Castro ME

Jo SK

Kiss János

White JM

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-“ta ,.  iS é » t4  ;- ! a  J ~L«r ¿g; ._ _,__  , .^_ ^—  American Chemical Society Symposium Series No. 482 (1992) 310. 'Chapter 20Mass Effects in Photodissociation of ChloromethaneQuenching by Substrate MetalS. K. J o 1,2, J . K issu , M. E. C astro1, and J. M. W hite1‘C en ter for M aterials Chem istry and  D epartm ent of Chem istry, Universityof Texas, Austin, TX 78712-1167The rates of photolysis of CH3CI and CD3CI on Pt(l 11) at 100 K were measured using a conventional mercury arc lamp. The rate is significantly faster for the lighter halide and, quantitatively, the rate ratio is CH3C1:CT)3C1::1.27:1. This difference is explained by the Menzel-Gomer-Redhead (MGR) model, which predicts a mass-dependent variation of the competition between substrate quenching and dissociation. The rate ratio corresponds to the successful dissociation of 2.4-8% of the excited molecules.This study extends cur earlier work on CH3CI surface photochemistry (1) and investigates how mass influences the probability of photon-driven dissociation of the C-Cl bond, thereby quantifying the extent of substrate quenching. In the earlier work (1), we confirmed and extended the pioneering investigations of Cowin and co-workers (2 ,3) who showed that the C-Cl photodissociation can be accounted for by dissociative attachment of substrate-excited photoelectrons. That work employed multilayer CH3CI on N i(l 11) (2) and monolayer CH3CI on H2O-precovered N i(l 11) (3) to elucidate the underlying mechanism. After 248 nm (5 eV) laser pulses, they monitored time-of-flight (TOF) CH3 signals and post-irradiation temperature-programmed desorption (TPD) areas of CH3CI as measures of photolysis for various initial CH3CI and H2O spacer-layer coverages. As a function of layer thickness, they observed an initial increase and subsequent decay of both the photolysis cross section (s) and the CH3 TOF signal, and concluded the importance o f both a metal-to-adsorbate charge-transfer (dissociative election attachment, DEA) mechanism and a strong perturbation (resonance quenching) of the CH3Q  photolysis by the substratemetaL The decay at high coverages was ascribed to the attenuation of electrons coming from the substrate through several layers.2Current address: Microelectronics Sciences Laboratories, Columbia University,New York, NY 100273Visiting scholar from Reaction Kinetics Research Group, Hungarian Academy of Sciences, University of Szeged, P.O. Box 105, Szeged H-6701, Hungary0097—6156/92/0482—0310S06.00/0 ©  1992 American Chemical Society20. JO ET AL. Mass Effects in Photodissociation of Chloromethane 311Recently, we studied the same multilayer (la) and D2O-precovered monolayer CH3CI systems (lb), but on Pt(l 11) and with a CW Hg-arc light source. We directly measured the yield of ejected photoelectrons during irradiation, and successfully correlated it with the photolysis rate, confirming that DEA alone is sufficient to account for all the dissociation chemistry. Only those photons with energies greater than 4.3 eV were effective, indicating that hot electrons with energies in the range between the Fermi level and -0.2 eV below the vacuum level are ineffective.One remaining issue is the extent to which the substrate metal quenches the photolysis (lb). The measured photolysis rate coefficient (k ' in d[CH3Cl]/dt = -k '-[C H 3Cl]), although approximately constant over small coverage ranges, generally drops with increasing CH3CI coverage (1). We have, however, pointed out that the coverage-dependent variation of either the cross section or the (pseudo-) rate coefficient (k') is not an appropriate parameter for an assessment of quenching because, as the coverage increases, the exponentially decaying electron flux contributes to their variation (la, 4). Taking account of the exponentially decreasing photoelectron yield gave a rate coefficient (k), which was higher for low coverages than for high coverages (la). This was attributed to the overwhelming effect, for molecules at the metal surface, of promotion over quenching. The possible origins for the net enhancement of the low-coverage photolysis, as discussed earlier (1), are: (i) an additional contribution from direct photon excitation of the "metal-adsorbate complex" consisting of the surface metal layer and first adsorbate layer (5)\ and, more likely, (ii) enhanced tunnelling attachment of "hot" electrons excited to levels between the vacuum level and -0 .2  eV below it.Our earlier results (1), however, still could not provide a quantitative estimate erf the probabilities of quenching and dissociation. This very important branching ratio issue is addressed in this paper. Excitation and quenching both involve coupling between the adsorbed molecule and the metal (6). This coupling takes the form of induced-dipole/induced-dipole coupling, electron-hole pair formation in the metal, and charge transfer mediated processes. Direct dissociation times along repulsive potential energy curves can be as short as 10*13 to 10* 14 s (7, 8); thus, quenching must operate on an equal or faster time scale to inhibit bond cleavage. Depending upon the resonant or non-rcsonant character of the relaxation process, theory predicts characteristic quenching times relatively close to this, lying between 10" 15 and 10" 13 s (9,10).From an experimental point of view, one way to gain insight into the branching ratio is to compare photon-driven rates for hydrogenated molecules with those for deuterated ones. Within the MGR model (11,12), more quenching, and therefore a smaller dissociation cross section, is expected for heavier isotopes. As applied to surface photochemistry, this method was used by W olf et al. (13) with the H 20/Pd(l 11) system. Here we report results for CH3G  and CD3CI on Pt(l 11). The measured dissociation rate coefficient is a factor of 1.27 larger for CH3CI than for CD3G . Depending on the effective reduced mass chosen, we estimate that, of the excited CH3CI molecules, between 2.4 and 8 % dissociate. The implications are discussed in relation to previous results (1-3).E x p e rim e n ta l.A UHV (2.5x10" 10 torr) chamber equipped with X-ray photoelectron spectroscopy (XPS), ultra-violet photoemission spectroscopy (UPS), quadrupole mass spectroscopy (QMS), and an ion gun w used. The substrate, a Pt(l 11) single crystal (8 mm diam. and 1 mm thick), could be resistively heated to 1500 K with a linear ramp (0 .1 -5 0  K/sec) and subsequently cooled to 50 K within 5 min by a312 SURFACE SCIENCE OF CATALYSISclosed-cycle He cryostat. A collimated doser containing an inner pin-hole conductance (2 m), mounted on a linear motion device, provided reproducible, uniform and localized (to the crystal surface) exposures of the adsorbate molecules. The crystal temperature was kept at 60 K during dosing and irradiation. The full emission (£5.4 eV) of a high-picssure Hg-arc lamp was used to irradiate the adsorbate-covered surface. At these wavelengths, both CH3CI and its deuterated counterpan are transparent The ultraviolet (UV) light was incident (from outside the chamber through a UV-grade sapphire window) at 45° off the surface normal. Post-irradiation temperature-programmed desorption (TPD) of molecular CH3CI and CD3CI was used to quantify the photolysis. Line-of-sight TPD was performed with a 6 K/sec ramp rate. CH3CI (Linde; 99.9%) and CD3CI (MSD-Isotope; 99.9 atom % D) were dosed without further purification. Gas purities were confirmed using the QMS.R esu lts  an d  D iscussion.Assuming the photolysis is first order with respect to the surface concentration of C H 3CI (CD3CI), the rate equation can be written as d l/d t = -k '-I or Ln{I/Io> = - k 't ,  where Io and I are the parent molecular TPD areas after 0 and t min irradiation, respectively, and k ' is the rate coefficient. Thus, the rate coefficient (k') can be obtained from a semilogarithmic plot of the TPD area ratio (post- and pre­irradiation) as a function of irradiation time (t).Figure 1 summarizes the results in first-order kinetic form. The nicely linear fits demonstrate the adequacy of our first order analysis for the C-Cl dissociation reaction. To obtain each data point, the clean (as judged by XPS) surface was covered with a full chemisorbed layer (±3%) and irradiated at 60 K for the time indicated. As described earlier, one monolayer coverage was taken as that of maximum TPD area without multilayer desorption. After irradiation, the residual parent TPD signal was measured and integrated to give, after division by the initial TPD area, the ordinate o f Fig. 1. Gearly, with increasing irradiation time, the number of remaining parent molecules goes down, reflecting the photodissociation (other work shows die formation of Q  and CH3 bound to P t( l l l )  (1)). The slopes of the two curves give the relative cross sections; that for CH3CI is 1.27 times larger than that for CD3CI. This is the new, key observation reported here.We now turn to an interpretation and analysis of this isotope effect We employ a model that is often used to describe election stimulated desorption (ESD) (15-17). In 1964, Menzel, Gomer (11) and Redhead (12) proposed a model (MGR) for these kinds of processes that is analogous to direct photodissociation of a gas phase diatomic molecule. An adsorbate is assumed to be electronically excited to a repulsive excited state potential energy curve. After excitation, bond elongation (i.e. nuclear morion) occurs and can lead to dissociation. The presence of the metal surface opens up efficient quenching channels that are not available in the gas phase, and these operate to bring the adsorbate back to its ground state. The probability of successful dissociation is then proportional to a product of two parameters: the initial excitation probability, times the probability of survival on the excited state potential energy surface (PES) long enough to gain the required kinetic energy (bond elongation).There is a mass dependence in these considerations. First, it is assumed that the dissociative, repulsive potential energy surfaces of negative ion states are the same for both isotopes. Then, the mass dependence occurs because of timing; that is, it takes longer to move the heavier (deuterated) methyl group through the distance required for dissociation. Since the electronic couplings are assumed to be the same and since these are responsible for the quenching, we expect the effective average quenching times to be identical and, thus, the average distance travelled by CD3 to20. JO ET AL. Mass Effects in Photodissociation of Chloromethane 313Figure 1 : Ln (I/Io) as a function of irradiation time for 1 ML initial coverage of CH3G  and CD3G  cm Pt(l 11). Iq and I are the CH3G  (or CD3G ) TPD areas before and after irradiation, respectively. The ratio of the slopes (rate coefficients, k ') is also shown. See the text for details.be smaller. On the average then, the dissociation probability is higher, as observed, for the H-labelled, compared to the D-labelled, chloride.This can be described in mathematical form. Following the formulation by W olf et ai. (13) and Madey et al. (15), the probability, Pd, of C-Cl bond dissociation from the adiabatically excited negative ion (CH3CT*) is given bywhere x is the coordinate of the bond being broken, xo is the starting value (equilibrium C-Cl distance) and Xc the critical value beyond which dissociation occurs, t is the quenching lifetime, and v is the velocity of the methyl fragment. The mass dependence appears when Pd is rewritten in terms of the potential energy change that occurs between Xo and xç.Pd= exp1 fXc 'dxV 2 ! *W V(xo)-V(x)Xo /(2)where m is the reduced mass of the dissociating quasi two-particle system, and V(xo) and V(x) are the potential energies at xo and xc. The Cl ends of CH3CI and CD3CI are known to be oriented toward the Pt(l 11) substrate (1,14). Thus, it is reasonable to take the Pt into account in the reduced mass calculation (p=mi-m2/[m l +m2]; tni=Ptn+Cl (n-*°°) and m2=CH3 or CD3). Since, then, m i is so large compared to m2, the u value can be approximated as m2 (15 amu for CH3CI and 18 amu for CD3CI). At the other extreme, ignoring the Pt mass will give a reduced mass of 10.5 amu for CH3CI and 11.9 for CD3CL These can be used to establish limiting dissociation probabilities within the MGR formulation.3 1 4 SURFACE SCIENCE OF CATALYSISAs noted above, the observable total cross section for dissociation, a j ,  is a product:od = Pd-oex (3)where ocx is the excitation cross section. The repulsive negative ion states of CH3CI and CD3CI are assumed to be the same; thus, the aex are the same for the two isotopes. Taking the ad ratio for two isotopes, after inserting the mass dependent formula from Eq. (2) and cancelling mass independent terms, leads to:ad(H)/ad(D) = Pd(H)l-M , (4)where M = {wynn} Using the ad(H)/ad(D) of 1.27 obtained from the rate coefficient ratio in Figure 1 and M = (18/15)1/2 or 1.095, we obtain Pd(H) = 0.08. Using M = ( 10.5/ 11 .9) 1/2 gives Pd(H) = 0.024. The values for Pd(D) are 0.06 and 0.02, respectively. These calculations, as noted above, set bounds on the dissociation probability within the framework of the MGR model and indicate that dissociation and quenching are competitive.This range of Pd, 2.4~8%, is comparable to that (14%) obtained by W olf et al. (13) for H2O  on Pd(l 11). The results of this work provide an important quantitative assessment of substrate quenching (>90%) and confirm the operation of strong quenching of the excited negative molecular ion by the substrate metal, in agreement with the suggestion of Cowin et al. (2 ,J). Nonetheless, the photolysis rate coefficient in the submonolayer is higher than in the multilayer regime, and we have suggested the importance of enhanced tunneling attachment of hot electrons within 0.2 eV of the vacuum level (1-3).Summary.Based on measurements of photon-driven (<5.4 eV photons) dissociation for CH3CI and CD3CI on Pt(l 11), we have established that the dissociation cross section for CH3CI is 1.27 times that for CD3CI. Within the framework of the Menzel-Gomer-Redhead model, and assuming limiting values for the reduced mass of the Pt-methyl chloride system, we conclude that between 2.4 and 8% of those molecules excited actually dissociate. The remaining 92 to 98 % are quenched on a timescale that precludes dissociation.Acknowledgments.SKJ thanks Martin W olf for useful discussions. This work was supported in part by the National Science Foundation, grant CHE9015600.Literature Cited:[1] (a) Jo, S. K.; White, J.M. J. Phys. Chem. 1990, 94, 6852; Jo, S. K.; Zhu, X.-Y.; Lennon, D.; White, J. M. Surf. Sci. 1991, 241, 231.(b) Jo, S. K.; White, J. M. Surf. Sci. submitted.[2] Marsh, E. P.; Gilton, T. L.; Meier, W.; Schneider, M. R.; Cowin, J. P. Phys. Rev. Lett. 1988, 61, 2725.[3] Marsh, E. P.; Tabares, F. L.; Schneider, M. R.; Gilton, T. L.; Meider, W.; Cowin, Ji>. J. Chem. Phys. 1990,92, 2004.[4] Jo, S. K.; White, J. M. J. Chem. Phys., in press.20. JO ET AL. Mass Effects in Photodissociation o f Chloromethane 315[5] White, J. M., in Chemistry and Physics of Solid Surfaces V777; Betz, G.; Varga, P„ Editors.; Springer Verlag; Berlin, 1990, p. 48.[6] Avouris, Ph.; Walkup, R. E. Ann. Rev. Phys. Chem. 1989, 40, 173.[7] (a) Marsh, E. P.; Tabares, F. L.; Schneider, M. R.; Cowin, J. P. J. Vac Sei. Technol. 1987, A5 , 519.(b) Marsh, E. P.; Schneider, M. R.; Gilton, T. L.; Tabares, F. L.; Meier, W.; Cowin, J.P. J. Chem. Phys. 1990, 92, 2004.[8] Bersohn, R.; Zewail, A. H. Ber. Bunsenges. Physik. Chem. 1988, 92, 373.[9] Jennison, D. R.; Stechel, E. B.; Bums, A. R. In Desorption Induced by Electronic Transitions (DIET) III; Stulen, R. H.; Knotek, M. L., Eds. Springer-Verlag; Berlin, 1988, p. 167.[10] (a) Stechel, E. B.; Jennison, D. R.; Bums, A. R. ibid., p. 136.(b) Bums, A. R.; Stechel, E. B.; Jennison, D. R. Phys. Rev. Lett. 1987,58, 250.[11] Menzel, D.; Gomer, R. J. Chem. Phys. 1964 ,41, 3311.[12] Redhead, P. A. Can. J. Phys. 1964 ,42 ,886 .[13] Wolf, M.; Nettesheim, S.; White, J. M.; Hasselbrink, E.; Ertl, G. J. Chem. Phys. 1991, 94, 4609.[14] (a) Henderson, M. A.; Mitchell, G. E.; White, J. M. Surf. Sei. 1987,754 L325.(b) Roop, B.; Lloyd, K. G.; Costello, S. A.; Campion, A.; White, J. M., J. Chem. Phys. 1989 ,97 , 5103.[15] Madey, T. E.; Yates, Jr., J. T.; King, D. A.; Uhlaner, C. J. J. Chem. Phys. 1970 ,52 , 5215.[16] Leung, C ; Vass, M.; Gomer, R. Appl. Phys. 1977, 14, 79.[17] Madey, T. E. Surface Sei. 1973 ,36, 281.Received August 26,1991

MASS EFFECTS IN PHOTODISSOCIATION OF CHLOROMETHANE - QUENCHING BY SUBSTRATE METAL

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MASS EFFECTS IN PHOTODISSOCIATION OF CHLOROMETHANE - QUENCHING BY SUBSTRATE METAL

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