6100
J. Phys. Chem. 1984, 88, 6100-6103
the excitation of the CH, carbon atom producing a relatively greater yield of CF3 mass. Fragmentation on excitation of the fluorine atom yields patterns similar to the fluorinated carbon pattern with little CF3+mass produced. A difference in the relative yields of the CF+ mass can be seen on excitation of the direct ionization process or excitation to the bound state. This first report of site-selective specific fragmentation should provide a foundation for further work in the field of photofragmentation of molecules at high energy.
Acknowledgment. We thank Prof. A. Bradshaw (FHI and BESSY) for the allocation of measurement time, Dr. W. Braun for optimizing the monochromator, and Mr. M. Fischer and M. R. Stoiber for technical assistance. Funding of this research from the Bundesministerium fur Forschung and Technologie (BMFT) is gratefully acknowledged. L. A. Chewter thanks the Royal Society (London) for financial support. Registry No. CF,CH3, 420-46-2.
UV Photodissociation and Photodesorptlon of Adsorbed Molecules. 1. CH,Br on LiF(001) E. B. D. Bourdon, J. P. Cowin,+I. Harrison, J. C. Polanyi,* J. Segner,t C. D. Stanners, and P. A. Young Department of Chemistry, University of Toronto, Toronto M5S I A l , Canada (Received: October I , 1984)
A LiF(001) surface with submonolayer coverage of CH3Br was irradiated by pulsed ultraviolet (UV) radiation. Products were examined normal to the surface by time of flight (TOF) and by mass spectrometry. The source of CH, leaving the surface has been shown to be the single-photon photodissociation of methyl bromide in the adsorbed state: CH,Br(ad) CH,(g) + Br(ad/g). Photodesorption of CH3Br arose from UV absorption by the crystal. The two pathways gave rise to products with different peak energies, E*, and fwhm energy widths, AE: for photodissociation E*ph= 1.5 eV and AEPh= 0.6 eV and for photodesorption E*d = 0.060 eV and AE, = 0.14 eV. The dynamics of these photoprocesses are discussed.
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In recent years it has become established e~perimentallyl-~ and theoreticallp that the resonant vibrational excitation of adsorbed molecules by infrared radiation can result in molecular desorption from clean dielectric and metal surfaces. Very recently the first dynamical study of such a process has shown (for N H 3 adsorbed on Cu( 100) at T, = 90 K) that the molecular species is desorbed with a Boltzmann distribution over translational energy, at a desorption temperature Td 5 T,.7 It appears that infrared absorption channels sufficient energy from the adsorbate to the adlayersurface bonds and/or to the substrate, to give desorption dynamics that resemble thermal desorption, where Td 5 T , has also been reported.8-10 Though dynamical studies of UV photolysis of adsorbates have been lacking, Chen and Osgood" have found evidence for UV photolysis of adsorbed molecules in experiments in which Cd(CH,), at 1 torr in 1000 torr of argon deposited aligned particles of metal on carbon. Nishi et a1.12 have examined the dynamics of photodissociation and photodesorption from various crystalline ices of N H 3 and H 2 0 that absorb UV radiation in a one- or two-photon process, thereafter emitting ions or neutrals from their surfaces. In other work, multiphoton UV photofragmentation of adsorbed molecules was predicted to occur with enhanced cross section on rough surfaces13and has been observed to take p l a ~ e . ' ~ * ' ~ In this work we give first results of a dynamical study of the effect of ultraviolet (UV) absorption by molecules adsorbed on a clean dielectric surface. The system studied was CH3Br adsorbed on a LiF(001) surface. The major finding is that electronic excitation of the adsorbate leads to photodissociation on a sufficiently short time scale that the dynamics differ markedly from thermal desorption. The LiF single crystal was approximately 10 mm X 14 mm X 3 mm thick. It was mounted on a tantalum sample holder, temperature controlled to approximately f O . l K. The crystal (Harshaw Chemical Co.) was supplied already hardened by yradiation for ease of cleaving. It was cleaved in air and then mounted in the UHV chamber at lo-" torr. The crystal was 'Department of Chemistry, University of Santa Barbara, C A 93106. * Institut fur Physikalische Chemie der Universitat Meunchen, Munich, West Germany.
0022-3654/84/2088-6100$01 .SO10
baked at 200 OC for at least 12 h prior to an experiment.16J7 Auger spectroscopy (which was employed rarely, to avoid damage to the crystal) indicated that the fresh surface was clean. After prolonged irradiation of CH3Br(ad) (- lo7 laser pulses) a black discoloration appeared on the illuminated area of the crystal surface that was presumed to be carbon.'* The findings reported here were therefore checked on a freshly cleaved surface and also (1) For a recent review see T. J. Chuang, Surf. Sci. Rep., 3, 1 (1983). (2) J. Heidberg, H. Stein, E. Riehl, and A. Nestmann, Z . Phys. Chem. (Wiesbaden), 121, 145 (1980); J. Heidberg, H. Stein, and E. Riehl, Phys. Reu. Lett., 49, 666 (1982); Surf. Sci., 126, 183 (1983). (3) T. J. Chuang, J . Chem. Phys., 76, 3828 (1982); T. J. Chuang and H. Seki, Phys. Reu. Lett., 49,382 (1982); H. Seki and T. J. Chuang, Solid State Commun., 44, 473 (1982); T. J. Chuang, J . Electron. Spectrosc. Relat. Phenom.. 29. 12s (1983). (4) J.'Lin'and T. F. George, Surf. Sci., 100, 381 (1980); J . Phys. Chem., 84. 2957 ~- (1980). --,. (5) D. Lucas and G. E. Ewing, Chem. Phys., 58, 385 (1981). (6) H. J. Kreuzer and D. N. Lowy, Chem. Phys. Lett., 78, 50 (1981); Z. W. Gortel, H. J. Kreuzer, P. Piercy, and R. Teshima, Phys. Reu. E Condens. Matter, 27, 5066 (1983). (7) T. J. Chuang and I. Hussla, Phys. Rev. Lett., 23, 2045 (1984). (8) K. C. Janda, J. E. Hurst, C. A. Becker, J. P. Cowin, L. Wharton, and D. J. Auerbach, Surf. Sci., 93, 270 (1980). (9) D. S. King and R. R. Cavanagh, J . Chem. Phys., 76, 5634 (1982). (10) W. L. Guthrie, T.-H. Lin, S. T. Ceyer, and G. A. Somorjai, J . Chem. Phys., 76, 6398 (1982). (11) C. J. Chen and R. M. Osgood, Phys. Reu. Lcti., 50, 1705 (1983). (12) N. Nishi, H. Shinohara, and T. Okuyama, J . Chem. Phys., 80, 3898 (1984). (13) A. Nitzan and L. E. Brus, J. Chem. Phys., 75, 2205 (1981). (14) G. M. Goncher and C, B. Harris, J . Chem. Phys., 77, 3767 (1982); G. M. Goncher, C. A. Parsons, and C. B. Harris, J . Phys. Chem., 88, 4200 (1984). (15) M. Moskovits and D. P. DiLella in "Surface Enhanced Raman Scattering", R. K. Chang and T. E. Furtak, Eds., Plenum Press, New York, 1982, p 243. M. Moskovits and R. Wolkow, "Ninth International Conference on Raman Spectroscopy", M. Tsuboi, Ed., The Chemical Society of Japan, Tokvo. 1984. I) 874. (16) J. Eitil, H. Hoinkes, H. Kaarman, N. Nahr, and H. Wilsch, Surf. Sci., 54, 393 (1976). (17) G. Boato, P. Cantini, and L. Mattera, Surf. Sci., 55, 141 (1976). (18) A. P. Baronavski, V. M. Donnelly, and J. R. McDonald in "Laser Induced Processes in Molecules", K. L. Kompa and S. D. Smith, Eds., Springer-Verlag, West Berlin, 1979, Springer Series in Chemical Physics, No. 6, p 213. ~Z
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0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 25, 1984 6101
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PHOTOLYSIS (a)
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Figure 1. (a) TOF spectrum for photofragment CH3at high and low fluence. E? gives the TOF for molecules having the maximum allowable energy in the gas, and E? the same for the surface. Increasing fluence by 2.04X increased photodissociation yield by 1.95X. (b) The energy distribution for photofragment CH, normalized to a common peak height, showing invariance of P(EPh)with fluence, 3. (c) TOF spectrum for photodesorbed CH3Br at high and low fluence. Increasing fluence by 2.5X increased photodesorption yield by 2.6X. (d) Energy distribution for photodesorbed CH,Br normalized to common peak height. P ( E d ) is invariant with 3.
as a function of time. There was no change in the results; neither the photolysis nor the desorption behavior was dependent on the presence of the contaminant. Under operating conditions the crystal was held at 115 K. The crystal was dosed by means of a stainless steel capillary which gave rise to an estimated gas density over the crystal corresponding torr. Under typical experimental conditions, with a time to interval of 0.1 s between laser pulses, this corresponded to a dose of -0.01 langmuir. The yield of CH, photoproduct per laser pulse measured in this work (making the simple approximations of isotropic emission of CH,, UV absorption by CH3Br with the normal gas-phase absorption cross section of -0.01 A*/molecule at 222 nm,19 and unit quantum yield) was consistent with the presence of -0.01 monolayer of CH3Br at the surface. A Lumonics TE 861-4 excimer laser was used to irradiate the crystal at glancing incidence (approximately 85' to the normal) with unpolarized radiation. The laser gas mixtures were KrCl for 222 nm and XeCl for 308 nm. Powers used ranged from 1 to 6 mJ/pulse focused to an area of -0.30 cm2 at the crystal surface, corresponding to a power of 0.3-2.0 MW/cm. The repetition rate was varied from 0.1 to 50 Hz. The molecules and radicals coming from the surface were measured in the present work at a fixed angle of 5' to the normal. No ions of any mass were detected. The detector was a differentially pumped Extranuclear quadrupole mass spectrometer, whose ionizer was located 15.8 cm from the crystal surface. The times of arrival of ions at the 21-stage Cu/Be ion multiplier were recorded on a 20-MHz LeCroy transient digitizer with 1024 channels of channel width 50-5000 ns. Signal averaging, of typically 100-1000 shots, was performed on a DEC LSI-11 computer. The cracking pattern for CH3Br in the mass spectrometer, at our 100-eV electron energy, gave >90% CH3+with measurable CH3Br+. The observed time-of-flight spectra at 15 amu, corresponding to the mass of C H 3 radicals, is recorded in panels (a) and (c) of
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Figure 1, for X = 222 nm. The LiF(001) crystal face was dosed with -0.01 monolayer of CH,Br for these experiments, as noted above. It is evident that the CH3 signal exhibits a fast and a slow component (Figure l a and IC, respectively). Since both the yield and the time interval for the slow components (Figure IC) is greater, we have changed the scale on the ordinate and abscissa. The yield of the slow component is approximately lOOX the yield of the fast one; this ratio is variable due to the variable yield of the slow component, as will be explained. When the quadrupole was tuned to 94 amu, corresponding to CH3Br, no signal was observed in the time frame encompassed by Figure la, but a signal was observed that closely corresponded in its time profile to the slow-speed 15-amu signal of Figure IC. We therefore attribute the fast C H 3 to products of the photodissociation of CH3Br that arrive as CH3 at the ionizer of the quadrupole, whereas the slow-moving CH, is attributed to cracking of CH3Br in the ionizer; Le., the species arriving at the slow speed is CH3Br. As confirmation, we note that the fast CH,, if it were to come from the cracking of fast-moving CH3Br, would require the presence of CH,Br with 10 eV of energy, whereas our photon energy at 222 nm is only 5.58 eV. In the light of this discussion we conclude that two processes are occurring as a consequence of the illumination of the submonolayer of CH3Br by 222-nm radiation: photodissociation and photodesorption. Figure la,b relates to the former process, and Figure lc,d to the latter. We shall now discuss these processes in turn. The chosen excimer wavelength for photolysis studies, X = 222 nm, lies close to the peak of the absorption spectrum of gaseous CH,Br.l9 By analogy with CH31, which has been well (1 9) H. Okabe, 'Photochemistry of Small Molecules", Wiley-Interscience, New York, 1978, p 300. (20) S. J. Riley and K. R. Wilson, Faraday Discuss. Chem. Soc., No. 53, 132 (1972).
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6102 The Journal of Physical Chemistry, Vol. 88, No. 25, 1984
1
PHOTOLYSIS
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Figure 2. Change in yield of (a) CH3 photoproduct and (b) CH3Br photodesorption product before (broken line) and after (solid line) activation of the crystal by a burst of radiation (A = 222 nm).
we anticipate sin@e-photon absorption to a repulsive electronic state that dissociates "directly" Le., in 6 1 ps, to CH3 Br. We selected a case of direct photodissociation for study in order to minimize quenching of the electronically excited species by the surface, back to its ground electronic state. Rapid dissociation, on the time scale of half a vibrational period, should also minimize energy transfer from the recoiling species to the surface. In the gas phase, where CH, recoils from Br in isolation, conservation of momentum sets an upper limit of E F = 2.2 eV to the energy of the CH3 radical at 222 nm. The translational energy distribution of the CH3 from gas-phase photolysis extends to lower energies than this since some of the photon energy is channeled into electronic excitation of the halogen atom (the excitation energy for Br is E(Br*;*P112)- E(Br;2P3 2) = 0.46 eV) and some into vibration and rotation of the Cd3.24 For the case of gaseous CH3Br the precise partitioning into these degrees of freedom is not known. Figure 1b is the translational energy distribution for methyl formed by photodissociation in the present study. The peak of the distribution is E*ph= 1.5 f 0.1 eV and the breadth (fwhm) is AEph= 0.6 eV. It is evident that (a) the energy distribution is narrow, as if CH, recoiled directly away from the surface without extensive collisional energy loss, (b) the observed limit of the energy distribution, E T , lies slightly above the value of E F . = 2.2 eV as if some CH3 were recoiling from a more massive particle than an isolated Br atom, and (c) the value of E T approaches the theoretical limit of E Y = 2.6 eV which is obtained by subtracting the bond dissociation energy in CH3-Br (2.98 eV) from the energy of the 222-nm photon (5.58 eV), where E? corresponds to the energy of CH, recoiling from an infinite mass. An effective increase in mass of the Br end of CH3Br, if confirmed, would indicate that this end is attached to the surface. Thermal desorption studies performed in our apparatus give a bond energy of approximately 8 kcal/mol (0.3 eV) for the Br-S bond (S = surface). For the methyl that we are observing, leaving a t high speed almost normal to the surface, we can infer that the CH3-Br bond is at right angles to the surface, and we surmise from the substantial physisorption energy that the Br is located over Li+ rather than over F.This CH3-Br bond direction is consistent with the lack of encounters between CH3 and the surface during photorecoil. In the following paragraphs we consider some alternative mechanisms for the formation of the high-energy CH3. The evidence that this is photodissociation rather than thermal dissociation comes from the fact that the methyl radical translational energy distribution, P(Eph), is invariant with a factor of 2X change in fluence, 3, falling on the crystal; compare the solid
+
(21) T.Donohue and J. R. Wiesenfeld, Chem. Phys. Lett., 33,176 (1975). (22) M.Shapiro and R. Bersohn, J . Chem. Phys., 73, 3810 (1980). (23) R. E. Sparks, K. Shobotake, L. R. Carlson, and Y. T. Lee, J. Chem. Phys., 75,3838 (1981). (24) H. W. Hermann and S. R. Leone, J . Chem. Phys., 76,4866(1982). (25) S.Y.Lee and E. J. Heller, J . Chem. Phys., 76,3035 (1982). (26) V. Engel and R. Schinke, Mol. Phys., 51, 189 (1984). (27) G.N. A. Van Veen, T. Baller, and A. E. deVries, Chem. Phys., 87, 405 (1984).
and broken lines in Figure lb. Additionally we have observed that the same laser fluence at 308 nm yields no detectable flux of high-energy CH3 radicals; a simple heating effect would not be so wavelength specific. We have observed that the high-energy methyl is obtained when the wavelength is close to the peak of the CH3Br gas-phase absorption spectrum (222 nm) but is undetectable at a UV wavelength where the gas-phase absorption would only be -0.1% as large (308 nm). We conclude that the photoprocess channels its energy into the methyl through the methyl bromide molecule, rather than through the substrate. To test whether an adsorbed layer of CH3Br was necessary for photoproduction of CH3, we performed experiments with the LiF crystal at T, = 400 K, at which temperature little methyl bromide adsorbs as we know from thermal desorption studies performed here. Neither the photolysis nor the photodesorption peak was observed at T, = 400 K. This leaves the possibility that the observed photolysis could be occurring above the crystal (perhaps nanometers above) as the desorbed CH,Br expanded into the vacuum chamber. This possibility was eliminated by inducing a large variation in the density of desorbing gas over the crystal and checking for evidence of a corresponding change in the yield of photoproducts. Figure 2 shows, at the right, that the yield of the molecular species was altered by 3X and, at the left, that the yield of photoproducts remained constant despite this alteration. We conclude that the CH,(g) photodissociation occurs at the surface; Le., CH,Br(ad) Br(ad/g). The evidence that the observed high-energy CH3 originates in photolysis, rather than in the formation of a plasma at the LiF surface, comes from several sources. In the first place, we are working at 10-4X the damage threshold for LiFS2* Secondly, we cannot detect any ions coming from the surface. Thirdly, we find that 222 nm (absorbed by CH,Br) gives high-energy CH,, whereas 308 nm (which is not absorbed by CH,Br) does not. Finally, it is significant that the maximum energy in the CH, corresponds to the energy of the photon absorbed; a plasma would not be subject to this energy limit. We have not detected the Br atomic product of photodissociation (79 amu). Since its recoil velocity is expected to be low, and its yield must be low compared with molecularly desorbed CH,Br, it will tend to be obscured by the lOOX larger molecular peak at 94 amu which yields 79 amu fragments. The photodissociation of CH,Br(ad) involves absorption of a single photon of 222 nm, as evidenced by the linearity, with unit slope, of laser fluence vs. total yield of photoproduct, shown in Figure la. We have made a preliminary study of the effect of increasing surface coverage on the yield of photoproducts. Coverage was altered either by changing the pressure over the crystal or by changing the time interval between successive laser pulses. In either case the yield appeared to increase linearly with coverage in the submonolayer range. This is, again, consistent with
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+
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(28) J. F. Ready, "Effects of High-Power Laser Radiation", Academic Press, New York, 1971,p 290.
The Journal of Physical Chemistry, Vol. 88, No. 25, 1984 6103
Letters CH,Br(ad) photolysis as the source of energetic CH,(g). The tests described above were also applied to the molecular photodesorption peak pictured in Figure IC. The translational energy distribution of CH3Br is shown in Figure Id. The maximum energy is E F = 0.5 eV, the peak of the distribution is E*d = 0.060 eV, and the breadth is A E d = 0.14 eV. Increasing the laser fluence by 2.5X resulted in a 2.6X increase in desorption yield (see Figure IC); i.e., the dependence is linear. The change in wavelength from 222 to 308 nm that eliminated photodissociation had no observable effect on photodesorption. Adsorbing Xe on the crystal in place of CH3Br, we found that Xe desorbed with an energy distribution resembling that of desorbed CH,Br (the Xe P(E,) distribution peaked at -0.01 eV higher energy, due perhaps to a weaker surface bond for Xe). We conclude that desorption originates in a photoprocess that is mediated by the crystal. A further valuable clue as to the nature of the desorption process comes from the fact that we can enhance the quantum efficiency of photodesorption by irradiating the crystal (free of adsorbate) with a short burst of laser pulses ( N 100 H z for -0.1 s) immediately prior to an experiment at 1-Hz repetition rate. This was the procedure used in altering the yield of CH3Br as recorded in Figure 2b. The enhanced efficiency of molecular desorption decays exponentially with time following a burst of laser pulses, exhibiting a lifetime of rc 30 min. This implies that the return of the crystal to its “normal” state involves the surmounting of an Arrhenius energy barrier of approximately 0.4 eV (for a preexponential factor of lo1, and a crystal temperature of 115 K) * We surmise that UV absorption in color centers, at the surface of the crystal and/or within it, causes dislocations to occur29which render the crystal more strongly absorbing.30 Visual inspection showed that the crystal glowed strongly with a blue color at 222 nm and more faintly at 308 nm. The F-center absorption spectrum in LiF, at 4 K, peaks at 5.102 eV and has a fwhm of 0.596 eV; the 222 nm (5.58 eV) and 308 nm (4.02 eV) wavelengths therefore fall to either side of the peak of the LiF a b ~ o r p t i o n . ~ ~ One can only speculate as to the mechanism responsible for molecular desorption. The sudden (-lo-* s) deposition of electronvolts of energy in the color centers, and the ensuing atomic motion, could cause a photoacoustic shock to pass through the crystal.27 If it were coming from within the crystal, the shock would reach the surface across a median distance of 1 mm in 0.2 ps, a time short compared with the observed time of flight of CH,Br. The CH3Br or Xe may be ejected from the surface by the arrival of the shock. The process of desorption is unlikely to
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(29) R. Smoluchowski, 0. W. Lazareth, R. D. Hatcher, and G. J. Dienes, Phys. Rev. Lett., 27, 1288 (1971). (30) N. Itoh, Adu. Phys., 31, 491 (1982). (31) H. Rabin and M. Reich, Phys. Reu., 135, 101 (1964).
be simply heating of the surface, since the energy distribution of the desorbing molecules does not shift to higher mean energy with a factor of 3X increase in the power absorbed (see Figure Id). The amount of desorption increases with power, so the constancy of the energy distribution does not appear to derive from the fact that all molecules desorb in some narrow temperature range. (The bulk of the crystal remains at constant temperature to approximately fO.l K.) The mechanism by which phonons induce desorption of adatoms has been the subject of theoretical discussion over the past decade32-35as well as experimental s t ~ d i e s . ~ ~The , , ~mechanism by which radiation can be coupled into ionic crystals has been studied over a still longer period; for reviews see ref 28 and 30. The process responsible for the type of desorption investigated here probably differs not only from thermal desorption but also from that normally discussed under the heading of “PSD” (photon stimulated desorption) which has been traditionally concerned with the effects of high-energy radiation on surfaces and on adlayers. The mechanism of PSD3*+ has been the subject of detailed study. It can be summarized by the acronym DIET,“I desorption induced by electronic transitions. The species ejected by PSD have been largely fast ions, in contrast to the slow neutrals observed here. The identification of the UV photodissociation of adsorbed species yielding neutral free radicals is very likely to be followed by the observation of photoinduced surface reactions between adsorbed species. Such reactions would be restricted as to ranges of collision energies, collision angles, and impact parameters. The study of reactions under these conditions could materially assist in the characterization of the molecular motions underlying surface reaction. Acknowledgment. E.B.D.B. thanks the Natural Sciences and Engineering Research Council of Canada (NSERC) for the award of a Postdoctoral Fellowship, I.H. for the award of a NSERC Graduate Scholarship, and C.D.S. for a Summer Studentship. J.S. is indebted to the Deutsche Forschungsgemeinschaft for the award of a Fellowship. This work was made possible by generous assistance from NSERC, the University of Toronto, and Venture Research Unit, BP Canadian Holdings Limited. (32) B. Bendow and S. C. Ying, Phys. Reu. E : Solid State, 7,622 (1973). (33) F. 0. Goodman and I. Romero, J . Chem. Phys., 69, 1086 (1978). (34) M. S . Slutsky and T. F. George, Chem. Phys. Lett., 57, 474 (1978). (35) 2.W. Gortel, H. J. Kreuzer, and R. Teshima, Phys. Rev. E: Condens. Matter, 22, 5655 (1980). (36) P. Taborek, Phys. Rev. Lett., 48, 1737 (1982). (37) M. Sinvani, P. Taborek, and D. Goodstein, Phys. Lett. A , 95A, 59 ( 1983). (38) T. E. Madey in “Inelastic Particle-Surface Collisions”, E. Taglauer and W. Heiland, Eds., Springer-Verlag, West Berlin, 1981, p 80. (39) D. Menzel, J . Vac. Sci. Technol., 20, 538 (1982). (40) M. L. Knotek, Phys. Today, 37, 24 (1984). (41) “Desorption Induced by Electronic Transitions, DIET I”, N. H. Tolk, M. M. Traum, J. C. Tully, and T. E. Madey, Eds., Springer-Verlag, West Berlin, 1983.
