J. Phys. Chem. 1988, 92, 6859-6861
6859
Photoejection of Clusters from HBr Adsorbate: (HBr),, n 5 4 C.-C. Cho, J. C. Polanyi,* and C. D. Stanners Department of Chemistry, University of Toronto, Toronto, Ontario M5S 1 A I , Canada (Received: September 30, 1988)
Clusters of (HBr),, n I4, have been formed by irradiating multilayers of HBr adsorbed on LiF(001) with an ArF 193-nm excimer laser. The desorbed species were detected by angularly resolved time-of-flight mass spectrometry. The desorption yield for HBr monomer was found to peak normal to the surface whereas clusters (HBr),, n = 2-4, exhibited a peak at approximately 40’ to the normal. These contrasting angular distributions, together with the observed comparable yields, and differing desorption thresholds showed that clusters were desorbed as such from the surface rather than formed in adiabatic expansion of the desorbate.
Introduction The interactions between UV radiation and molecules adsorbed on surfaces have attracted increasing attention recently. Studies were motivated by their potential applications in material fabrication,’ and with a view to elucidating the dynamics of photoprocesses in the adsorbed state.2 A variety of photoprocesses-photodissociation, photoreaction, photodesorption, and photoejection-have been reported in CH3Br3s and H2S3b adsorbed on a LiF(001) surface. The photochemistry of HBr under submonolayer conditions has also been ~ t u d i e d . ~More recently, photodissociation has been studied on various metal
substrate^.^ In the present work, the effects of UV laser irradiation upon HBr adsorbed on LiF(001) at high coverage have been examined. Clusters of HBr have been found to desorb with angular distributions differing from that of the monomer. Although weakly bound van der Waals clusters have been extensively studied by nozzle expansions$ they do not appear to have been reported in either laser or electron-impact desorption. Our findings indicate that these HBr clusters were ejected from the surface as (HBr),, n > 1, rather than being formed in the course of adiabatic expansion away from the surface. In contrast to the angular distributions of other molecules desorbed by laser i r r a d i a t i ~ n , ~ ~which ~ , ’ - ~ exhibited a maximum yield at the surface normal, the angular distributions of HBr clusters showed an off-normal peak at 40’ to the surface normal. The off-normal angular distribution of the ejected clusters is thought to be due to collision cascades along preferred directions in the deposited crystalline HBr layer, as has been postulated for the directed emission observed in ion sputtering processes.1D-12 (1) Ehrlih, D. J.; Tsao, J. Y. J. Vac.Sci. Technol. 1983,E l , 969. Chuang, T. J.; Surf. Sci. Rep. 1983,3, 1. Osgwd, R. M.Jr. Annu. Rep. Phys. Chem. 1983, 34, 77. Chuang, T. J. Surf. Sci. 1986, 178, 763. (2) Bourdon, E. B. D.; Cowin, J. P.; Harrison, I.; Polanyi, J. C.; Segner, J.; Stanners, C. D.; Young, P. A. J . Phys. Chem. 1984,88,6100. Bourdon, E. B. D.; Das, P.; Harrison, I.; Polanyi, J. C.; Segner, J.; Stanners, C. D.; Williams, R. J.; Young, P. A. Faraday Discuss. Chem. SOC.1986,82, 343. ( 3 ) (a), Harrison, I.; Polanyi, J. C.; Young, P. A. J . Chem. Phys. 1988, 89, 1475. (b), Harrison, I.; Polanyi, J. C.; Young, P. A. J. Chem. Phys. 1988, 89, 1498. (4) Bourdon, E. B.D.; Cho, C. C.; Das, P.; Polanyi, J. C.; Stanners, C. D., to be submitted for publication. ( 5 ) Bartasch, C. E.; Gluck, N. S.; Ho, W.; Ying, Z . Phys. Reu. Lett. 1986, 57,1425. Domen, K.;Chuang, T. J. Phys. Rev. Lett. 1987,59,1484. Chuang, T. J.; Domen, K.J. Vac. Sci. Technol. 1987, AS, 473. Marsh, E. P.; Tabares, F. L.;Schneider, M.R.;Schneider, J. P.J. Vac. Sci. Technol. 1987, A5, 5 19. Celii, F. G.; Whitmore, P. M.; Janda, K. C. Chem. Phys. Lett. 1987, 138, 257. Ying, 2.;Ho, W. Phys. Reo. Lett. 1988, 60, 57. Zhou, Y.; Feng, W. M.; Henderson, M. A.;Roop, B.;White, J. M. J . Am. Chem. Soc. 1988,110,4447. ( 6 ) Castleman, A. W. Jr.; Keesee, R. G. Annu. Reu. Phys. Chem. 1986, 37, 525. Ryali, S. B.; Fenn, J. B. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 245. Hegena, 0. F. In Molecular Beams and Low Denstiy Gasdynamics; Wegener, P. P.,Ed.; Marcel Dekker: New York, 1974; Chapter 2. (7) Cowin, J. P.; Auchbach, D. J.; Becker, C.; Wharton, L. Surf: Sci. 1978, 78, 545; 1979,84(E), 641. (8) Comsa, G.; David, R. Surf. Sci. Rep. 1985, 5, 145. (9) Natzle, W. C.; Padowitz, D.; Sibner, S . J. J. Chem. Phys., in press.
0022-3654/88/2092-6859$01.50/0
Experimental Section The experiments were carried out in an ultra-high-vacuum Torr) equipped with (UHV) chamber (base pressure 2 X LEED/Auger and a differentially pumped quadrupole mass spectrometer (Extrel). The mass spectrometer could be rotated around the sample to provide angular resolved thermal desorption spectra and time-of-flight mass spectra. In the time-of-flight (tof) studies, a LiF(001) crystal was cooled to ca. 80 K, dosed with HBr (Scott Scientific Gas, 99.95%), and irradiated by an excimer laser (Lumonics TE-861-4). The flight distance of the photodesorbed species was 16.9 cm. The tof signal, triggered by the laser pulse, was recorded in analog mode on a Lecroy 200-MHz transient recorder, and averaged with the aid of an IBM XT compatible computer. The LiF(001) sample (Harshaw Chemicals) was cleaved in air and annealed at 700 K for at least 12 h in the UHV chamber prior to the experiments. The sample was heated radiatively by a tungsten filament and cooled by a closed-cycle helium refrigerator (Air Products). Gaseous HBr was purified by several freezepumpthaw cycles with liquid nitrogen and chloroform/liquid N2 baths before it was introduced into the UHV chamber. The LiF (001) surface could be dosed either by a stainless steel capillary directed at the crystal or by a leak valve for background dosing. An ArF (193 nm) excimer laser beam was used to irradiate the crystal at 6’ glancing incidence, at right angles to the rotation plane of the mass spectrometer. The angles reported in the following section have been transformed from the rotation angles of the mass spectrometer into polar angles, to allow for the 6O tilt of the crystal. Typically, in these experiments, the laser illuminated area was 0.3 cm2, the repetition rate was 2 Hz, and the laser fluence ranged from 0.3 to 5 MW/cm2. Results and Discussion Previous photochemical studies of adsorbates on LiF surfaces have focused on the low-coverage region (0.001-30 langmuirs; ~ the experiments presented here, 1 langmuir = 10” Torr s ) . ~ ,In the effect of higher doses (80-12000 langmuirs) was investigated. In addition to the photoproducts observed in submonolayer studies, namely H, H2, Br2, and photodesorbed HBr monomer, desorption of HBr clusters was observed. These included dimer, trimer, and tetramer. Large clusters are also likely to have been formed, but their masses are beyond the range of the mass spectrometer. The desorption yield of HBr monomer is dependent on coverage, laser fluence, and laser wavelength. (Dependencies will be documented more fully in a subsequent comm~nication.)~ At submonolayer coverage in other studies, the desorption yields of HBr, ~
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(10) Sigmund, P. In Sputtering by Particle Bombardment I; Behrisch, R., Ed.; Springer-Verlag: New York, 1981; Chapter 2. (11) Andersen, H. H. Nucl. Instrum. Methods 1987, B18, 321. (12) Wittmaack, K. In Inelastic Ion-Surface Collisions; Tolk, N. H., Tully, J. C., Heiland, W., White, C. W., Eds.; Academic Press: New York, 1977; pp 153.
0 1988 American Chemical Society
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6860 The Journal of Physical Chemistry, Vol. 92, No. 24, 1988 POLAR ANGLE (DEGREE) 0
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Figure 1. Angular distributions of HBr monomer, dimer, trimer, and tetramer, from the first (solid line), second (dash line) and third (dotted line) batches of five laser shots, in an experiment using a 900-langmuir HBr dose and 3 MW/cm2 laser fluence. The distributions have been normalized to unity at the peak (relative yields are given in the text).
CH3Br, and H2S monomer were found to increase with increasing coverage and laser fluence; however, at multilayer coverage the yields could drop The desorption yields of HBr dimer, trimer, and tetramer studied here also grew with increasing coverage and laser fluence and diminished when the coverage was high. The sharply reduced desorption yield at high coverage, for a given laser fluence, suggested the existence of a desorption threshold FD which increased as the surface coverage became larger. With a 900-langmuir dose and laser fluence of 3 MW/cm2, the desorption yields were around 100 langmuirs/pulse for the first few laser shots. Desorption of the clusters, but not monomer, disappeared after 20 laser shots. Presumably the remaining surface coverage was not great enough for a detectable number of clusters to be produced. The minimum suxface coverage required to produce HBr clusters was approximately 50 langmuirs. At a given coverage and laser fluence, the HBr clusters of smaller size showed higher velocity and lower translational energy. For example, at 6" to the surface normal with a 900-langmuir dose and 3 MW/cm* laser power, the most probable velocities for HBr monomer, dimer, trimer, and tetramer were 810, 590, 550, and 530 m/s, respectively, corresponding to peak translational energies of 0.24,0.24,0.36, and 0.46 eV. Regardless of the desorption yields, the translational energies of the desorbates formed by the first laser shots were always the highest. The average energies decreased as the number of the laser shots increased, Le., as surface coverage and desorption threshold decreased. (The variation of desorption thresholds will be discussed
in more detail later.) The translational energies also decreased with increasing polar angle. This is commonly observed in de~orption.*~~J~ The integrated relative tof signals of monomer to dimer to trimer to tetramer were typically 1.00.0.01:0.01:0.01. The relative ionization efficiency of the clusters is not known. Assuming comparable ionization efficiency, these signals imply constant relative populations far n = 2,3, and 4,in marked disagreement with the exponentially declining population characteristic of cluster formation in adiabatic expansion of a gas.I4 The yields of HBr monomer, dimer, trimer, and tetramer at different angles are shown in Figure 1. Each point in the figure represents the average yield of a batch of five laser shots. The monomer showed a narrow radial distribution peaked at the surface normal. The result for HBr dimer (Figure lb) was strikingly different: the maximum yield appeared at about 40° to the surface normal. The angular distributions from the first, second, and third batches of laser shots (each comprising five shots) were similar. The second group of laser shots produced a higher yield of dimers than the first. For HBr trimer and tetramer (Figure 1, c and d), both an off-normal and a normal peak were detected. The off-normal peak in the first batch of shots appeared around 25", approaching 40' (13) NoorBatcha, I.; Lucchese, R. R.;Zeiri, Y. J. Chem. Phys. 1987,86, 58 16. (14) Soler, J. M.;Garcia, N.; Echt, 0.;Sattler, K.; Recknagel, E. Phys. Rev. Lett. 1982, 49, 1857.
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The Journal of Physical Chemistry, Vol. 92, No. 24, 1988 6861
in the second and third batches of shots. The normal peak of the trimer decreased with increasing number of laser shots whereas the off-normal peak showed its maximum yield in the second batch of laser shots. The maximum yields of tetramer appeared in the second group of laser shots for the normal peak and in the third group for the off-normal peak. The formation of the off-normal peaks of clusters seems likely to be related to the structure of the HBr layer. A parallel may be drawn to ion bombardment in which it is found that particles emitted from a single-crystal target tend to be ejected in the low-index The sputtering is found to take place predominantly along the closest packed crystal orientation. This is pictured as being due to the fact that linear chain collisions, termed ’linear collision cascades”, are involved in the ejection process. If the deposited HBr layer has a crystalline structure, similar collision cascades may be involved in the photoejection of HBr clusters. Crystalline HBr of orthorhombic lattice structure and hydrogen-bonded zigzag chains has lattice constants 5.56, 5.64, and 6.06 Linear collision cascades along the [Ol 11 direction of the HBr crystal would result in a peak directed at about 45’ to the normal. This is approximately what is observed. Specialized dynamics for photoejection are also required to account for the fact that some 95% of the clusters have a translational energy in excess of that required to rupture the HBr-HBr bond (i.e., >0.05 eV).20 The major photoejection pathway for multilayer HBr yields monomer. This monomeric HBr peaks normal to the surface (there is marginal evidence for a lesser peak of monomer at approximately 40° in the second batch of laser shots; Figure la). It would appear that the majority of the HBr monomer, since it does not require concurrent ejection of two (or more) HBr, need not be desorbed by the specialized process of collision cascade along a closest packed HBr crystal direction. Instead it is desorbed by a process of photoejection resembling that observed at monolayer .&.‘8919
coverage^.^,^ Theoretical and experimental studies of ion-induced cluster formation have attempted to distinguish between direct emission of pre-existing groups of particles as opposed to recombination processes in the gas phase.11s21z In the case of laser-induced HBr (neutral) cluster formation studied here, if the clusters were formed in the gas phase (as in a nozzle expansion or in Knudsen-layer collision^'^*^^) not only would we expect exponentially declining yields of successive members of the (HBr), series (see above), but the radial distributions of (HBr),, n = 2-4, should peak normal to the surface as does HBr. The observed comparable yields of clusters and also the very different angular distributions of (HBr), as compared with monomer both strongly suggest the ejection of pre-existing clusters. A third persuasive piece of evidence supporting this interpretation is the existence of different dependencies on number of laser (15) Wehner, G. K. Phys. Rev. 1956, 102, 690. (16) Staudenmaier, G. Radiat. Eff.1973,18, 181. (17) Shulga, V. I. Radiat. Eff. 1984,82, 169. (18) Savoie, R.; Anderson, A. J . G e m . Phys. 1966,44, 548. (19) Sandor, E.; Johnson, M. W. Nature 1968,217, 541. (20) Pine, A. S.; Howard, B. J. J . Chem. Phys. 1986, 84, 590. The zero-pint dissociation energies of HF and HC1 dimer are 0.129 and 0.053 eV, respectively. (21) Wittmaack. K. Phvs. Lett. 1979.69A.322. (22j Winograd, h.;Gairison, B. J.; Harrison, D. E. Jr. J . Chem. Phys. 1980,73, 3473. (23) Kelly, R.; Dreyfus, R. W. Nucl. Instrum. Methods 1988,B32, 341.
shots for HBr monomer as compared with clusters. For example, under 900-langmuir HBr dose and 2.5 MW/cm2 radiation, at 6O to the surface normal, the HBr monomer showed the maximum yield from the first five laser shots, but the trimer showed the maximum from the second five, Le., at a higher deposited energy. If the clusters were formed in the gas phase (with monomers growing into clusters), the yields of monomer and trimer should have been at its greatest for the same batch of laser shots. Most of the tof spectra obtained in these experiments could be fitted by a Boltzmann function with a superimposed stream ve10city.~This was not true of HBr trimer and tetramer in the first batch of five laser shots. These tof spectra exhibited distributions with a tail extending toward lower energy (rather than toward higher energy as would be the case for a Boltzmann distribution). This distribution could be generated from a Boltzmann function by a weighting factor favoring higher energies. A plausible explanation for this effect would be the existence, for the first group of laser shots, of an energy barrier to photoejection of clusters that exceeds the energy of the final state; this energy difference could appear as translation in the ejected cluster. For the second and third batches of laser shots the clusters exhibited a Boltzmann distribution over translational energy. Since these later batches were characterized by higher yield of clusters (see Figure 1) we believe that the HBr adlayer may be partially fractured by the first batch of laser shots resulting in decreased barrier to cluster photoejection; the yield of clusters is then enhanced and the full Boltzmann distribution is observed. By contrast, HBr monomer exhibited a Boltzmann distribution even for the first batch of laser shots, suggesting that there is no similar barrier to monomer desorption. In earlier work we distinguished photodesorption from photoejection; the former was crystal mediated ( F centers in the LiF substrate absorbed radiation over a broad wavelength region) and the latter was adsorbate mediated (the incident radiation was absorbed by the ad~orbate).~” We have evidence that the clusters studied here are photoejected. If the laser wavelength was changed from 193 to 248 nm no clusters were observed. This can be understood in terms of the small absorption coefficient of HBr at 248 nm (1/500 of that at 193 nm, in the gas phase).24 When a multilayer of H 2 0ice was interposed between the LiF crystal and the HBr(ad) multilayer deposit, irradiation with 193 nm still ejected both monomer and clusters. This indicates that the processes being studied here are unlikely to be mediated by the LiF crystal. It should be noted that irradiation of the pure H 2 0ice at 193 nm gave no H 2 0ejection. This too is as expected, since H 2 0 has only a small extinction coefficient at 193 nrn24and hence cannot undergo photoejection. Finally we note that the peak translational energy of the clusters (-0.3 eV) exceeds by a substantial margin the peak energy that was observed for photodesorption of HBr monomer from LiF (-0.04 eV).4 Once again the experimental evidence points to photoejection of the clusters rather than photodesorption. Acknowledgment. We thank J. B. Fenn of Yale University for a helpful discussion of cluster formation in jets. We are much indebted to NSERC of Canada, the University of Toronto, the Venture Research Unit of BP Canadian Holdings, and the Ontario Laser and Lightwave Research Centre for their support of this work. (24) Okabe, H. Photochemistry of Small Molecules; Wiley-Interscience: New York, 1978.
