third-generation synchrotron sources at wavelengths of 0.1 nm and is only a few hundred
at 100 nm. For the x-ray laser, it will be of order 109. This increment of seven to ten orders of magnitude
in an important source parameter will have revolutionary scientific impact. The x-ray laser is not just a
better source than a traditional synchrotron; it is a qualitatively different kind of source. It would be more
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appropriate to call these sources first-generation x-ray lasers, than to call them fourth-generation
synchrotron sources. In fact, for most of the venerable experimental methods of hard and soft x-ray
research, it is essential that photons interact singly with the sample. Synchrotron facilities will continue to
be the workhorses of the field for some time to come. As will become apparent, the new experimental
methods appropriate for an x-ray laser have more in common with optical laser methods and many have
no analog at existing synchrotron sources.
Seeded Beams. In the preceding paragraph, we have described the unseeded beam parameters
essentially as a starting point for discussing the improvements obtained by seeding, as we propose with
high-harmonic generation from conventional Ti:sapphire lasers. The longitudinal structure of the
unseeded pulses is complex; the structure is not transform-limited and it is statistically noisy. Seeding has
the potential to reduce or eliminate these shortcomings, producing pulses that are transform-limited with
duration perhaps below 1 femtosecond, with correspondingly lower pulse power. The science we envision
will be greatly enhanced by, and in some specific cases entirely dependent on, these improved
characteristics. Successfully implementing this technology is challenging, but it will pay huge scientific
dividends. There appear to be no fundamental technical barriers. It looks straightforward at the longer
wavelengths, with increasing difficulty as one approaches the hard x-ray regime. The synergy generated
by the technical challenge, followed by the scientific pay-off of producing stable, reliable, transform-limited pulses approaching 0.1 nm wavelength, will be the intellectual driving force during the early
operation of this facility.
Fundamental Lengthscales and Timescales. Over the centuries, improvements in the optical
quality of photon sources have enabled measurements with improved resolution. With the development of
x-ray laser sources, an enormous step is now possible before physical limits are reached. Consider spatial
resolution. A laboratory x-ray tube enables x-ray images with about 0.1 mm resolution, typical of a
medical x-ray. The introduction of synchrotron sources moved that resolution to a few microns, and third-
generation undulator sources have the potential to resolve features below 100 nm. A concept exists for a
30 nm nanoprobe facility at Argonne National Laboratorys Advanced Photon Source. An x-ray laser
will have full transverse coherence, which is the equivalent of achieving the diffraction limit. In that limit,
a microscope is theoretically possible with resolution equal to the wavelength of the radiation.
In extracting static structure information, reciprocal space methods, commonly called x-ray
diffraction techniques, are perhaps even more important than real-space microscopy. The challenge of
extending diffraction methods to larger length scales is the conjugate of the problem, described above, of
extending real-space methods to shorter length scales. The full transverse coherence of an x-ray laser
beam will permit the perfect diffraction experiment, that is, one in which the resolution of the
diffraction peaks could potentially be the inverse of the size of the beam illuminating the sample. This is a
direct consequence of the uncertainty principle, and the concept of diffraction limit. The implications of
the situation are simple and powerful. With full transverse coherence, it will be possible to elect either
real space methods or reciprocal space methods over the entire range of spatial length scales (say 0.1 nm
to 0.1 mm) relevant to condensed matter. Indeed, in this limit, the most powerful approach is not to
separate imaging and diffraction, but to combine them in one general method referred to as coherentimaging. Implications for the study of non-periodic structures and, particularly, individual molecules, are
profound.
We now turn to an analogous discussion in the time domain, where the characteristics of the
beams we propose, particularly those seeded to approach or reach the transform limit, will be
revolutionary. Consider photoemission, or inelastic x-ray or neutron scattering. In these techniques, the
probe exchanges energy with the sample and the measurement of that energy transfer, as a function of
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momentum transfer, gives fundamental information on the dynamics of electrons, spins, phonons and
other quasi-particle excitations in condensed matter systems. The accessible region of energy resolution
for x-rays is from a few electron volts to 1 meV, with increasing difficulty at the best resolutions. For
neutrons, the available resolution range is shifted to smaller energies by a factor of 10 to 100, but the
limited flux of neutron beams means large samples are required. Neutron techniques cannot be widely
applied to microscopic samples, monolayers, or small particles containing a few atoms, molecules, or
clusters.
The x-ray laser would open a vast new dynamical regime by permitting studies to be carried out
in real time. There will be two distinct advantages of this capability. First, energy ranges accessible to
conventional inelastic x-ray and neutron scattering can be probed in real time, rather than in the energy
domain. In many complex systems, the dynamics are not well-described or understood in terms of
harmonic quasi-particle excitations whose excitation energy is greater than their inverse lifetime. In
many cases, the critical motions are large conformational changes involved, for example, in protein
function or chemical reactions. Second, and equally important, real-time methods will naturally extend to
times that correspond to energy resolution well outside the range of traditional energy-domain methods,
for example to nanoseconds (i.e. microvolts), or microseconds (i.e., nanovolts). Anticipating success in
seeding beams to get pulse lengths of 1 femtosecond, we will have a probe capable of accessingessentially all relevant timescales, from those of atoms and molecules to those in condensed matter in
virtually all forms. Not only that, but such a probe, to the extent that its wavelength approaches 0.1 nm,
will be generally sensitive to the structural character of the dynamics.
1.2 SCIENCE
Over the last decade, but with increasing activity in the last five years, the prospect of accelerator-
based x-ray laser sources has generated immense excitement in a growing scientific community. We
estimate that over 100 workshops have been held leading to the development of four 100-plus page
scientific cases for proposed new facilities: the Linac Coherent Light Source (LCLS) at Stanford, the
TESLA XFEL at the Deutsches Electron Synchrotron Laboratory (DESY), the SASE-FEL project at the
Berlin Electron Storage Ring for Synchrotron Radiation (BESSY), and the 4GLS project at the DaresburyLaboratory in the UK. High-level committees in the respective countries have favorably reviewed these
cases. Virtually all the work proposed in these documents can be undertaken at the proposed MIT x-ray
laser facility. Our website (http://mitbates.mit.edu/xfel/index.htm) contains these documents in their
entirety. Furthermore, beyond the paper studies, three demonstration facilities have been built to
produce radiation in the range of 100 nm providing the incentive for the development of real experimental
programs, all of which could also be carried out at the proposed MIT facility.
In consideration of all of this activity, we have concluded that the general case for the x-ray laser
has been made clearly and convincingly. Our goal in this document is not to repeat that work, but rather
to move toward the development of more specific experimental concepts and approaches that are driven
by compelling scientific opportunities. During the proposed study we intend to establish and execute a
process that will greatly expand the involvement of the external community, and result in conceptualdesigns for a specific set of about ten initial beamlines to be included in the construction proposal. This
process is described in more detail in Section 1.3 (User Program) and Section 5 (Deliverables and
Proposed Scope of Work). We want to emphasize here our intent that this process (1) be closely
integrated with the design of the machine so as to maximize the science it will enable, (2) be based on a
broad outreach activity involving many workshops and a public proposal solicitation process, and (3)
employ peer review for all proposals using well-established criteria to determine scientific merit.
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To this end, we present below eight contributions by scientists and faculty at MIT, Brandeis,
Yale, Argonne, and the Stanford Synchrotron Radiation Laboratory. It is essential that the project team
have a scientifically strong cadre of users with whom to interact on a regular basis as the technical
design of the facility is developed. It is also essential to have such a nucleus of interested and motivated
users to organize the planned workshops and generally provide leadership to the community to develop
and prepare the more detailed technical and scientific cases for each beamline included in the construction
proposal. Five of these contributions are proposals in themselves (their authors are collaborators on thisproposal) aimed at specific work that would be funded and performed under this study. The other three
contributions (Sections 1.2.4, 1.2.7, and 1.2.8) have a somewhat different flavor. They are concepts for
very innovative and in some cases speculative instruments in different wavelength ranges of the proposed
facility. In two cases, concept development and R&D are underway at Argonne and Stanford with
separate funding. For x-ray lithography, more work would be necessary before a specific proposal could
be made to develop realistic instrument concepts. But at this stage, we do want to think broadly and
entertain some high-risk concepts. Taken together, all of these ideas, we believe, give a flavor of some
extremely exciting possibilities. But not all science must wait for new machines to be constructed. We
also include a brief overview in Section 1.2.9 of science proposed or underway at existing 100 nm
sources.
1.2.1 Nanoscale Dynamics with X-ray Transient Grating Spectroscopy
1.2.1.1 Introduction
The development of time-resolved coherent laser spectroscopy has ushered in a new
understanding of condensed matter dynamics, including the time scales for electronic and vibrational
decoherence and relaxation, liquid-state molecular dynamics and chemical reactions, and collective
structural rearrangements in a variety of complex media. The technology and methods used have been
exploited extensively for practical applications as well, ranging from advanced materials characterization
and metrology to optical coherence tomography and other biomedical assessment techniques to the
coherent optics used routinely in photonic switching and communications. These applications, with their
myriad benefits to society, were developed and are being extended continually by the scientists andengineers whose graduate and postgraduate research was devoted to inventing time-resolved coherent
optics and spectroscopy.
Another scientific and technological revolution will be ushered in as ultrafast coherent optics and
spectroscopy are extended to wavelength scales 100 times shorter than those used today. Here we propose
a study of collective structural change in condensed matter using four-wave mixing or transient grating
techniques with coherent x-rays. Transient grating fringe spacings of = 1-100 nm, corresponding to
coherent scattering wavevectors q = 2/ extending to nearly the edge of the Brillouin zone, will be used
to examine acoustic modes, nonoscillatory density dynamics, polarization, and other order-parameter
responses on the same length scales. These are the mesoscopic correlation lengths whose fluctuations
underly the great majority of collective structural dynamics that we now measure through coherent
spectroscopy on much coarser length scales [68]. Thus our current measurements tell us a great dealabout the time scales for collective structural change, but little about the length scales or the connections
between the two.
When we measure the multiple time scales for density (i.e. structural) relaxation in viscoelastic
polymer liquids or polarization relaxation in mixed ferroelectric crystals, we are observing on coarse
length scales (far longer than the correlation lengths for these variables) the integrated outcomes of
fluctuations whose dynamics may in fact vary sharply with (mesoscopic) length scale. Thus, we are left to
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speculate: are the fastest components of polymer relaxation associated with motions of polymer end
groups, intermediate components with side chains, and slowest components with backbone and whole
molecule motions? If so, then how do we understand the similar hierarchy of time scales observed in the
structural relaxation dynamics of glass-forming van der Waals liquids that have no obvious corresponding
hierarchy of structural correlation lengths [912]? Are the fastest dielectric relaxation components in
mixed crystals like KnbxO3/Kta1-xO3 near ferroelectric phase transitions [1315] associated with the
smallest polarized nanodomain regions, those formed around isolated impurity ions? Are the intermediaterelaxation time scales associated with those nanodomains that have encountered each other and merged
together as the temperature has cooled toward Tc and the polarization correlation length around each
impurity ion has grown? Are the slowest time scales associated with clusters of randomly situated
impurity ions whose surrounding polar regions have merged? Or does collective relaxation in complex
systems of this sort involve sequences of steps that inherently give rise to complex dynamics? These
questions are of practical as well as fundamental interest, since the design of these materials, used widely
in ferroelectric DRAMS, capacitors, and piezoelectric actuators, for high-bandwidth applications is
intimately connected to the association between local structure and dynamics.
Unambiguous association of the time and length scales of collective structural rearrangements
requires direct and simultaneous measurement of both. This will be enabled through coherent opticalspectroscopy using x-ray wavelengths. At the same time, the development of coherent optical methods
that operate on nanometer length scales will open a wealth of new possibilities. Coherent x-ray machining
and lithographic fabrication of nanometer features in advanced devices, x-ray optical trapping and
organization of atoms, molecules, and nanomaterials into assemblies with nanometer spacings, x-ray
coherent scattering metrology of ultrathin films, and other advanced materialsa new world of
applications will emerge under the leadership of the scientists and engineers whose graduate and
postgraduate research involves development of ultrafast coherent x-ray optics and spectroscopy.
1.2.1.2 The MIT/Bates X-ray Laser
The proposed x-ray laser will provide unprecedented capabilities for coherent ultrafast x-ray
spectroscopy, coupled with an exceptional degree of access through construction of a beamline dedicatedfor the purpose. The anticipated output parameters are extraordinary: approximately 1 mJ energy per
pulse at x-ray wavelengths, with pulse duration in the range of 100-200 femtoseconds. The optical four-
wave mixing measurements that we have conducted over the years on viscoelastic materials and crystals
that undergo structural phase transitions [9,11,12,1624] have typically involved pulse energies of tens of
microjoules, focused to spot sizes of tens to hundreds of microns in samples that are essentially
transparent to the excitation light. The proposed facility will provide comparable energies at the sample,
and far higher intensities if desired since the x-ray spot could be focused to far smaller sizes. More likely,
comparable spot sizes will be used and the energy may need to be reduced. Transient grating experiments
can be conducted with nearly any hard x-ray wavelength, and the availability of various wavelengths may
be exploited to enlarge the range of coherent scattering wavevectors. The pulse duration only needs to be
short compared to the fastest acoustic oscillation period, which will be on the order of 0.5-1 THz. If
seeding is used to produce shorter pulse durations, they will also be suitable.
1.2.1.3 X-ray Time-Resolved Transient Grating Spectroscopy
The time-resolved four-wave mixing setup illustrated in Figure 1 yields a transient grating fringe
spacing and corresponding coherent scattering wavevector magnitude q given by the wavelength of
and the angle between the excitation pulses, = 2/q = /2sin(/2). As in conventional light scattering
spectroscopy, the measurement length scale is limited to the order of the light wavelength. Time-resolved
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optical four-wave mixing measurements have provided an effective means for probing structural
relaxation dynamics in both liquid and solid materials, including elucidation of dispersive responses
involving thermal, acoustic, or optic phonon-polariton modes [9,11,12,1624]. An example is provided in
Figure 2, which shows data from a polymer liquid recorded as the temperature is cooled such that the
characteristic structural relaxation time (an average value used for characterization of nonexponential
relaxation dynamics) moves through the range of the acoustic oscillation period, i.e. through the range
ac 1 where ac is the acoustic frequency given by the acoustic (transient grating) wavevectormagnitude qac and the acoustic velocity vac(ac) through the dispersion relation ac/qac = vac. The data
show clearly the acoustic anomalies (velocity dispersion and attenuation maximum) that occur around the
region where ac 1. The data also show slower, nonoscillatory components at lower temperatures that
reveal structural relaxation dynamics occurring on time scales slower than the acoustic oscillation period.
This method, known as impulsive stimulated thermal scattering (ISTS) is based on the sudden sample
heating resulting from the excitation pulses and the subsequent thermal expansion that launches the
acoustic and slower responses observed through coherent scattering of the probe light. Data of this sort
have been used extensively for study of supercooled liquid dynamics [9,11,12,1921].
In other studies, excitation of acoustic waves or coherent optic phonons through impulsive
stimulated Brillouin scattering (ISBS) or Raman scattering, respectively, has been used to generatematerial responses relevant to collective structural change including viscoelastic relaxation or structural
phase transitions [1619], again with the time-dependent responses probed through coherent scattering. In
general, coherent time-domain spectroscopy of collective modes has proved particularly valuable in cases
where the frequencies of the modes are low or the damping (or dephasing) rates are high, including the
overdamped regime, and where the low-frequency spectrum may contain several contributions from, for
example, structural or polarization relaxation in addition to acoustic or optic phonons. In these cases,
conventional light-scattering spectroscopy may be unsuitable since the frequency shift may be
prohibitively low, the Stokes and anti-Stokes lines may broaden and merge, or additional central peak
FIGURE 1 Setup for four-wave mixing. (a) A binary phase mask pattern is used to generate two excitation
pulses that are recombined at the sample. The variably delayed probe pulse also is split to generate a
reference field for heterodyne detection. (b) Adaptation of the setup for x-ray wavelengths. A crystalline
grating will be used to split the incoming excitation and probe beams, and a second crystalline grating will be
used to recombine the pulses at the sample.
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FIGURE 2 Four-wave mixing data from polypropylene glycol. As T is reduced, the viscosity increases andthe structural relaxation time scale lengthens, passing through the ac 1 range at around 260 K wherethe acoustic damping rate is strongest. At lower T, slow components of structural relaxation are observed
directly. At long times (0.11 ms), thermal diffusion between the transient grating peaks and nulls is
observed.
features due to relaxation processes may obscure the acoustic or optic phonon features. The advantages of
the time-domain approach are often realized in association with collective structural rearrangements (e.g.,
structural phase transitions or structural relaxation in viscoelastic fluids), since they are characterized by
slow and complex collective dynamics, often involving coupled low-frequency modes [68,2024].
Recent x-ray Brillouin scattering measurements [25], while revealing interesting behavior at high
wavevectors, also show clearly the difficulties posed by strongly damped or overdamped responses aswell as additional central peak spectral features.
In x-ray four-wave mixing measurements, we expect to be able to adjust the transient grating
spatial period through the region of characteristic structural correlation lengthsL, i.e., we expect to gain
full access to the wavevector regime where qL 1. Acoustic anomalies (maximum in the damping rate,
dispersion in the velocity) similar to those that occur when ac 1 should be observed, in this case
directly revealing the correlation lengths rather than the correlation times involved in structural relaxation.
Observation of the slower, nonacoustic dynamics observable on time scales longer than the
acoustic period (which is very fast at high q) with variable wavevector will provide a direct window into
the connection, if any, between the structural correlation length and time scales. If the broad distribution
of relaxation times, with average value , is associated with a distribution of correlation lengths withaverage value L, then as the transient grating wavevector is varied from the qL < 1 regime through the
qL 1 range, the dynamical response will progressively lose its slower relaxation components until, for
qL > 1, the structural relaxation dynamics are filtered out completely because the measurement is being
made on a length scale shorter than that of the observed structural relaxation processes. In crystalline
solids such as the mixed ferroelectrics mentioned earlier, very similar considerations hold since strain is
linearly (piezoelectrically) coupled to the polarization which is the order parameter [26]. Examination of
the soft optic phonon branch [7,17] (whose displacements give rise to the polarization) in the high-
wavevector range should also be revealing. If nanodomain polarization correlation lengths L are
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associated with the correlation times measured, the results should be analogous to those described above
for acoustic modes, namely a stiffening of the phonon response at high wavevectors as the qL 1 range is
reached and exceeded. Thus x-ray four-wave mixing measurements of samples that under collective
structural rearrangements will directly reveal the relevant time and length scales and the association
between them. Other related issues such as the wavevector range at which acoustic modes in disordered
or partially disordered materials become overdamped will also be addressed directly.
1.2.1.4 Beamline Concept
The details of the beamline to be constructed for x-ray coherent scattering experiments will be
formulated during the proposed grant period. However, the beamline will certainly need apparatus for
generation and delivery to the sample of excitation and time-delayed probe x-ray pulses. This could be
done using crystalline grating interferometers as shown in Figure 1. Reflective optics may also be used
for beam delivery to the sample. A separate beamsplitting step, prior to the one shown in Figure 1, is
necessary for generation of the probe pulse, which must be variably delayed and then directed to the
sample either through reflective or diffractive optics. The latter, as illustrated in the figure, permit
heterodyning of the coherently scattered signal field with a reference field that originates from the probe
beam. This methodology [27] is used extensively for optical four-wave mixing measurements.
The beamline will also require an amplified femtosecond laser system with which time-resolved
optical probing can be used following x-ray excitation. The optical wavelengths cannot be used for
coherent scattering off the x-ray generated transient grating, but they can be used to assess electronic and
other responses to x-ray excitation irrespective of length scale. An understanding of the x-ray excitation
process and what sample dynamics are initiated by it will be a crucial element in the development of
coherent scattering methodology at x-ray wavelengths.
1.2.1.5 Proposed Work
A study of the experimental feasibility and theoretical description of time-resolved four-wave
mixing measurements conducted with coherent x-ray wavelengths on collective structural dynamics is
proposed. This will include preliminary design of the experimental beamline at the proposed facility andestimates of the expected signal levels and time-dependences that might be observed. One of the science
collaborators on this proposal, Keith Nelson, has been involved in formulation of x-ray four-wave mixing
experiments in connection with the LCLS project. This project, if supported and brought to successful
operation, will provide the necessary coherent x-ray pulses, but will offer far more than the energy
required for the proposed experiments. Also its single experimental station will be used for a number of
other experiments as well as research on condensed matter of the sort proposed here.
Professor Nelson also is involved currently in an experimental collaboration with Professors
Henry Kapteyn and Margaret Murnane at the University of Colorado, attempting to conduct four-wave
mixing measurements at soft x-ray wavelengths generated through high harmonic generation of
femtosecond optical pulses [28]. These measurements, if successful, will provide access to a range of
wavevectors considerably wider than the range accessible through visible wavelengths, but still farsmaller than the range reached with hard x-rays, and confined by absorption to near-surface regions. The
measurements also will teach us much about how to conduct this class of experiment with coherent
x-rays. For example, the experimental setup depicted in Figure 1(b) is being tried, not with crystalline
gratings, but with 100-nm grating structures fabricated through electron-beam lithography [29].
Prof. Shaul Mukamel and his group at the University of Rochester has undertaken
[3032] theoretical treatment of x-ray four-wave mixing applied to the study of molecular response.
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However, the experiments of interest here deal explicitly with condensed matter collective responses that
have not been considered to date. Perhaps the most important questions deal with the mechanisms
through which such responses will be generated by the x-ray excitation pulses. What are the relevant
mechanisms through which hard x-ray excitation pulses will interact with the samples? At optical
wavelengths, there are essentially two excitation mechanisms that give rise to sudden, spatially periodic
stress in the sample, thereby inducing acoustic, and in some cases, slower density responses. In ISTS,
optical absorption and rapid thermalization produce a sudden temperature rise and a step-function appliedstress. In ISBS, the initial, spatially periodic stress takes the form of an impulse function, which drives a
transient acoustic response. The combination of driving forces and time-dependent responses has been
treated in detail, and both mechanisms have been exploited for study of acoustic behavior associated with
collective structural change [9,11,12,16,1924]. Following excitation through either mechanism, the time-
dependent density dynamics are monitored through coherent scattering of probe light.
At x-ray wavelengths, absorption of crossed excitation pulses leads to ionization at the
interference maxima (i.e., the transient grating peaks), producing local currents and resulting in a spatial
distribution that may be influenced by electron mobility. At the high transient grating wavevectors q that
will be reached, thermal diffusion from grating peaks to nulls (the rate of which increases as q2 in the
hydrodynamic limit) may be fast compared to some or all components of the density response, in whichcase the temporal profile of the applied stress may approach that of an impulse function rather than the
step-function profile exerted at optical wavelengths. Apart from this, ionization of molecules at the
grating peaks will lead to a separate step-function stress since the steady-state density will be different for
partially ionized regions of the sample than for pristine regions. These effects all arise from x-ray
absorption and are likely to be the dominant contributions to acoustic and other density responses.
Additional interactions that give rise to stress in a manner analogous to stimulated scattering also must be
considered. Excitation of phonons in crystalline solids through similar mechanisms will be treated as
well. For example, liberation of valence electrons by ultrashort pulses reduces screening within the unit
cell, inducing coherent optic phonon responses in semiconductors even with visible excitation
wavelengths [24,33]. X-rays should produce similar responses in insulating crystals as well. Finally, the
detection of time-dependent responses through coherent scattering of x-ray probe pulses must also be
treated.
Apart from the light-matter interactions, density dynamics at high wavevectors and short times
will be modeled approximately so that the time-dependent responses likely to be observed can be
simulated. The purpose of the effort proposed is not to undertake a full simulation of the collective
dynamics at high wavevectors, but to anticipate the types of time-dependent responses expected in view
of plausible material response functions and the different excitation mechanisms at play, and on that basis
to elaborate experimental strategies for observing the responses of interest in the most incisive manner.
Support for a postdoctoral associate in the Nelson group will be used to undertake the theoretical
and feasibility study described above. Professor Shaul Mukamel has indicated a strong interest in working
collaboratively on the theoretical effort. His experience in treating x-ray four-wave mixing in molecules
specifically [32] and nonlinear optical spectroscopy generally [34] will accelerate the effort markedly.Through treatment of the x-ray excitation and probing processes and consideration of the collective
sample responses of interest, we will be able to estimate reliably the signal levels and dynamical features
to be expected from x-ray four-wave mixing measurements conducted at the proposed accelerator, the
information content that the experiments can be expected to provide, and the theoretical modeling that
will be necessary in order to extract that information content. Using realistic parameters for the output
generated by the proposed system, we will be able to assess critically the feasibility and scientific value of
the experiments.
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1.2.2 Nanoscale Dynamics with X-ray Photon Correlation Spectroscopy
1.2.2.1 Introduction
Complete understanding of a condensed matter system depends not only on knowledge of its
static properties and structure, but also on how it changes in time in response to time-varying applied
fields, or, simply, to thermally-induced spontaneous fluctuations. In addition, these sample dynamics maybe crucial for deciding how suitable a material may be for a particular application, and surely are key for
any materials processing required to achieve a useful final product. In many cases, we still do not
understand how molecular interactions and motions at the Angstrom scale give rise to often-complex
collective dynamics at mesoscopic (nanometer) and macroscopic (micrometer and longer) length scales.
What is needed is a powerful and general means of monitoring the full dynamic response over a range of
length scales for materials of all sorts, from proteins and long-chain polymers to ferroelectrics and
glasses.
1.2.2.2 The MIT/Bates X-ray Laser
In recent experiments, it has been shown that the new technique of x-ray photon correlationspectroscopy (XPCS) is capable of studying the slow dynamics of strongly-scattering samples at smaller
length scalesof the order of 30 nmthan can be achieved with laser PCS [3541]. In principle, XPCS
yields the intermediate scattering function (IFS) for the electron density of a sample, S(Q,t), and thus
should be a general and powerful method for characterizing the small-scale dynamics of condensed
matter. However, the time and length scales that can be studied via XPCS even at third-generation sources
is limited by the source brilliance, so that to extend XPCS studies to shorter times and into the nanometer
range demands even more brilliant sources. Specifically, the coherent flux, required for XPCS, is directly
proportional to the source brilliance. Thus, the proposed MIT/Bates x-ray laser source represents an
extremely exciting opportunity for novel XPCS studies.
1.2.2.3 Beamline Concept
Generally the proposed 1 kHz time structure of the source must be carefully considered in the
design of XPCS experiments, particularly the duty cycle, which may range from 10 to 100%. Our goal,
under this proposal, is to develop some of the methods and instruments that will permit us to optimally
perform XPCS experiments largely independent of the details of the duty cycle. An XPCS beamline will
have the following key features. First, it will have an x-ray beam splitter/delay line that will separate a
single pulse into two and introduce a variable time delay between the arrival times of the pulses at the
sample. We envision the beam splitter using eight thin diamond crystals to give two more-or-less
symmetric legs, each with a variable x-ray path length. The difference in x-ray path length between the
two legs then determines the delay time between pulses via the speed of light - a maximum path length
difference of 3 m would allow a maximum delay time of 10 ns. Two equivalent legs permit the path
length and delay time to be nulled straightforwardly. Second, it will have a slit system that will allow the
beam size on the sample to be accurately controlled, while introducing as little extraneous slit scatteringas possible. Third, it will have the means to orient the sample and detector as desired. Importantly, in
order to resolve speckle, we anticipate an unusually large sample-to-detector distance of 10 m or more.
Finally, it will have a fast-CCD based x-ray detector, with sufficient resolution to resolve speckle and
sufficient speed to keep up with the x-ray laser repetition rate.
To realize XPCS at the proposed x-ray laser, there are several issues that must be addressed,
including sample damage issues, and the construction of the x-ray beam splitter and delay line. We will
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FIGURE 3 Schematic intensity autocorrelation functions for a polymer melt for four wavevectors, increasing
from A to D.
Another as-yet untested, defining aspect of the reptation model concerns the lineshape of the ISF
versus tfor times neard. The lineshape is predicted to consist of a sum of exponential decays [46,54,55].
Figure 4, for example, shows reptation-model-based predictions for the mode amplitudes and mode
relaxation rates for a highly asymmetric diblock copolymer [55].
Careful XPCS measurements at the proposed x-ray laser source will allow detailed lineshape
analyses to test these sorts of predictions. But beyond tests of existing theory, new XPCS experiments
have the potential to yield key insight into resolving the puzzle of what determines the entanglement
length in blends and melts.
In addition to the polymer melts and blends discussed so far, there are many other polymeric
systems, in which the polymer conformations can be quite different and can exhibit novel dynamics.
Thus, we can anticipate that XPCS studies of a variety of polymer systems extending down to nanometer
length scales will prove an exciting and fruitful research area.
The second method for implementing XPCS at the proposed x-ray laser facility is a significant
departure from established methods, but being able to study condensed matter dynamics on time scalesfrom 1 picosecond to 10 nanosecond timescales is the potential reward. Now, we must rely on an x-ray
FIGURE 4 Specific predictions for the relative mode amplitudes and corresponding relaxation rates for
compositional fluctuations within an asymmetric (f=0.05) diblock copolymer melt. Note that d=4t*.
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pulse splitter and delay line to create from one pulse of the x-ray laser two x-ray pulses with a variable
separation in time (t). The two pulses are then permitted to scatter in succession off of the sample, and
the scattered x-rays are detected by a CCD area detector.
Each split pulse will generate a speckle pattern (given that the pulse width is shorter than any
sample dynamics) with the second speckle pattern corresponding to the samples configuration at a time
tafter the first. If the samples configuration is essentially unchanged in t, the two patterns will be thesame. On the other hand, if the samples configuration has completely changed after a delay t, the two
speckle patterns will be uncorrelated. The speed of light (3108 ms-1) together with the reasonable range
of delay line lengths (0.3 mm to 3 m, say) implies that it may be possible to create pulses with t=10-12s
to 10-8 s.
Although this range of times is similar to the range that can be achieved with NSE, it is pertinent
to recall that only a handful of NSE instruments exist in the world, and that with XPCS it will be possible
to study much smaller samples of difficult-to-manufacture materials or the dynamics of thin films. One
exciting avenue of research, for which being able to study small sample volumes will be an important
asset, will be to study the dynamics of internal motions within protein molecules. XPCS studies
performed on protein crystals, where crystallographic order orients the vibrational/relaxational modes,may prove especially informative in assessing the key motions of these fundamental molecular machines.
It is not possible, however, to separately read out CCD images from pulses separated by
10-12-10-8 s. Nevertheless, the statistical properties of the intensity distribution in the combined image
reveal whether the two contributing speckle patterns are correlated or uncorrelated, and indeed the extent
of their correlation [5659]. Although the conclusions are the same for partial coherence, for clarity we
will consider a perfectly coherent incident x-ray beam. In the case of a sample with slow dynamics
compared to t, it is as though there is a single speckle pattern, and the variance of intensity distribution
relative to the mean intensity squared - that is, the contrast of the speckle pattern - is 1. By contrast, the
sum of two uncorrelated speckle patterns has a contrast of 0.5. If t is varied over the range of
characteristic times of the samples dynamics, one will go from a constrast of 1 for small tto a contrast
of 0.5 at large t. Clearly, the variation of the contrast versus tdetermines the characteristic relaxationtimes of the sample. Thus, the second method of carrying out XPCS involves measuring the speckle
contrast for different settings of the delay line.
1.2.2.5 Proposed Work
In both of the XPCS methods that we have outlined, it is necessary to read out batches of
100 successive CCD images each separated one from another by 1 ms. These raw data must then be
processed to yield either correlation functions (method 1) or the speckle contrast (method 2). Because of
the very high data rates, a development effort to realize the data acquisition and data reduction envisioned
is necessary. Under this proposal, we will develop a prototype fast CCD detector system suitable for both
methods.
To this end, the first item that we need is a CCD camera capable of 1 ms readout. This is state-of-
the art for current technology. Specifically, we propose to purchase a CCD camera under this proposal
the Dalstar 1M60 from Dalsathat can read out an array of pixels in 0.86 ms. Mochries group has
already shown that this relatively inexpensive, commercial camera may be straightforwardly modified to
become a successful detector for XPCS [60].
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In the context of XPCS experiments at the proposed x-ray laser64 1024 pixels per
1 millisecond repeated 100 times per secondthe overall data rate is about 6 Megabytes per second, or
22 Gbytes per hour, or 500 Gbytes per day. It would be theoretically possible to store these quantities of
data for later data reduction and analysis. Practically, however, it is clear that such a data storage rate
would be difficult to maintain for extended periods, and if it becomes necessary to interrupt data
acquisition to do data storage, some of the gain in the signal-to-noise ratio from parallel detection in many
channels is lost. In addition, in order to be able to make sensible decisions during the experimental run, itis far preferable to reduce the data in real time, so that the correlation functions are available during the
experiment. Anticipation of CCD cameras with yet faster readouts, leading to increased data rates, also
directs one towards real-time data reduction. Thus, we propose to develop a CCD detector system, based
on the Dalstar 1M60, read out by 4 frame grabbers in 4 fast computers, one for each of the cameras
4 taps. The computers, in turn, will calculate intensity correlations versus time (method 1) or the
speckle pattern contrast (method 2) in real time. The modular nature of the system means that it should be
straightforward to upgrade to a faster detector, or faster frame grabbers, or faster computers as they
become available and/or required.
Next, we require computer code that carries out the necessary calculations. Writing, debugging,
verifying, and refining this software is an essential part of the work of this proposal. A key advantage ofour software-based data reduction scheme is that it will be possible to continuously improve, for example,
how g2 is calculated, if improved algorithms become available. A software-based implementation also
facilitates other methods of processing data should that be desired. Others have implemented CCD-based
PCS (multi-speckle PCS) in real time [61,62]. The distinction here that warrants a significant
development effort is that the raw data rates are hundreds of times higher than what was achieved
previously.
Finally, it is worth remarking that, although the camera system we propose is a prerequisite for
carrying out XPCS at the proposed x-ray laser, the real-time data reduction that we will implement will in
addition very greatly improve the user-friendliness of XPCS at third-generation rings, providing
experimenters with the same sort of immediate review of the data as is available in light-scattering
experiments. Thus, our proposed detector system development effort has the potential to greatly increasethe pool of scientists who may be interested in carrying out XPCS experiments now and in the future at
the proposed x-ray laser.
1.2.3 Femtosecond Spectroscopy in Solution-Phase Chemical and Biophysical Systems
1.2.3.1 Introduction
In large part, the advances made in chemistry and other molecular sciences during the twentieth
century involved molecular structure: its description, determination, creation, and manipulation. Atoms
were described as the building blocks of molecules, and the construction of molecules from diatomics to
DNA was essential to describing their chemical, physical, biological, and material properties. Changes to
molecular structure through reactions (or mutations) are thus used to influence such properties orfunction. Through this development, much of molecular structure has also been largely described in time-
invariant or static terms. Part of the reason for this may be that many of the tools we use to characterize
structure are time-averaged measurements. However, every chemist inherently deals with structural
change, even at levels as simple as drawing arrows to indicate the motion of electrons in chemical
reactions. Increasingly, researchers in the molecular sciences are emphasizing the importance of
describing, watching, and controlling the time-evolution of molecular structure. This is of broad
importance, whether to watch and control changes in molecular bonding during a chemical reaction, to
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understand the folding of a protein into its physiologically active structure, or to describe structural
transformation or phase transitions in molecular materials.
For studies of molecular dynamics in solution there are no broadly applicable tools to characterize
how structure changes on appropriate timescales. For that reason, the study of solution-phase chemical
and biophysical problems would greatly benefit from new structurally sensitive experimental methods for
describing molecular and collective dynamics. Femtosecond x-ray spectroscopy and scatteringexperiments offer the promise of unique sensitivity to atomic, molecular, and collective structure with
time resolution exceeding that of nuclear motions. Such techniques thereby offer tremendous
opportunities for characterizing molecular dynamics in liquids and other amorphous systems. The
proposed MIT/Bates x-ray laser project represents the creation of a center of intellectual activity that will
move this under-developed area of science forward, training and developing scientific personnel in an
emerging discipline within the chemical and biophysical sciences.
For the exploratory work under this proposal, a variety of possible experimental applications of
femtosecond x-ray pulses to studies in the liquid phase will be evaluated, for their use in the study of
near-equilibrium and non-equilibrium dynamics in solution. These vary by the nature of the chemical
system studied, the x-ray interaction employed, and the type of distance scales to be probed. The resultsof these feasibility studies will form a primary input to the design of a beamline for such experiments. In
the following sections, a general overview of methods applicable to chemical problems in solution is
described, followed by a more detailed description of four experiments. These four examples represent
specific experiments whose feasibility can be evaluated quantitatively through modeling, and which
overlap with other complementary time-resolved spectroscopies.
1.2.3.2 The MIT/Bates X-ray Laser
Several aspects of the proposed x-ray laser make it a unique as a facility for the study of solution-
phase molecular dynamics. The pulse length of 100 fs for unseeded beams makes it useful to all but the
fastest studies that involve observation of the motion of nuclei, opening experimental possibilities over
time scales from femtoseconds to milliseconds. The ability to seed beams to pulse lengths of 1 fs or lesswill then cover all the relevant timescales. The extremely high peak brilliance of this source compared
with others is vital to time-domain spectroscopies. All such methods use multiple pulses to interrogate a
sample, and generally involve detecting small changes in what would otherwise be already small signals.
Perhaps the most powerful attribute of the proposed x-ray laser source is the broad tunability over the soft
to hard x-ray energy range. For chemists this translates into the ability to spectroscopically probe distance
scales from the very local with atom specific excitation, to the mesoscopic with scattering experiments
that interrogate the 0.1-100 nm distance scales relevant to the study of numerous complex condensed
phases and biological systems.
1.2.3.3 Time-Resolved X-ray Spectroscopy of Molecular Dynamics in Solution
Studies of molecular dynamics in solution can be broken into two broad categories. One set of problems involve studies of dynamics near equilibrium, such as the changing collective structure of
liquids, conformational fluctuations and large amplitude motions in biopolymers, and dynamical
heterogeneity in supercooled liquids and glasses. Perhaps more experimentally challenging is finding
ways of describing the complex structural changes accompanying non-equilibrium processes, such as the
changes in nuclear and electronic configurations during chemical reactions, or large-scale conformational
changes in biophysical processes, such as protein folding or binding of substrates. Whether near or far
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from equilibrium, a broad predictive understanding of these types of processes requires experimental
methods that probe the evolving structures in these systems.
Generating high-brilliance coherent femtosecond x-ray pulses opens the door to numerous
possible techniques that could be used to study solution phase problems. These techniques will require
two or more pulsed electromagnetic fields, of which one or more act initially as a pump, to prepare a
system by initiating a chemical process or perturbing the system from equilibrium, and one acts as aprobe, to detect a time-dependent change. These can be broadly classified as optical-pump/x-ray probe
and multiple-pulse x-ray experiments. Each in turn can involve time-resolved x-ray absorption
spectroscopy (XAS) on core level electrons, scattering, or diffraction processes. A system can also be
probed by x-ray emission, although with limited control over the time scale of the dynamics to be probed.
The spectroscopic theory for such experiments is rapidly gaining momentum, and in the perturbative
limit, can formally be related to third-order nonlinear spectroscopies [63,64].
Optical-pump/x-ray probe experiments can be used to follow photo-initiated chemical processes
or molecular relaxation processes with structural selectivity. Resonant optical pumping can be used to
photoinitiate chemical reactions varying from charge transfer to bond rearrangements. Near-infrared
pumping of solvent overtones can be used to induce rapid temperature jumps for protein folding studies.Nonresonant optical pumping can be used as an impulse perturbation to an equilibrium systema method
often used in study of collective relaxation in complex liquids. Femtosecond x-ray absorption or
scattering can then be used as a structurally sensitive probe of these initiated dynamics [65,66].
Multiple pulse or nonlinear x-ray absorption or scattering experiments can be used to follow the
dynamics initiated by an x-ray absorption or scattering event. For pulses resonant with the absorption
bands of core electrons, such experiments include techniques broadly used in the optical range such as
pump-probe or hole-burning experiments, and transient gratings, yet with the additional control over
molecular scale excitation wave-vectors [64]. Such experiments would be sensitive to the dynamics of
molecular relaxation processes viewed through the core electron levels. While the lifetimes of core level
holes are much shorter than the planned 100 fs pulse length [67], other relaxation and transport processes
can be probed [68]. To the degree that inhomogeneous or dynamic line broadening may be present, x-raytransient photochemical hole burning or pump-probe could be used to probe structural dynamics or
heterogeneity present in the broadening of absorption lineshapes. Transient grating experiments would
follow relaxation of excitations prepared with a well-defined wave-vector on distance scales
corresponding to molecular and collective structures. Coherent multi-pulse experiments detected with
wave-vector selective scattering offers the ability to follow molecular dynamics through their influence
on the interference between fields scattered from two delayed pulses. For the case of nonresonant
excitation and probing, this is an x-ray photon correlation spectroscopy that will allow the large-scale,
collective motions of liquids or macromolecules and the heterogeneity to be probed as a function of time
and wavevector [69].
1.2.3.4 Beamline Concepts and Proposed Work
Building on the afore-developed general concepts, four types of experiments are discussed here
for their use in solution phase molecular dynamics, from the collective structure in liquids to chemical
reaction dynamics in solution and rapid protein folding events. The goal of this project is to investigate
the feasibility of these experiments theoretically and computationally, and provide input to the design of
beamlines for such experiments. The goal is to undertake specific investigations and to communicate a
conceptual and theoretical framework with which to describe and model such femtosecond x-ray
experiments. The feasibility studies will also permit subsequent investigations into the experimental
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systems that lend themselves to femtosecond x-ray studies and complementary optical and infrared
experiments.
X-ray Probed Optical Kerr Effect Spectroscopy (XOKE). The femtosecond Kerr effect
spectroscopies used in the optical regime have been widely used to study collective structural relaxation
processes in molecular liquids, supercooled liquids, and other amorphous condensed phases [7072].
OKE experiments in two-pulse or transient grating configurations have proven effective at characterizingthe time scales for collective dynamics by measuring the decay of a transient orientational or spatial
anisotropy induced by an optical pulse [73]. The lack of microscopic spatial selectivity makes separating
processes such as reorientation, collective librations, density fluctuations, and heterogeneous relaxation
difficult to distinguish.
The selectivity of OKE experiments to collective liquid relaxation can be extended to
characterizing the distance scales associated with relaxation processes, when combined with x-ray
probing. Figure 5 shows a schematic of this XOKE experiment. By exerting a torque on the liquid, a
strong femtosecond optical pulse creates an orientational anisotropy that decays with collective relaxation
processes. A femtosecond x-ray probe pulse can be scattered off this anisotropy to follow the relaxation
as a function of wave-vector, revealing the time scale of relaxation over different spatial dimensions.Probing with wavelengths in the 0.3 to 30 nm range effectively allows all relevant distance scales for
intermolecular liquid motions to be probed. This is a fundamental step in revealing the nature of
collective relaxation processes in liquids and other amorphous condensed phases. Particularly in
supercooled liquids, wave-vector dependent relaxation experiments will throw insight into whether non-
exponential relaxation behavior arises from heterogeneous dynamics in single nanometer scale domains.
Fragile supercooled organic liquids and isotropic phase liquid crystals are both excellent
candidates for probing structural correlation lengths and wavevector-dependent dynamics [71,74].
Theoretical evaluation of this experiment can work with both atomistic and hydrodynamic descriptions of
the signals, revealing how molecular and collective reorientation as well as density fluctuations are
observed in the experiment. Also the role of the polarization of the fields involved on the motions
observed should be addressed [72].
Photoinitiated Chemical Reaction Dynamics in Solution. Femtochemistry refers to the study
of optically initiated chemical reaction dynamics, which have most often been performed on simple
chemical reactions with femtosecond electronic spectroscopy of the low-lying valence states involved
FIGURE 5 An optically induced anisotropy in a liquid sample is probed by time-delayed small angle x-ray
scattering using a femtosecond x-ray pulse. The change in the scattering function at different wavevectors for
a given time delay S(q,) will gradually return to an isotropic shape with collective relaxation processes.
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[75]. In the case of photodissociation of small molecules, the knowledge of potential energy surfaces and
the ability to directly relate the wavelength of observation to a point on the reaction coordinate allows
much to be said about the dynamics of the nuclei and electronic states involved [76]. For increasingly
complex reactions, other structurally selective probing methods are required. Time-resolved x-ray and
electron diffraction experiments are beginning to be used in this context to study the gas phase reaction
dynamics of small molecules [77], or for probing of phase transitions in crystalline substances [78,79].
While scattering experiments in an isotropic solution may have some use in following the
reactants, transition states, products, time-resolved XAFS provides an alternate approach that gives local
information on changes in electronic and nuclear structure [79,80]. The applicability of this method has
recently been demonstrated. XAFS probing of optically induced charge-transfer reactions have shown the
ability of such methods to probe the structural changes accompanying excited state processes, [65,81].
Such techniques have broad applicability to the study of electron and proton transfer processes,
isomerization, photodissociation and more complex reactions in solution. XAFS would also be a sensitive
probe of the structural changes accompanying optically initiated spin state transition in iron complexes
[82].
We plan to evaluate is the use of x-ray probes of femtosecond UV photodissociation of CO frommetal carbonyls in solution. Photodissociation of metal dicarbonyls and hexacarbonyls in hydrocarbon
solvents have been widely studied and show rich dynamical behavior and solvation effects [83]. Also,
these lend themselves to complementary femtosecond infrared vibrational spectroscopies [84]. It is of
interest to not only follow the time-scale for the leaving of the ligand and solvation of the fragments, but
to determine what femtosecond x-ray spectroscopy can say about the relative geometries of fragments and
solvent during such a process. Femtosecond XAS or XAFS probing of the metal L-edge and K-edge of
the O will be tested as a structural probe of metal and ligand during dissociation and solvation.
Transient Photochemical Hole Burning of X-ray Absorption Line Shapes. Little is known
about the dynamic or static line broadening mechanisms for x-ray absorption features. Since the lifetime
of the core hole is exceedingly fast (
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larger scale, questions exist regarding the heterogeneity of denatured states, the degree of native structure
and compactness of intermediates, and how these are reflected in the topology of protein folding free
energy landscapes. These types of questions can be addressed by directly observing folding process and
heterogeneity at a molecular level on all relevant time scales. Probing of protein folding or denaturing
processes in solution with femtosecond x-ray scattering would open up the possibility of probing local
and large-scale structures on the now largely inaccessible time-scales between 100 fs and 1 s that govern
the initial collapse of denatured states [88,89].
In crystalline systems, time-resolved x-ray diffraction has been demonstrated as effective in
probing the localized dynamics of ligands and sidechains following picosecond excitation of
photoinitiated protein dynamics [90]. The more conformationally flexible, large amplitude motions in
solution have been probed through time-resolved small angle x-ray scattering (SAXS) on s to second
time scales, [66,91]. Such measurements have been used to follow the change in the radius of gyration for
the protein. To access the formation of protein structure in solution on various distance scales and over
shorter time scales, time-resolved SAXS can be used to probe folding induced by an optical pulse [89,92].
Perhaps the most general approach is folding initiated by a nanosecond temperature-jump experiment on a
cold-denaturing protein. A nanosecond near-infrared pulse can be used to rapidly heat a sample through
solvent overtone absorption, allowing the nanosecond to microsecond time scales of protein folding ordenaturing to be studied [93]. For a single excitation pulse, a broad range of time scales can be probed
with a sequence of micropulses from the x-ray laser pulse train. Equivalently, binding experiments can
also be initiated with optical pulses [94]. For instance, the binding of divalent zinc or calcium ions can be
initiated by optically releasing these ions from a photolabile ligand [95]. Binding of the ions by proteins
or peptides could be probed either with SAXS or XAS. As with the XOKE experiments, analysis of the
induced scattering change as a function of time and wave-vector can potentially be used to establish the
timescales for formation of secondary and tertiary structure and the degree of compactness of the protein.
It is possible that the analysis of transient and temperature-dependent equilibrium scattering patterns can
be used to quantify structural heterogeneity of the folding proteins.
1.2.4 Trace Chemical Analysis in the Particle Counting Limit
1.2.4.1 Introduction
Trace particulate analysis is at once an important and a difficult area of forefront analytical
science research. The importance is exemplified by, but not limited to, the needs of the semiconductor
industry to analyze devices whose features will shortly approach 100 nm and for which one impurity atom
can dramatically affect device performance. The surface composition of submicron sized particulates is
crucially important in assessing their environmental impact. Similarly, microbiologists attempt to measure
the three-dimensional location and abundance of single altered molecules contained in a single cell. In the
future, where nanotechnology promises to become an important tool, such measurements are likely to
become even more important. The difficulty in the trace measurement of such samples arises in