Infrared Laser-Induced Photodesorption of Adsorbed and Condensed Phases

Journal of Electron Spectroscopy and Related Phenomena, 38 (1986) 65-74 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

65

INFRARED LASER-INDUCED PHOTODESORPTION OF ADSORBED AND CONDENSED PHASES

Ingo Hussla IBM Almaden Research Center, K33!801, 650 Harry Road, San Jose, California 95120-6099, U.S.A.

ABSTRACT Infrared laser-induced photodesorption occurs via pulsed excitation of internal vibrational modes of molecules adsorbed on surfaces. A review of these experiments is given. Excitation of the second overtone of the molecule-surface bond has been proposed theoretically as alternative photodesorption channel. Experimental results are reported for CO-Cu(lOO). No desorption was detected even though the overtone occurred within the CO 2 laser range and incident intensities up to 100 MW!cm 2 were used. PHENOMENON AND SYSTEMS VISITED Infrared laser-induced photodesorption (lR-LIPD) is a phenomenon involving the desorption of molecules from adsorbed and condensed phases after excitation of internal vibrational adsorbate modes with a pulsed infrared laser. The IR-LIPD phenomenon has been studied since 1978 and has now been observed from a variety of substrates including ionic crystals, semi-conductors and metal surfaces, employing time-resolved mass spectrometry as detection method. Photodesorption experiments, using tunable pulsed CO 2 laser sources, have been reported for numerous adsorbates and condensates, including SF6-NaCI(l00) (ref. I), CH 3F-NaCl(100) (ref. 2), CH3F-NaCl(film) (ref. 2-8), C2H6!CH3F-NaCl(film) (ref. 9-10), CO-NaCl (ref. 11,12) by Heidberg et al., and CsHsN-KCl (ref. 13-15), CsHsN!CsDsN-KCI (ref'.lS), SF 6-Si(l11) (ref. 15), CsHsN-NHfoi!) (ref. 13), CsHsN-Ag(llO) (ref. 15), CsHsN-Ag(film) (ref. 16,17), CsHsN!CsDsN-Ag(film) (ref. 15) by Chuang et al... Preliminary results for the systems CO!CH3F-Cu(polycrystal) (ref. 18) and NH 3-Cu(lOO) (ref. 19) have also been presented. Other CO2 laser photodesorption experiments included the work by Hess et al., (ref. 20-24) on condensed thick molecular layers and bulk materials, such as CH 30H, CD 30H, CD 30D, CH 30D, CCI 4 , D20, CH 3F and C6HsCHO. Using a HF!DF laser, LIPD of H 20 from optical surfaces like CaF2 , NaCl, KCl, SiO, Cu has been claimed (ref. 25). In a recent publication we reported IR-LIPD of NH 3-Cu(l00) and NH3-Ag(film) after

66

exciting the N-H stretching modes (ref. 26-30) with a tunable pulsed laser operating in the 2.5-4.2 /-1m region. Mixed ND 3-NH3 molecular layers adsorbed on Cu(lOO) and Ag(film) were also investigated (ref. 28-30), as well as Xe-NH 3 co-condensates on Ag(film) and NaCl(film) substrates (ref. 29). IR-LIPD data are measured with a time-resolved mass spectrometer as the rise and decay of the mass of the desorbed molecules after laser adsorbate-interaction. These desorption signals monitor the velocity distribution of photodesorbing species and can be fit to translational temperatures by using a Maxwell Boltzmann least square fit. In most cases linear IR-absorption spectra of the adsorption system are first recorded in order to optimize coupling of the IR-laser frequency resonantly to an internal mode. In other cases, where the linear IR-spectrum

of

the

adsorption-substrate

system

has

not

been

measured,

an

absorption-desorption spectrum can be obtained by measuring the laser frequency dependence of the desorption yield. Frequency and intensity dependence of the desorption yield depend strongly on system parameters like coverage and substrate temperature.

The intensity

dependence of the desorption yield measured at maximum infrared absorptivity provides information about the rate of IR-LIPD, which can then be compared with those obtained by semi-empirical model calculations. Using a master equation approach, rates have been published by Kreuzer et al., for the adsorption systems CO-Cu(lOO)(ref. 31), CO-W(llD) (ref. 31), CO-NaCI(ref. 32) and CH 3F-NaCl(ref. 32, 33,48). The importance of proper spectral widths (for laser and system) input in these calculations has been emphasized (ref. 7,43,38). Anharmonicity of the excited vibrational modes must also be included in order to obtain better agreement between experimental and theoretical data (ref. 43,38,39). Time-of-flight spectra of photodesorbing species have been calculated (ref. 34). Lucas and Ewing have calculated the desorption rate and lifetimes of vibrationally excited adsorbates (ref. 35). More recently, a lineshape function which specifically applies to one-photon desorption has been presented (ref. 36). Actually, there are not many adsorption systems known where the absorbed photon energy exceeds the adsorption energy. In this regard CO-NaCI (ref. 37) and (NH 3) multiiayer-Cu(lOO) (ref. 30) are model systems for one-photon IR-LIPD. Comparison of experimental and theoretical data on the desorption rate of both' single and multi photon situations became available recently for NH 3/ND3-Cu(l00) (ref. 30).

Frequency dependence of the photodesorption yield due to

homogeneous and inhomogeneous line broadening has been recently calculated for CH 3F-NaCI. (ref. 38,39) A phase-dissipative mechanism was investigated and found to assist the laser-driven active mode in LIPD (ref. 56).

Very recently an extended theory for

laser-stimulated surface processes was presented by Beri and George (ref. 50,51,52). Results and conclusions for the systems studied are reviewed below. Other laser-induced surface processes such as diffusion (ref. 55), dissociation and ionization can be induced by

67 IR-radiation. The intensities observed for these surface reactions are smaller than for the same molecules in the gas phase (ref. 19). Laser-induced thermal desorption (LITD) occurs when the incident laser beam heats the substrate.

This desorption mechanism competes with

IR-LIPD, especially when the IR-reflectivity of the substrate is low and the absorptivity is high. Laser-induced surface reactions in general, and resonant vibrational adsorbate coupling in particular, have been reviewed recently in great detail by Chuang et 01., (ref. 28,29,44,49) and others (ref. 11,45,46,47,53). EXPERIMENTAL CONSIDERATIONS Custom-built IR cells and apparatus for laser solid-interaction under DHV have been developed (ref. 57,58). The instruments, procedures and experimental difficulties of the pulsed laser-induced desorption experiment have been discussed in detail for the case of LITD, where the molecularly desorbing species are also detected by time-resolved mass spectrometry (ref. 59).

To obtain real-time desorption signals, a laser-in, laser-out optical set-up has to be

achieved. Otherwise, laser light will scatter from the chamber walls, causing spurious effects and signal broadening. Fast ion current amplification of the mass spectrometer probe must be accomplished. For this reason, slow electrometer amplification of the mass spectrometer has to be avoided, necessitating the use of a large bandwidth operational amplifier (ref. 59). Best results have been obtained using a differentially pumped mass spectrometer chamber equipped with cooled slits and discrimination devices to make sure that only molecules without any wall collisions reach the electron impact ionizer. (ref. 59). The desorption time-scale is zeroed by the actual laser pulse event. In most cases a signal from the thyratron-triggered laser pulse is used to start the desorption signal acquisition device, e.g., storage oscilloscope, transient recorder or signal averager. Since the time scale of detection is in the microsecond range, laser jitter in the ns range does not affect the measurements. If the surface coverage is controlled by constant backing pressure exposure of the substrate to the gas molecules (ref. 59) rather than by a single dosage, a different trigger method is used (ref. 59, 60): A shutter blocks the laser output for a certain time, chosen to obtain the desired Langmuir exposure. The shutter is timed by the trigger output of the laser, which is operating at a 1 Hz repetition rate for optimum output stability. When the laser actually hits the surface, the light is reflected onto a fast (photondrag) IR detector, whose signal output is used to trigger the data acquisition. This procedure also allows measurement of the reflected laser energy simultaneously to each desorption event, providing a single beam single reflection-absorption spectrum of the adsorbate. A difficult experimental LIPD problem is the spatial movement of the IR beam spot on the crystal when the laser is tuned (grating, monochromator) through its frequency range. Therefore, all measurements have to be reproduced at different optical alignments.

68

REVIEW OF RECENT RESULTS ON IR-LIPD IR-LIPD can take place from both dielectric and metal surfaces at surface coverages as low as a monolayer. Excitation of different types of internal vibrational modes (stretch, bend, ring) leads to desorption (ref. 16,19). The photoactive mode has a much higher vibrational frequency than the modes directly associated with the molecule-surface bond and to first order is generally considered as being decoupled. Resonant photodesorption with strong wavelength dependence in a certain intensity range of the incident laser radiation is observed for both dielectric and metal substrates. In principle, the frequency dependence on the desorption yield matches the IR-absorption spectrum of the adsorbate, but examples of smaller linewidth (FWHM) due to multiphoton absorption have been reported. The importance of measuring both the IR-(reflection) absorption spectrum and the IR-absorption-desorption spectrum simultaneously must be emphasized. IR-LIPD can be obtained with IR-laser pulses of 6 ns. Most experiments are done with a CO 2 laser having a pulse duration between 60 and 200 ns. No LIPD experiment using CW radiation has been performed successfully to date, most likely because induced thermal contributions are overuling any resonant effect (ref. 72). Incident Iaser intensities for IR-LIPD are in the order of 0.5-2 MW/cm

2

•

At higher laser

fluences and elevated substrate temperatures, the resonant feature of IR-LIPD can be washed out by given or induced thermal effects (ref. 29). Effects of high laser power density, such as alteration of surface properties or ablation of subtrate material are not expected in the intensity range mentioned above (ref. 71). High vibronic excitation of the adsorbate, however, can induce IR-photoionization and IR-photofragmentation of the adsorbed molecules (ref. 18). The desorption yield appears to be higher on dielectric substrates than on metal substrates. Typical quantum yields of 2xlO- 1 and 5xl0-4 are reported respectively (ref. 4,27). However, since no systematic study of yield vs. pulse duration has been performed, these different numbers might be also caused by the different laser systems used in ref. 4 and ref. 27, respectively. Only a very few adsorption systems are known where one-photon vibrational absorption energy exceeds the adsorption energy of an adsorbed molecule. Therefore most of the systems investigated so far require multiphoton absorption. The situation is different in condensed layers, where the evaporation energy is in the order of the energy of a CO 2 photon. The intensity dependence of the desorption yield is of course different for the two cases (ref. 30). Fluence or intensity "thresholds" of LIPD have been reported.

However, since all

experiments performed so far have used quadrupole mass spectrometry in

~

time-of-flight

69

mode, which has poor sensitivity mainly due to low electron impact ionization efficiencies and a small solid angle, these "thresholds" may be an artefact due to lack of instrumental sensiti vity. The velocity distribution of the desorbed particles appears "close" to a Maxwell-Boltzmann distribution, but only very few real-time desorption signals have been reported.

The

measured translational temperatures for molecules photodesorbing from metal surfaces are close to the substrate temperature or higher (ref. 30,18). Molecules photodesorbing from ionic crystals are found to be colder than the subtrate temperature (ref. 3).

No

spatial-resolved IR-LIPD data have been published to date. IR-LIPD experiments with co-adsorbates (multilayer) show no significant enhanced isotope selectivity in the desorption yield within the 25% experimental uncertainty (ref. 30), while preferential desorption from co-adsorbates with one IR active component has been reported (ref. 9, 18). Conditions such as low substrate temperature, low coverage and a non-metal subtrate (= low thermal contribution to the desorption rate) will rise the probability of sucessful isotope separation experiments via IR-LIPD (ref. 30). A quantum statistical theory of IR-LIPD has been derived based on the master equation describing the time evolution of the occupation of the vibrational states of the adsorbed molecule. Transition probabilities are calculated according to Fermi's golden rule for the laser-induced dipole transitions in the molecule, for the phonon-induced cascades in the surface potential molecule, for the resonant heating mechanism, and for the elastic and inelastic tunneling processes into continuum states leading eventually to desorption (ref. 7,30-33). The numerical results of desorption rates are in agreement with the experiments, in particular if one considers spectral line widths and resonant heating mechanism including phonon and electron damping (ref. 7, 30). A phenomenological description of thermally assisted IR-LIPD is proposed: The primary process is the excitation of a localized adsorbate vibration.

Bound to bound state

transitions play essential roles in channeling the absorbed photon energy into the localized vibrational levels in the surface potential via electron or phonon-mediated aid resulting in the thermal excitation of the surface potential. This thermal excitation can enhance the desorption probability when the molecule is also internally excited or when it is coupled with the elastic and inelastic tunneling processes, but the selectivity will suffer. Application: IR-LIPD might become a surface analytical tool in order to obtain Ik-absorption-desorption

spectra,

e.g.

from

catalysts.

Successful

separation

of

co-adsorbates via IR-LIPD can be obtained from dielectrics and in co-adsorbates where resonant heating cannot cause desorption of the unexcited species. cleaning" might become available.

"Selective surface

70 SECOND OVERTONE EXCITATION IN CO-Cu(tOO)

In this section an alternative IR photodesorption channel is considered:

the direct

vibrational excitation of the surface-molecule bond. This process has been theoretically treated by George et al., (ref. 61-64) and ledrzejek e: al., (ref. 65-66). The frequency of this mode is normally low (300-500 cm-

I

).

Only in a few adsorption systems, such as H-W (ref. 67), can

a pulsed CO 2 laser be used for direct coupling into this mode. ledrzejek et al., suggested the possibility of a second overtone excitation of the Cu-C bond using a high power pulse laser at 921 cm- I . This frequency value was calculated based on the measured values of the fundamental Cu-C vibration of 339 ern (ref. 68) and the experimental value of 69.7 kf /rnol (ref. 69) for the isosteric heat of adsorption.

One-Dimensional

Microscopic

Quantum

Mechanical

Theory

ill

Photodesorption

in

CO-Cu(IOO)(ref. 65,66). Thermal desorption is caused by phonon energy transfer from the lattice to the vibration of the chemisorptive bond and can be increased by laser vibrational coupling into this mode. Let the transition rates be WR_m and Wh_m for phonon and laser-driven transitions respectively. Then the probability Pnet) that the atom is in a state at time t, is given by the "master equation":

(1)

The total transition rate Wn-sm is the sum of the phonon and laser contributions

(2) The use of the master equation involves certain implicit assumptions: (a) The "Markovian approximation" is invoked by assuming that the rate of dissipation of energy in the phonon heat bath is larger than the transition rates Wn-sm' (b) The off-diagonal elements of the reduced density matrix of the adsorbed particle are neglected. The computation of W~_m includes multiphonon effects (spacing between Em and En

>

Debye energy), the effect of

oscillator-anharmonicity (increase of WR_m when continuum close and e.g. n .... n+2 transitions are possible), the effect of bound-to-continuum transitions (!!!! desorption) and finally, transition rates between all levels. Wh_m are evaluated using the golden rule fomula

(3)

71

Here I' is the absorption line-width for the n-s-rn transition, p. is the dipole moment operator for the chemisorptive bond, and E is the local electric field due to the laser. The matrix element is taken between states

I n>

and

1 m>

of the Morse oscillator representing the atom

bound to the surface by the average lattice-atom interaction.

Several of these quantities

cannot be calculated with any precision. It is difficult to compute the local field accurately even for a perfectly flat surface. The

incident laser field is modified by local fields emanating from the metal and the surface molecules. If phenomenological Maxwell equations are used, the polarization of the metal is described by the Fresnel formula. At infrared frequencies the metal is close to being a perfect conductor, and the local field is roughly twice that of the incident laser. However, the Fresnel equations break down at points located too close to the surface and then the accuracy of these equations is unknown. The presence of any kind of roughness may also modify the local field substantially. The effect of the polarization of the neighboring molecule is equally difficult to determine in a satisfactory manner. Assuming maximum photon absorption by a perpendicular dipole,

1EJ.1 2 is

proportional

to the local light intensity 1 and is equal to 21/ 'oocon ('o=vacuum dielectric constant, c velocity of light, '1/ = refractive index):

(4)

where the constant C is defined by C = 1/(10'1/). (Large C = large local field.) Other uncertainties are the dependence of the dipole moment on the length of the chemisorbed bond, the role of anharmonicity (n - n ± 1 transition are off resonance or are achieved in the harmonic case) and finally, the role of the width f nm the line width of the adsorbed species. The presence of I'nm in eq. (4) is required by the existence of dissipative processes coupled to the anharmonic oscillator. If f nm - 0 a 6-function appears in eq. (4), and the photon is active only if its frequency is equal to (Em - En)/II within the smaller power broadened absorption bandwidth. The presence of Tnm allows all the n - m transitions to have some participation even if they are not in resonance, though they are less and less effective as t.l-(Em-En)/Ii is increased.

W~=ml/fnm'

On the other hand if I'nm has excessively large values, then l

Numerical results have been reported using: EeEo/1i = 339 cm- , potential depth D = 16.6 Kcal/mol, bond distance Qo = 2.27A, potential width a = 2.464; laser energy II, pulse width 2 200 ns, I'nm = 30 cm -1, (E 3-Eo) = 921 cm- l and laser intensity 1 = 50 GW/cm ( !) . The desorption rates have also been given as a function of .local laser field parameter C.

72

Experimental Results The described photodesorption mechanism was probed in the system CO-Cu(lOO) using CO 2 laser intensities up to 100 MW!cm

2

.

The whole accessible CO 2 laser frequency range

was applied. The experiments were carried out in an UHV system, which has been used for successful LITD experiments in CO-Cu(lOO) (ref. 70), laser-induced ablation of copper (ref. 71) and very recently, for IR-LIPD in CH 3F!CO mixtures adsorbed on Cu (polycrystal) (ref. 18). Substrate temperature of the clean (Ar + sputtered) Cu(lOO) single crystal was 90K. To 8

assure that highest coverages are obtained, CO backing pressures up to 10- mbar were applied. The CO surface coverage was controlled by LITD (ref. 70).

Angles of incidence of the

p-polarized pulsed CO 2 laser (FWHM 60ns) were 450, 67.5· and 86·; the detection angle of the mass spectrometer was 0·, 22.5· and 0·, respectively. Time-of-flight distance was 23 em. Up to 64 desorption events were sampled. More details of the experimental set-up can be found in ref. 18. Laser energy measurements were taken on a pulse-to-pulse basis (ref. 59) before and after reflection from the single crystal. No desorption of up 10 100 MW!cm

2

co

from Cu(lOO) single crystals could be obtained applying laser intensities of

under the above described conditions. The whole of the CO 2 laser frequency l

range from 910-1090 cm- was used. Also ion sputtered rough crystal surfaces were used.

DISCUSSION AND CONCLUSION Photodesorption by direct excitation of the surface-molecule via second overtone excitation 2

proposed on theoretical grounds, does not occur with intensities of up to 100 MW/cm • Clearly, this implies that phonon-assisted relaxation and damping of any vibrational excitation are extremely efficient in this metal adsorbate system.

The necessity of applying laser

2

intensities higher than 100 MW!cm to cause photodesorption makes it quite impossible to handle this desorption channel experimentally (surface damage).

On the other hand, this

experiment shows that CO desorption from clean Cu(lOO) surfaces is not achieved via laser thermal desorption because of the high reflectivity of copper for incident IR radiation. This latter result is in accordance with our recent findings for CH 3F!CO.Cu co-adsorbate where desorption of CH 3F has been obtained via CO 2 laser excitation of internal modes, while CO does not desorb (ref. 18).

Finally, attention should be drawn to the microreversible

phenomenon of IR-LIPD: adsorboluminescence. If LIPD is induced by vibrational excitation of internal modes of the adsorbate, one expects a release of IR radiation when adsorption occurs. A preliminary experiment has been reported for the adsorption system CO-Ni (film) (ref. 73), where IR adsorboluminescence has been observed after admission of CO to a nickel film at 17K. The time resolution of this early investigation was Is only and has to be improved in order to obtain information about the dynamics of LIPD. Also, the vibrational and rotational states of the photodesorbing species should be probed in future experiments. .

73

ACKNOWLEDGEMENT I would like to thank Professor R. Viswanathan, Department of Chemistry, Beloit College, Beloit, Wisconsin, for his help with the experiments. I am grateful to Professor Eric Weitz and Professor Peter Stair, Chemistry Department, Northwestern University, Evanston, Illinois, for providing laser and ultra high vacuum facilities. Thanks to Dr. M. R. Philpott for critical reading of the manuscript. References 1. 1. Heidberg, H. Stein, A. Nestrnann, E. Hoefs and I. Hussla, in "Laser-Solid Interactions and Laser Processing," AlP Conference Proc. 50, eds.: S. D. Ferris, H. J. Leamy and J. M. Poate, American Institute Physics, New York 1979, p. 49-54. 2. J. Heidberg, H. Stein, E. Riehl and A. Nestrnann, Z. Physikalische Chern. N.£. 121, 145 (1980). 3. J. Heidberg, H. Stein and E. Riehl, in "Vibrations at Surfaces," R. Caudano, J. M. Gilles, A. A. Lucas, eds. Plenum Press, New York 1982, p. 17. 4. J. Heidberg, H. Stein and E. Riehl, Phys. Rev. Letters 49, 666 (1982). 5. J. Heidberg, H. Stein and E. Riehl, Surf Sci., 126, 198 (1983). 6. J. Heidberg, H. Stein, E. Riehl and I. Hussla, in "Surface Studies with Lasers," F. R. Aussenegg, A. Leitner and M. E. Lippitsch, eds. (Springer Verlag, Berlin, 1983) p. 226. 7. J. Heidberg, in Symposium Laser Controlled Chemical Processing of Surfaces," Materials Research Society Proceedings, A. Wayne Johnson and D. J. Ehrlich, eds. (North-Holland, 1984), p. 333. 8. J. Heidberg, H. Stein, Z. Szilagi, D. Hoge and H. Weill, in "Dynamics on Surfaces," B. Pullman et aI., eds. (Reidel Publishing, Dortrecht, Holland) 1984, p. 329. 9. J. Heidberg and I. Hussla, J. Electron. Spectr. and ReI. Phenom. 29, 105 (1983). 10. J. Heidberg and I. Hussla, Appl. Physics 829, 184 (1983). 11. J. Heidberg, H. Stein, E. Riehl, Z. Szilagi and H. Weill, Surf Sci. 158, (1-3) 553 (1983). 12. J. Heidberg and H. Stein, private communication, October 1984. 13. T. J. Chuang and F. A. Houle, J. Vac. Sci. Technol. 20,603 (1982). 14. T. J. Chuang, J. Chern. Phys. 76,3828 (1982). 15. T. J. Chuang, J. Electron. Spectr. ReI. Phenom. 29, 125 (1983). 16. T. J. Chuang and H. Seki, Phys. Rev. Lett., 49, 382 (1982). 17. H. Seki and T. J. Chuang, Sol State Cornrn. 44,473 (1983). 18. I. Hussla and R. Viswanathan, J. Vac. Sci Technol. B 3(5), 1520 (1985). 19. I. Hussla and T. J. Chuang, Ber. Bunsengesell. Phys. Chern. 89(3),294 (1985). 20. M. Mashni and P. Hess, Chern. Phys. Lett. 77,541 (1981). 21. M. Mashni and P. Hess, Appl. Phys. 829,205 (1982). 22. B. Schafer and P. Hess, Chem. Phys. Leu. 105,563 (1984). 23. B. Schafer, M. Buck and P. Hess, Infrared Phys. 25,245 (1985). 24. B. Schafer and P. Hess, Appl. Phys. B 37, 197 (1985). 25. S. D. Allen, J. O. Porteus and W. N. Faith, Appl. Phys. u« 41(5),416 (1982). 26. I. Hussla and T. J. Chuang, Laser Controlled Chemical Processing of Surfaces," Materials Research Society Proceedings, Wayne Johnson and D. J. Ehrlich, eds., (North Holland; 1984) p. 341. 27. T. J. Chuang and 1. Hussla, Phys. Rev. u« 52,2045 (1984). 28. T. J. Chuang and I. Hussla, in "Dynamics on Surfaces," B. Pullman et al., eds. (Reidel Publishing, Dortrecht, Holland; 1984) p. 313. 29. T. J. Chuang, H. Seki and I. Hussla, Surf Sci. 158, (1-3) 525 (1984). 30. I. Hussla, H. Seki, T. J. Chuang, Z. Gortel, H.-J. Kreuzer and P. Piercy, Phys. Rev. B 32 (6),3489 (1985). 31. H.-J. Kreuzer and D. N. Lowry, Chern. Phys. Lett. 78,50 (1981).

74 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73.

Z. W. Gortel, H.-I. Kreuzer, P. Piercy and R. Teshima, Phys. Rev. 827,5066 (1983). Z. W. Gortel, H.-I. Kreuzer, P.'Piercy and R. Teshima, Phys. Rev. 828,2113 (1983). Z. W. Gortel and H.-I. Kreuzer, Phys. Rev. 829,6926 (1984). D. Lucas and G. E. Ewing, Chem. Phys 58,385 (1981). F. G. Cellii, M. P. Casassa and K. C. Janda, Surf Sci. 141, 169 (1984). 1. Hussla, Thesis, Friedrich Alexander-Universitat, Erlangen-Nurnberg, 1980. Z. W. Gortel, P. Piercy, R. Teshima and H.-J. Kreuzer, Surf Sci., in press. Z. W. Gortel, P. Piercy, and H.-J. Kreuzer, Surf Sci., in press. Z. W. Gortel, P. Piercy, H.-J. Kreuzer, this proceed. G. S. Wu. B. Fain, A. R. Ziv and S. H. Lin, Surf Sci., 147,537 (1984). B. Fain, Chem. Phys. Lett. 118(3),283 (1985). C. Jedrzejek, J Vac. Sci. Techno/. D, 3(5), 1431 (1985). T. 1. Chuang, Surf Sci. Reports, 3, 1 (1983). G. S. Selwyn and M. C. Lin, in press. J. Lin, W. C. Murphy and T. F. George, I&EC Product Research and Development, 23 334 (1984). T. F. George, Industrial Chemical News, in press. Z. W. Gortel and H.-J. Kreuzer, Surf Sci. L359 (1983). T. J. Chuang, J. Vac. Sci. Techno/. B 3(5), 1408 (1985). A. C. Beri and T. F. George, J. Chem. Phys. 76(6), Part II, 4288 (1983). A. C. Beri and T. F. George, J. Chem. Phys., 83 2482 (1985). A. C. Beri and T. F. George, Zeitschrift Physik D 60,73 (1985). T. F. George, J. Lin, A. C. Beri and W. C. Murphy, Progress in Surface Science, 16, 1984, p. 139. J. Lin, M. Hutchinson and T. F. George, in Advances in Multiphoton - Processes and Spectroscopy, ed. S. H. Lin, (World Scientific Publishing Company, Singapore), 1985, p.105. W. C. Murphy, X.-Y. Huang and T. F. George, Chem. Phys. Leu. 104(4),303 (1984). X.-Y. Huang, T. F.' George, J.-M. Yuan and L. M. Narducci, J. Phys. Chem. 88, 5772 (1984). 1. Heldberg, 1. Hussla, E. Hoefs, H. Stein and A. Nestmann, Rev. Sci. Instrum. 49, 1571 (1978). D. R. Burgess, Jr., 1. Hussla, P. C. Stair, R. Viswanathan and E. Weitz, Rev. Sci. Instrum. 55,1771 (1984). J. Heidberg, H. Stein, and E. Hoefs, Ber. Bunsenges. Phys. Chem. 85,300 (1981). R. Viswanathan and 1. Hussla, Rev. Sci. Instrum. 56(7), 1468 (1985). M. S. Slutsky and T. F. George, Chern. Phys. Leu. 57 (1978); J. Chem. Phys. 70, 1231 (1979). J. Lin and T. F. George, Chern. Phys. Leu. 66,5 (1979). J. Lin and T. F. George, J. Chem. Phys. 72,2554 (1980). J. Lin and T. F. George, Surf Sci. 107,417 (1981); 108,340 (1981). C. Jedrzejek, K. F. Freed, S. Efrima and H. Metiu, Surf Sci. 109, 191 (1981). C. Jedrzejek, in "Surface Studies with Lasers," eds. F. R. Aussenegg, A. Leitner, M. E. Lippitsch, Springer Series in Chemical Physics 33, Springer Verlag (Berlin 1983), p. 230. Y. J. Chabal and A. J. Sievers, Phys. Rev. Lett. 44, 944 (1980). B. A. Sexton, Chem. Phys. Leu. 63,451 (1979). J. C. Tracy, J. Chem. Phys. 56,2736 (1972). D. R. Burgess, Jr., I. Hussla, P. C. Stair and E. Weitz, J. Chern. Phys. 79 (10), 5200 (1983). R. Viswanathan and 1. Hussla, in "Laser Processing and Diagnostics," eds. D. Bauerle, Springer Series of Chemical Physics 39, 2984, p. 148. J. Mercier, Ph.D. Thesis, Cornell University (1983). J. Heidberg, I. Hussla, K.-H. Stammberger, Thin Solid Films, 90,209 (1982).