Effects of introducing a gas into the free-electron laser
R. H. Pantell, A. S. Fisher, J. Feinstein, A. H. Ho, M. Ozcan, and H. D. Dulman
Department of Electrical Engineering, Stanford University, Stanford, California 94305
M. B. Reid
NASA Ames Research Center, Moffett Field, California 94305
Received November 3, 1988; accepted February 10, 1989
Many interesting applications of the free-electron laser (FEL) require the extension of the operating wavelengthinto the ultraviolet region of the spectrum. The introduction of a gas into the wiggler section of a FEL alters thephase velocity of the electromagnetic wave and so changes the synchronism condition relating wavelength to wigglerparameters and beam energy. This provides a means for tuning the frequency of an oscillator, and with the additionof 200 Torr of hydrogen gas the wavelength of a FEL operating in the near infrared without gas was reduced by 0.73,um. The plasma generated from ionization of the hydrogen molecules by collisions with the electron beamdiminished the oscillator gain, but this effect was eliminated by the addition of less than 0.1% of an electronattachment gas. Gain is also reduced by multiple scattering of the beam electrons, but this effect is not severe for a1-m wiggler length. When hydrogen is used, a FEL with fixed wiggler parameters and electron energy can be tunedfrom the near infrared to -1200 A, and with helium the wavelength can be reduced to 600 A.
INTRODUCTION
Because of the many potential applications for free-electronlasers (FEL's) that are easily tuned and can operate at wave-lengths shorter than the fundamental attainable in currentlypractical wigglers with reasonable gain, it is of interest toconsider possible laser physics techniques that could extendthe wavelengths of FEL's toward the ultraviolet withoutrequiring an adjustment of wiggler parameters or electronbeam energy. In this paper we describe a technique that hasbeen demonstrated to have significant promise in this re-gard: adding a gas to the wiggler.
Adding a gas to the wiggler section of a FEL reduces thephase velocity of the electromagnetic wave and therebychanges the synchronism condition. If n is the refractiveindex of the gas, the new relationship is
(n-1) + = (1)xw 2'y2
where
X is the wavelength,X, is the wiggler period,a, is the rms wiggler parameter = 0.093 Be (kG) XA (cm),Bw is the wiggler field, andy is the ratio of electron energy to rest energy, >>1.
For n = 1, i.e., no gas, Eq. (1) is the usual FEL synchronismcondition, with the exception of the presence of the . Invacuum only the + is appropriate, corresponding to theelectron's slipping one optical period per wiggler period.With a gas, however, the phase velocity is reduced so thatanother synchronism condition exists in which the electronadvances one optical period per wiggler period, correspond-ing to the - in Eq. (1). Operation with the + has been
designated the phase-slip mode, and with the - as thephase-advance mode.
For most cases the preferred gas is hydrogen because thenthe multiple scattering is less than for any other gas whenthe refractive index is specified. The expression for (n - 1)for hydrogen is",2
-6p 3 /2(n - 1) = io- 6 P- 21.1+ 12.7 X 10-3
Xi2 /(2)
where
P is the pressure in atmospheres,T is the temperature in kelvins, andX is the wavelength in micrometers.
This expression is valid from the near infrared to -1200 A,which is a wavelength slightly longer than the vacuum-ultra-violet electron resonance in hydrogen. The simultaneoussolution for Eqs. (1) and (2) leads to the tuning curves for thegas-loaded FEL (GFEL) using hydrogen, which are shown inFig. 1 for the parameters X = 2.3 cm, a = 0.7, and -y = 80.Tuning is obtained only by changing gas pressure, with wig-gler parameters and beam energy held constant.
From Fig. 1 it is seen that as the electronic resonance isapproached the gas pressure decreases, which follows fromthe fact that the refractive index increases appreciably closeto resonance. This means that a given wavelength shiftrequires less pressure. It is also apparent from this figurethat at a given pressure there can be more than one synchro-nous wavelength. The wavelength at which oscillationwould occur depends on the net GFEL gain at each wave-length, which is a function of both the optical cavity lossesand the beam-wave interaction mechanism. Another fea-ture illustrated by Fig. 1 is that at a specified wavelength the
range from infrared to ultraviolet wavelengths.3 If the FELin vacuum has an infrared electronic gain of 36% per pass,then the GFEL gain varies between 13% and 42% throughthe visible and into the ultraviolet.
The requirements on beam quality can be estimated byallowing 7r phase slippage from the synchronism relationshipover the interaction length L. This gives an allowed angulardivergence AO of
A = (X/2L)'1 2 , (5)
which is the same as the expression for the vacuum FEL, andan energy acceptance
A = _ ,
y - 0 2Ny(6)
Pressure (atm)
Fig. 1. Oscillation wavelength for the GFEL as a function of hydro-gen-gas pressure. The parameters are X= 2.3 cm, a, = 0.7, and y =80.
phase-advance mode requires higher gas pressure than thephase-slip mode. Since multiple scattering increases withpressure this would tend to favor the phase-slip mode, but,as will be shown, for a planar wiggler the gain can be higherfor the phase-advance mode.
The advantages to adding a gas are as follows:
* The ability to tune the FEL over a large bandwidth.When hydrogen gas is used the wavelength range is from thenear infrared to '1200 A. With helium the wavelengthcould be as low as -600 A.
* A reduction in the cost and size of the accelerator for aspecified wavelength through a reduction in the requiredbeam energy. With the wiggler parameters shown in Fig. 1,in vacuum the beam energy is 400 MeV at X = 1200 A andwith hydrogen gas it is 40 MeV.
* An increased gain per unit length. Since the gainvaries inversely with a power of the beam energy, operationat a lower energy increases gain.
The small-signal gain expression for the GFEL with ahelical wiggler is identical to the expression for the conven-tional FEL. For a planar wiggler, the conventional wigglerhas an additional factor F:
F = [Jo(M) -J(M)],
which is replaced by F':
F' = [Jo(M) J(M)]2
(3)
(4)
for the GFEL, where M = Xwa 2/4Xy2. The minus in Eq. (4)is associated with the phase-slip mode, and the plus with thephase-advance mode. For the conventional FEL, M has amaximum value of 0.5, whereas for the GFEL M is unbound-ed. For example, for a GFEL with XA = 2 cm, X = 1.0 jim, aW2
= 1.3, and y = 80, we have that M = 1.0. For these parame-ters F' for the phase-slip mode is 0.11, and it is 1.45 for phaseadvance, so that the phase-advance mode may have thehigher gain even though the pressure, and therefore themultiple scattering, is greater.
Using typical parameter values in a model incorporatingthe effect of the gas, we have calculated the GFEL gain in the
where X0 is the wavelength in vacuum and N, is the numberof wiggler periods. Equation (6) is the same for the FELwhen X = X0 and is reduced when gas is introduced.
The GFEL is not without potential problems,4 which maybe categorized as alterations in the electron beam, alter-ations in the gaseous medium, and alterations in the electro-magnetic wave. Many of these problems are listed in Table1, and of particular concern are the beam-plasma instabil-ities that have been observed previously for beams passingthrough gases.5 An important distinction between theGFEL experiment and earlier experiments is that the dura-tion of the electron beam pulse, approximately 1 psec, is 3orders of magnitude less that the duration of pulses used inprevious studies. The buildup time for most beam-plasmainstabilities is longer than our pulse duration, so that theseinstabilities should not develop.
EXPERIMENTAL RESULTS
Experiments were performed in the following sequence:
A. The electron beam was passed through hydrogen gasto observe whether instabilities occurred.
Table 1. Potential Problems with the GFEL
A. Alterations in the electron beam1. Angular scattering in the gas and containing foil2. Energy loss and straggling owing to multiple collisions in the
gas and containing foil3. Ionization of the medium and resultant beam-plasma insta-
bilities4. Space-charge neutralization of the electron beam owing to
medium ionization, and resultant pinch5. Energy loss owing to reverse current excitation in the plasma
B. Alterations in the medium (change in refractive index)1. Temperature rise during the macropulse owing to inelastic
collisions with electrons2. Ionization of the gas owing to electron collisions3. Electromagnetic breakdown.4. Pressure variations associated with the pressure-control sys-tem
C. Alterations in the electromagnetic wave1. Breakdown2. Self-focusing3. Raman scattering4. Brillouin scattering
3.0
2.5
2.0
1.5
1.0
Pantell et al.
1010 J. Opt. Soc. Am. B/Vol. 6, No. 5/May 1989
B. A foil was placed in the path of the electron beamentering a FEL wiggler. For the GFEL experiment a foilwas used to restrict the gas to the region of the wiggler, i.e., toprevent the gas from filling the linac, and the purpose of thisexperiment was to measure the effect of the foil on the FELgain before the addition of gas.
C. A small amount of gas, -5 mTorr, was introduced intothe wiggler so the effect on FEL gain could be observed.
D. Gas pressure was increased to 200 Torr, the maxi-mum pressure for which the foil was tested.
Step A. Beam-Propagation ExperimentIn the first experiments an electron beam from a rf linac withthe time characteristics shown in Fig. 2 was passed through a1-m length of hydrogen gas. The beam energy was 42 MeV,and the pressure of the hydrogen gas was varied from 10-3 to1.25 atm in 0.25-atm increments. Measurements were madeof the current transmission, the beam size and position, andthe erenkov radiation. The observed increase in electronbeam size was attributable to multiple scattering, and noobvious plasma instabilities were observed. This does notnecessarily mean that FEL operation is unaffected by plas-ma effects, since FEL gain is highly sensitive to beam param-eters. However, the experiment demonstrated that the in-stabilities associated with longer electron pulses (kink, hose,sausage) did not occur.
Step B. Effect of a Containment FoilA thin boron nitride foil was used to prevent the gas fromentering the accelerator, with the arrangement shown in Fig.3. The foil was placed 50 cm upstream of the front end ofthe wiggler so as to be out of the path of the wave in theinterferometer and was mounted upon a gate valve for re-mote insertion and removal. The foil had a thickness of 1.3/m, with a diameter of 6 mm, and was tested to a maximumpressure difference of 200 Torr. This foil has been used formore than 100 h with the relativistic beam passing through itand with pressure differentials as great as 200 Torr. Dark-
Z
, 10
EC[$a)m
0 67
Time (msec)
Fig. 2. Time structure for the electron beam from the rf linac.
H2 inlet I - - 108 cm -. l
BN
/Gate valve
Fig. 3. Arrangement for the GFEL experiment, showing the loca-tion of the boron nitride containment foil.
Table 2. FEL Operating Parameters Used in theGFEL Experiment
Wiggler length, Lw 108 cmWiggler period, X 2.3 cmOptical wavelength (in vacuum), 4.18 gmWiggler parameter, a, 0.97Electron energy, y 73.6Normalized beam emittance 77r mm mradElectron beam diameter 1 mmPeak current 25-100 AMacropulse-averaged current 180 mAMicropulse duration 0.5-2 psecMicropulse repetition period 350 psecMacropulse duration 2.7 secMacropulse repetition rate 15 Hz
ening of the foil has been observed, presumably a conse-quence of the generation of color centers in the boron nitrideby the electron beam.
Typical parameter values for the electron beam and theplanar, hybrid wiggler are listed in Table 2. When the beampasses through the foil there is an increase in the angulardivergence of the beam, which is 0.5 mrad for 40-MeVelectrons and a 1.3-Atm thickness of boron nitride. Theeffect on emittance depends on the beam diameter, so that ifthe beam is focused to a small spot on the foil the incrementin emittance is small. It was calculated that with the foil inplace and no gas in the wiggler, the FEL electronic gainwould be reduced by 16%, i.e., from 30% per pass to 25% perpass, and the measured gain was close to this prediction.6
Step C. The Gas-Loaded Free-Electron Laser at Low GasPressureWhile the boron nitride foil was in place and the FEL wasoscillating with the parameters shown in Table 2, the pres-sure in the wiggler was raised from high vacuum to roughly 5mTorr (by opening a valve to a short length of copper tubingpreviously evacuated by a forepump, a preparatory step forhydrogen filling). Oscillation halted immediately and couldbe resumed only by refocusing the electron beam with quad-rupole magnets upstream of the wiggler. The change infocus was probably a consequence of a partial (-1%) neutral-ization of the electron beam's space charge by the plasmacreated by ionizing the gas.7 This effect was not observed inthe previous beam-propagation experiment because no mea-surements were taken at high vacuum and because FEL gainis a more sensitive indicator of changes in beam properties.
With 1.5 Torr of hydrogen in the FEL, the beam focusingchanges even more strongly. Without the beam's being re-focused, its diameter on a phosphor screen downstream fromthe wiggler doubled over the vacuum case. This increasemust be attributed to a plasma, as multiple scattering causesinsignificant growth at this pressure. Also, in this case theplasma density does not reach a steady state during themacropulse, and the quadrupole settings necessary to trans-mit the full beam current through the narrow vacuum cham-ber inside the wiggler varied throughout the pulse. Lasingcould not be maintained for more than a brief fraction of themacropulse.
Step D. Gas Pressures to 200 TorrAt pressures above 10 Torr a steady state is reached earlyenough in the macropulse that time-dependent focusing no
Pantell et al.
Vol. 6, No. 5/May 1989/J. Opt. Soc. Am. B 1011
longer presents a problem. However, plasma effects stillreduced the laser's gain. Small-signal gain was less thanpredicted from the theoretical model, and oscillation couldnot be obtained above 100 Torr. In addition, the opticalsignal did not build up over the entire duration of the elec-tron beam pulse, and it generally terminated before the endof the pulse. It was concluded that the plasma formed in thehydrogen gas altered the beam propagation owing to a pincheffect and also changed the group velocity of the wave duringthe macropulse so that there was a loss of overlap betweenelectron beam micropulses and optical micropulses.
The plasma effects result from the presence of electronsstripped from hydrogen gas in the path of the primary beam,and by attaching these electrons to a molecule the plasmaphenomena can be eliminated. A good molecule for thispurpose is perfluorocyclobutane, c-C4F8,8 since it is nontoxicand has a rather high electron attachment cross section forelectrons in the energy range of interest, 0.5 to 1.0 eV.
From Fig. 4 it is seen that a small fraction of c-F4F8
reduces the density of free electrons in the plasma by morethan 2 orders of magnitude. It is important that this frac-tion be small because of the increase in both multiple scat-tering and generation of additional plasma electrons fromthe dopant gas. For example, at 1% dopant concentrationthere is a 70% increase in scattering angle over pure hydro-gen, and the rise in plasma density at concentrations greaterthan 1% results from ionization of dopant molecules. Effec-tive reduction of plasma density for GFEL oscillation wasobtained with c-C4F8 concentrations in the range 0.015 to0.1%.
At 100 Torr of hydrogen gas, for example, without c-C4F8
the GFEL gain was highly sensitive to electron beam focus-ing, and the measured electronic single-pass gain was 16%.With the addition of c-C4F8 in the concentration range of0.15 to 0.1% the gain was relatively insensitive to focusing,and the electronic gain increased to 23%. Lasing continueduntil the end of the macropulse and reached saturation.
Figure 5 shows the measured and predicted electronic gaincurves, normalized to unity at zero pressure, as a function ofgas pressure with 0.06% of c-C4F8. With the attachment gasthere was no evidence of plasma effects, and there was good
151CO
I,
a)
Cd
14
1310O
1210
0.00 0.01 0.02
Dopant fractionFig. 4. Density of free electrons in the GFEL as a function of thefraction of dopant (c-C4 F8) added to the hydrogen gas. (Figurefrom Ref. 9.)
1.2
1.
0-.0
M.
Cu
0.
0.
0.
0.
0.
0~ -
.8 - I ", _ _
2 - __ _- _
. _ _0
- Theory* Experiment
100 200 300
H2 Pressure (Torr)
Fig. 5. Small-signal gain for the GFEL, normalized to the gain invacuum, as a function of the pressure of hydrogen gas. (Figure fromRef. 9.)
Table 3. Experimental Results Using Hydrogen With0.06% c-C4F8
EnergySaturated in the
Pressure Power Macropulse(Torr) (kW) (mJ)
0 6.5 1250 4.2 4
100 2.7 3150 2.4 3200 1.8 1.5
agreement between theory and experiment.9"10 The opticalsignal reached saturation at all pressures, and Table 3 liststhe characteristics of the emission at various pressures.
The reduction in small-signal gain with increasing pres-sure is largely a consequence of the increase in electron beamarea with increasing pressure owing to multiple scattering.In the experiment, as illustrated in Fig. 3, the boron nitridefoil was placed 50 cm in front of the wiggler. If a design wereused in which this region of additional scattering, which doesnot contribute to gain, could be eliminated, the gain wouldbe approximately constant with pressure.
Figure 6 illustrates the measured and calculated depen-dence of wavelength on pressure. From Eq. (1), the shift inwavelength in the presence of hydrogen from the wavelengthin vacuum, AX, is given by
AX = -X.(n - 1), (7)
where (n - 1) is obtained from Eq. (2). The range of wave-lengths shown by the bars on the data points results primari-ly from the variation in electron beam energy over the ma-cropulse.
The measured gain reported in Fig. 5 could be obtainedonly with the mixture of c-C4F8 and hydrogen gas flowingthrough the wiggler. When the flow stopped, in a period of1-2 min the optical power diminished -4 orders of magni-tude, and the original power level could be retrieved only byturning on the flow. Figure 7 shows this effect for 0.06% ofc-C4 F8, 50 Torr of hydrogen, and a 15-Hz macropulse repeti-tion rate.
This transient phenomenon is a consequence of the fact
) . . . . r-- -I -- I --1- - I I
i
I I
: I I
iI
I I i
* I I
I ' - I - - -1-- - --
A.
_and , ;
Pantell et al.
, .v
I
1012 J. Opt. Soc. Am. B/Vol. 6, No. 5/May 1989
Wavelengtl(4m)
4.2
4.0
3.8
3.6
3.4
- I X-~~~~ l l l
0 50 100 150 200
Pressure (Torr)
Fig. 6. Oscillation wavelength of the GFEL as a function of hydro-gen-gas pressure. The data points are superimposed upon the cal-culated tuning curve, shown as a solid curve. (Figure from Ref. 9.)
10' -I
as flow turned off
0 10
0~
0 10
E0 - Gas flow restarted0Z .410 -_ L
0 2 4 6
Time (minutes)
Fig. 7. Transient behavior of the optical output power when thegas is turned off and then back on. (Figure from Ref. 9.)
that some fraction (2%) of the -C4 F8 molecules frag-ments" into other molecules and ions when attachment oc-curs. Thus the -C4 F8 in the wiggler is depleted in a timeperiod calculated to be a few minutes, causing the density offree plasma electrons to increase and the recurrence of dele-terious plasma effects. When the gas is flowed, a constantsource of fresh -C4F8 is provided, and this phenomenon isavoided.
The transient reduction in optical power was not observedwhen the macropulse repetition rate was reduced from 15 to1.5 Hz. At the slower rate, there was adequate time betweenpulses to replenish the -C4F8 by diffusion from large reser-voirs of fresh gas located upstream and downstream fromthe wiggler.
EXTENSION TO THE VACUUM ULTRAVIOLET
The FEL on the superconducting accelerator (SCA) at Stan-ford University has operated at 0.5 m with the parameterslisted in Table 4. With relatively little modification, this
oscillator can be extended to -2000 A with the introductionof hydrogen gas into the wiggler. Figure 8 illustrates thearrangement of the FEL on the SCA along with modifica-tions for conversion to gas.
Wavelength dependence on gas pressure is shown in Fig. 9for the phase-slip and phase-advance modes. With thephase-slip mode, tuning into the vacuum ultraviolet can beaccomplished without exceeding 40 Torr of gas pressure.Assuming 10% gain for the vacuum FEL at X = 0.5 Am, thecalculated gain for the GFEL versus wavelength is drawn inFig. 10. Dielectric mirrors with losses much less than thepredicted electronic gain are available to wavelengths asshort as 2000 A. Below this wavelength, to achieve net gainwith aluminum mirrors, for example, the electronic gainwould have to exceed 20%, so that to cover the entire rangedepicted in Fig. 10 it would be necessary to quadruple thebeam current.
There are a number of interesting biophysical and bio-medical applications for vacuum-ultraviolet GFEL, whichcan provide tunable radiation of picosecond duration andhigh peak power. This type of source is used to drive thephotoablative effect,12 in which electronic transitions areexcited by the incident light, and then the stored energy istransferred to molecular dissociation. Selective dissocia-tion or bond breaking by this mechanism can be used, forinstance, to determine the functions of a particular molecu-lar subgroup in a protein. For example, ultraviolet absorp-tion by hemoglobin and the subsequent molecular dissocia-tion has been used to investigate the process of oxygen-ation.'3 In a similar manner the dynamics of DNA havebeen studied, 4 15 as has the role of rhodopsin in the retina.'6
Photoablation has also been used to perform fine surgicalcuts. Because of the high absorption coefficient of water,hemoglobin, and melanin to ultraviolet light (the absorptioncoefficient is >104 cm-'), the depth of photoablation is re-stricted to micrometer dimensions. In addition, the shortduration of the pulse limits the thermal damage to adjacenttissue, and photoablative cuts of -50-um width have beenachieved.'7,8
In summary, the GFEL should permit tunability from theinfrared to the vacuum ultraviolet, and this effect has beenachieved over a 7000-A interval in the near infrared. Wehave also demonstrated the feasibility of propagating pico-second electron beam pulses, in a stable manner, throughhigh-pressure gases. Plasma effects in the GFEL, whichresult from ionization of the gas by the primary electronbeam, were eliminated by the addition of a small fraction of
Table 4. Parameters for the FEL on the SCAWiggler length, L 4.66-mWiggler period, Xe,, 3.5 cmOptical wavelength (in vacuum), X0 500 nmWiggler parameter, a 0.69Electron energy, y 230Normalized beam emittance, cyare 57r mm mradPeak current, Io 2.8 AOptical cavity length, L 12.7 mMirror radii, R1 = R2 8.0 mOptical Rayleigh length, z 3.3 inEnergy spread, Ay/y 0.1%
Pantell et al.
Vol. 6, No. 5/May 1989/J. Opt. Soc. Am. B 1013
r / BN foil location
Wiggler - Q Q __Q Q magnet as inletl
location Mirror
-466 cm | 120 cm 1 55 cm | 185 cm IFig. 8. Layout of the FEL on the SCA showing the upstream end. The same beam optics and pipe dimensions are duplicated at thedownstream end. To convert this FEL to a GFEL it would be necessary to introduce a foil and provide a gas inlet.
0.
0.
0.
0.
5- _ l
4.Phase-slip mode I
3.-
Phase-advance mode
.0 . 5 . 20 25 50 75 100 125
Pressure (Torr)Fig. 9. Tuning characteristics for a GFEL using the superconduct-ing accelerator as a beam injector, as a function of hydrogen-gaspressure.
0
rO0C)UM,
2 l l
Gain in vacuum with no foil
Phase-slip mode
4
Phase-advance mode
2.
0_________5000 !O A
Wavelength (A)
Fig. 10. Electronic gain as a function of wavelength for a GFELusing the SCA as an injector.
attachment gas. The gas-containment technique, using a1.3-Arm-thick boron nitride foil, worked extremely well interms of withstanding high-pressure differentials and trans-mitting high-current, relativistic beams without damage.
ACKNOWLEDGMENT
This research is supported by the National Science Founda-tion under grant ECS88-19143.
REFERENCES
1. E. R. Peck and S. Huang, "Refractivity and dispersion of hydro-gen in the visible and near infrared," J. Opt. Soc. Am. 67, 1550-1554 (1977).
2. A. L. Ford and S. C. Browne, "Direct-resolvent operator compu-tations on the hydrogen-molecule dynamic polarizability, Ray-leigh, and Raman scattering," Phys. Rev. A 7, 418-426 (1973).
3. M. B. Reid, A. S. Fisher, R. H. Pantell, J. Feinstein, T. L.Deloney, and A. H. Ho, "Design of a gas-loaded free-electronlaser experiment," Nucl. Instrum. A 259, 133-135 (1987). Arelated discussion of the GFEL in another context is presentedin M. B. Reid, J. Feinstein, R. H. Pantell, and A. S. Fisher, "Gainin spatially varying optical fields: applications to high emit-tance beams and gas dielectric FELs," IEEE J. Quantum Elec-tron. QE-23, 1539-1544 (1987).
4. R. H. Pantell, J. Feinstein, A. S. Fisher, T. L. Deloney, M. B.Reid, and W. M. Grossman, "Benefits and costs of the gas-loaded, free-electron laser," Nucl. Instrum. Methods A 250,312-315 (1986).
5. A. S. Fisher, R. H. Pantell, J. Feinstein, T. L. Deloney, M. B.Reid, and W. M. Grossman, "Picosecond beam propagation forgas-loaded free-electron lasers," Nucl. Instrum. Methods A 250,337-341 (1986).
6. M. B. Reid, "The gas-loaded free electron laser," Ph.D. disserta-tion (Stanford University, Stanford, Calif., 1988).
7. A. S. Fisher, R. H. Pantell, J. Feinstein, T. L. Deloney, and M. B.Reid, "Propagation of a picosecond-duration, relativistic elec-tron beam through hydrogen at atmospheric pressures," J.Appl. Phys. 64, 575-580 (1988).
8. A. A. Christodoulides, L. G. Christophorou, R. Y. Pai, and C. M.Tung, "Electron attachment to perfluorocarbon compounds,"J. Chem. Phys. 70, 1156-1168 (1979).
9. M. B. Reid, A. S. Fisher, J. Feinstein, A. H. Ho, M. Ozcan, H. D.Dulman, Y. J. Lee, and R. H. Pantell, "Experimental elimina-tion of plasma effects in a gas-loaded, free-electron laser," Phys.Rev. Lett. 62, 249-252 (1989).
10. A. S. Fisher, R. H. Pantell, M. B. Reid, J. Feinstein, A. H. Ho, M.Ozcan, and H. D. Dulman, "Observations of gain and pressuretuning in a gas-loaded FEL," Nucl. Instrum. Methods A 272,
Pantell et al.
l
1
i667 Ilk2500
1014 J. Opt. Soc. Am. B/Vol. 6, No. 5/May 1989
89-91 (1988); M. B. Reid, J. Feinstein, R. H. Pantell, and A. S.Fisher, "Gain in the gas-loaded FEL," Nucl. Instrum. MethodsA 272, 268-274 (1988).
11. C. Lifshitz and R. Grajower, "Dissociative ionization in per-fluorocyclobutane," Int. J. Mass Spectrom. Ion Phys. 10, 25-37(1972/73).
12. J. L. Boulnois, "Photophysical processes in recent medical laserdevelopments: a review," Lasers Med. Sci. 1, 47-66 (1986).
13. D. Houde, J. W. Petrich, 0. L. Rojas, C. Poyart, A. Antonetti,and J. L. Martin, "Reactivity and dynamics of hemeproteins inthe femtosecond and picosecond domains," in Ultrafast Phe-nomena V, G. R. Fleming and A. E. Siegman, eds., Vol. 64 ofSpringer Series in Chemical Physics (Springer-Verlag, NewYork, 1986), pp. 419-422.
14. R. J. Robbins, D. P. Millar, and A. H. Zewail, "Picosecondtorsional dynamics of DNA," in Picosecond Phenomena II, R.M. Hochstrasser, W. Kaiser, and C. V. Chank, eds. Vol. 14 of
Springer Series in Chemical Physics (Springer-Verlag, Berlin,1980), pp. 331-335.
15. D. A. Angelov, G. G. Gruzadyan, P. G. Kryukov, V. S. Letokhov,D. N. Nikogosyan, and A. A. Oraevsky, "High-power UV ultra-short laser action on DNA and its components," in PicosecondPhenomena II, R. M. Hochstrasser, W. Kaiser, and C. V. Chank,eds., Vol. 14 of Springer Series in Chemical Physics (Springer-Verlag, Berlin, 1980), pp. 336-339.
16. T. Koayashi, H. Ohtani, and M. Tsuda, "Primary process ofvision: hypsorhodopsin," in Ultrafast Phenomena V, G. R.Fleming and A. E. Siegman, eds., Vol. 46 of Springer Series inChemical Physics (Springer-Verlag, Berlin, 1986), pp. 416-418.
17. S. L. Trokel, R. Srinivasan, and B. Braren, "Excimer laser sur-gery of the cornea," J. Ophthalmol. 96, 710-716 (1983).
18. R. Linsker, R. Srinivasan, J. J. Wynne, and D. R. Alonso, "Farultraviolet laser ablation of atherosclerotic lesions," LasersSurg. Med. 4, 201-206 (1984).
