State-specific surface scattering with laser-prepared molecules

transcript

Dynamics of Molecular Collisions

Asilomar July 2005

Outline INTRODUCTION: Evidence for breakdown of Born-

Oppenheimer Approximation for molecules at metal interfaces.

EXPERIMENTAL

Vibrationally promoted ejection of electrons from a surface

Relative importance of vibration and translation on trapping at a surface.

Eyring and Polanyi said…Let there be the Born Oppenheimer Potential Energy Surface Many talented theoreticians

work for 6-7 decades…

Many great experiments

Agreement between experiment and theory leads to “understanding”

Predictive power for new reactions • H. Eyring, J. Walter, G. E. Kimball, Quantum Chemistry

(John Wiley and Sons, New York, 1944).• H. Eyring, H. Gershinowitz, C. E. Sun, J. Chem. Phys. 3, 786 (1935). • J. Hirschfelder, H. Eyring, B. Topely, J. Chem. Phys. 4, 170 (1936).

H + H2 exchange reaction

“Über einfache Gasreaktionen”, H. Eyring and M. Polanyi, Sonderdruck aus Z. Phys. Chem., Abt. B, 12, Heft 4.

D-atom flux contour map from the H+HD reaction

Experiment TheoryH

H + HD → H2 + D

Rydberg Tagging of reaction products- by X. Yang

Full quantum dynamics on accurate PES - by R. Skodje

“For F+H2, What is the importance of multiple surface scattering?” …Cassavecchia

What happens when we apply this strategy to reactions at metal surfaces?

N-N recombination on Ruthenium: 2N(ad) →N2(v)(g)

L. Diekhoner, L. Hornekaer, H. Mortensen, E. Jensen, A. Baurichter, V. V. Petrunin, A. C. Luntz, J. Chem. Phys. 117, 5018-5030 (2002).

Surface by: M. J. Murphy, J. F. Skelly, A. Hodgson, B. Hammer, J. Chem. Phys. 110, 6954-6962 (1999).

Theory

Experiment

Early barrier

Dissociation of O2 on Aluminum

Experiments show activated process… Indicative of reaction barrier.

Behler, J., Lorenz, S., Reuter, K., Scheffler, M. and Delley, B., 2004

L.Österlund, I.Zoric,B.Kasemo, PRB 55 (1997) 15452

Born-Oppenheimer PES Calculated with DFT shows no barrier!

Both results imply: Important energy transfer between the reactive center and the surface N2 reaction on Ruthenium

Many vibrational quanta appear to be lost from the molecule during the 10’s of femto-seconds required for the product to leave the surface

O2 reaction on Aluminum Despite no electronically adiabatic barrier on the potential energy

surface, translation energy is need to induce the reaction

Possible important energy transfer couplings To phonons: vibration of the metal substrate To electrons: creating excited electron hole pairs (EHP’s)

Born-Oppenheimer Approximation at metal surfaces is less obviously correct The stronger attraction of a molecule to a surface, proportional to the inverse

third power of distance compared to the inverse sixth power in the gas phase, can lower the energies of more polarizable excited electronic states, bringing them nearer in energy to the ground state.

More dramatically, positively or negatively charged molecules at surfaces are

stabilized by an image potential proportional to the first power of the distance, frequently resulting in the crossing or avoided crossing of ionic and neutral potential energy surfaces.

Metals bind electrons more weakly (work functions are generally less than 5 eV) than gas phase molecules (Ionization potentials are generally more than 8 eV).

Finally, metal surfaces exhibit a continuum of electronic states, the conduction band, for which there is no energy separation whatever between electronic states. Electron-hole pair (EHP) transitions between electronic levels in the conduction band can provide a mechanism for energy transfer with an adsorbate molecule and perhaps even call into question the applicability of the concept of motion evolving on a PES.

Excellent review: J. C. Tully, Ann. Rev. Phys. Chem. 51, 153 (2000).

Of greatest interest: How is energy exchanged between nuclear motion and metal electrons near the Transition state?

Products (B-C)weak coupling to metallic electrons

Transition Statestrong coupling to metallic electrons!

Reactants (A-B)weak coupling to metallic electrons

Experimental Approach

Experimental Setup: NO(v=15) + Au(111)

Au(111)

Optical Pumping on Nitric Oxide

Capabilities Franck-Condon Pumping of low

vibrational states (v=0-10) Stimulated Emission Pumping of

high vibrational states (v<27, 5.1 eV) Laser induced fluorescence

detection Resonance enhanced multiphoton

Ionization detection A few percent of sample may be

excited.

Large amplitude vibration a mimic of the kind of motion near the transition state!

1.2 1.4 1.6

Numerical Solution to the Vibrational Schrödinger Equation for NO

The Stark Effect: Quantum Broom to Sweep Away the Ground State

)1()1(

)12/3(*2/3

2/3*2/3

ESO~120 cm1

)12/1(*2/1

2/1*2/1

NO: ~0.15 D

Ground Spin-Orbit State has weak Stark Effect

Excited Spin-Orbit State is unpopulated in a beam

SEP can prepare high v-state with large Stark effect

D. Matsiev, J. Chen, M. Murphy, A.M. Wodtke J. of Chem. Phys. 118 9477-9480 (2003);Chem. Phys. 301(2-3) 161-172, (2004)

Hexapole acts to focus highly vibrationally excited molecules selectively v=18 21/2

v=18 23/2

v=0 21/2

Fields of 150 kV/cm now routine

D. Matsiev, J.Chen, M. Murphy, A.M. Wodtke J. of Chem. Phys. 118 9477-9480 (2003)

Beam is re-focused in two dimensions to about 1-mm

75 cm downstream sample returns to ~1-mm size

observed simulated

Three-dimensional refocusing will be possible in the future.

When it is all put together, a machine that can…

…transport optically prepared molecules to UHV surface science chamber and refocus them.

…enrich beam in concentration of high-v molecules.

…select M-states for orientation studies.

…Provide vibrational-state specific dipole moments.

Scattering from a simple inert metal surface

NO(v=15) from Au(111) and comparison to LiF insulator

Multiquantum Vibrational Relaxation on Au(111)

15 14 13 12 11 10 9 8 7 6 50.0

0 -17.

Probabili

ty (ar

Evib (kJ/mole)

Final Vibrational State (v) Important Points <3% survival in v=15 (by summing histogram above) 150 kJ/mol most likely vibrational exchange Vibrational energy is transferred to the metal surface

Conditions Prepare NO(v=15) with SEP Ei= 0.05 eV

Ts = 300K Clean Au(111)

Y. Huang, S.J. Gulding, C.T. Rettner, D.J. Auerbach A.M. Wodtke, Science 290, 111 (2000)

Sub-picosecond time-scale for energy exchange

8090100110

v=11 Ei=0.05

v=8 Ei=0.30

v=10 Ei=0.30

v=11 Ei=0.30

v=13 Ei=0.30

Strongly peaked angular distributions independent of Eincidence

Independent of v Time-Scale of the Interaction is ~10-13 Sec

Prepared NO(v=15) + Au(111)

Quite different dynamics observed on insulators.

Without the Electrons: Scattering from cleaved LiF insulator surface Scattering of NO(v=12)

from LiF Limited vibrational

relaxation No Detectable vibrational

excitation

Phonon frequencies higher on LiF than Au

9 10 11 12 130.0

Vibrational State

Vibrational Population distribution after scattering

Ts=480K and Ein=72 kJoule/Mol

Survival probability vs. incidence energy on an insulator NO(v=12) survives LiF

Low Energy: Trapping Desorption

High Energy: direct scattering

0 10 20 30 40 50 60 70 80

Energy (KJ/mole)

A.M. Wodtke and Y. Huang, D.J. Auerbach, J. Chem. Phys.118(17) 8033-41 (2003)

Some ideas about mechanisms

Electron transfer appears to play an important role.

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5-7.3

1.0N + O + e

Electro

nic Energy

1.0 1.5 2.0 2.5

r >(v=1

rer< (v=15)

Electron Binding Energy (eV)

NO’s ability to bind an electron depends strongly on bond length

Image charge interaction allows charge transfer events

Newns, Surf. Science 171, 600 (1986) 1 2 3 4 8 10

Au) EA(NO)

1.1 eV

Image Charge

interaction ~1/R

The critical factors here are: Surface work functionElectron affinity

A picture is worth a thousand words

Monte Carlo Wave Packet Study of Negative Ion Mediated Vibrationally Inelastic Scattering of NO from Metal Surface Shenmin Li and

Hua Guo, J. Chem. Phys,

v117 2002

Qualitative features of this picture appear in theory.

Outer turning point electron transfer to NO

Electron re-transfer to metal at smaller bond lengths

Does this mean multiquantum relaxation on Au excites a single electron? If, for example, transient

NO is formed Dipole derivative for NO

during large amplitude vibration is quite large ~8 Debye/Å

Dipole function is quite linear. Might couple one vibration

at a time to many electrons as the anion equilibrates to the surface.

1.0 1.2 1.4 1.6

Inernuclear Separation (A)

NO Dipole moment

ccsdtqcisdt

Can we observe vibrationally induced electron emission from a low work function surface?

If you can convert such large amounts of vibrational energy to electronic excitation……Can you induce electron emission

form a low work-function surface?

Details of the Apparatus

Photoemission probe of Cs-doseBeam exposure to low work-function surface

View from the molecular beam

View looking down from above

Photoemission vs. Cs Dosing of Au(111)

0 10 20 30 40 50 60

Cs Dose (Seconds)

Working here~1.3-1.6 eV

Experimental observation of vibration induced electron emission from a solid Spectroscopic identification of the signal

Fluorescence Dip’s indicate the resonances where, NO(A-state) population is moved back to NO(X-state) (v=18)

Emission of electrons from the surface are also shown

Two pump lines indicated Q21(0.5) Q11(0.5)

461.5 462.0 464.0 464.5

Wavelength of Dump Laser (nm)

Fluorescence D

epletion

NO(v=18) scattered from Cs/Au at Ein=29 meV

NOA2(v=3)→X2(v=18) Resonances observed in two ways

Vibrational quantum number dependence of electron emission: Preliminary results

Emission probability high Previous reports of

electron emission due to exothermic surface reactions ~10-6-10-8

Threshold coincident with work-function

J. White, J. Chen, D. Matsiev, D.J. Auerbach and A.M. Wodtke, Nature 433(7025),503-505, (2005).

Oxygen Coverage Dependence: Indirect evidence for work function dependence

0.0 0.5 1.0 1.5 2.0 2.50

Oxygen Exposure (L)

Electron Emission on Oxygen Free Surface

Low Coverage maximum

Possible evidence of signal scaling with surface work function

J. White, J. Chen, D. Matsiev, D.J. Auerbach and A.M. Wodtke, J.V.S.T. 23, 1085-1089 (2005).

Vibrational and Translational Influence on Trapping

Few experimental measurements

v = 12

vibrational quantum number of NO(v)

z = distance of NO from Au(111)

r = interatomic distance of NO

Sharani Roy and John Tully: A new potential energy surfacee.g. plots for NO approaching an On-top Site

Charge on NO (a.u.)

z (Å)

Energy (a.u.)

z (Å)

Binding Energy = 0.72 eV

N-Metal distance ~ 3.0 Å

Charge on NO ~ -0.6 (z = 2.4 - 2.8Å r = 1.6 Å)

Experimental Logic

Calculated well depth is ~-0.5 - 0.7 eV. This means if trapping occurs, residence times exceed 30 s.

If trapping of vibrationally excited molecules occurs, thermal accommodation and desorption in ground vibrational state can be assumed.

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.800.0

Incidence Kinetic Energy (electron-Volt)

State Specific Survival Probabilities

Why don’t survival of v=2 and 0 look identical at high incidence energy?

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.800.0

Because v=2-1 relaxation increases in probability with incidence energy

After correcting for this…

One can obtain trapping probability vs. translational energy for v=0 and 2

Insensitivity of trapping at surfaces to molecular vibration

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

State-specific surface scattering with laser-prepared molecules

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