Photochemistry of CH3I Adsorbed on Al2O3 (0001)

J. Phys. Chem. B 1999, 103, 9717-9720

9717

Photochemistry of CH3I Adsorbed on Al2O3(0001) S. Y. Nishimura, D. N. Aldrich, M. T. Hoerth, C. J. Ralston, and N. J. Tro* Department of Chemistry, Westmont College, Santa Barbara, California 93108 ReceiVed: July 8, 1999; In Final Form: August 30, 1999

The photochemistry of CH3I adsorbed on Al2O3(0001) was studied using ultraviolet laser pulses at 266 nm. Desorbing photoproducts were detected using a mass spectrometer during photoexposure, and surface-adsorbed products were determined using thermal programmed desorption (TPD). The primary photoproducts were gas-phase iodomethane, gas-phase iodine, gas-phase methane, and gas-phase ethane. No other photoproducts were found in the gas phase or on the surface. The cross section for depletion of CH3I from the surface was 2.6 ( 0.2 × 10-19 cm2. The total photochemical yield of gas-phase photoproducts increased with increasing coverage up to a total coverage of approximately 5 ML and then remained constant with further increases in coverage. A simple stochastic model with random adsorption was used to model these data and indicated a reaction depth of approximately 2 ML.

Introduction The photochemistry of surface adsorbed molecules is significant in many processes such as photolithography, optical data storage, stratospheric ozone depletion, solar cell sensitization, and catalysis.1-5 This topic has been well studied in recent years, and there are several excellent review articles in the literature.6,7 Surfaces affect photochemical pathways in a number of ways. Metal surfaces, for example, often quench adsorbate excited states and reduce photochemical yields relative to the gas or solution phase.6,8 Some surfaces have been found to enhance chemical yields by absorbing energy from light and then transferring the energy to the adsorbate.9 The unique nature of the adsorbed phase may also produce physical constraints such as caging or orientational effects which may alter chemical pathways relative to gases or solutions.10 In this paper, we report on the photochemistry of iodomethane adsorbed on Al2O3(0001). We are particularly interested in how a dielectric surface affects the photochemical pathways relative to the gas and solution phases and how variables such as coverage alter photoproducts. We chose iodomethane because it has been well studied in the gas phase11-14 and in solution and can therefore be easily compared with our results on the surface. The lowest energy absorption band of iodomethane is associated with the promotion of a nonbonding p electron on the iodine to an antibonding σ* orbital on the C-I bond. The absorption is broad and continuous, ranging from about 220 to 350 nm, with an absorption maximum at about 258 nm (σ ) 1.5 × 10-18 cm2).11 Excitation of this transition in the gas phase and in solution results in the direct cleavage of the C-I bond. In the gas phase, this results in methyl radicals and iodine atoms with a quantum yield of unity. In solution, these radicals can recombine due to cage effects or can react with other species present in solution.11 The surface photochemistry of CH3I has been studied by several other groups.15-26 On Pd(100), exposure to UV light enhances the thermal decomposition of CH3I to CH4 and adsorbed iodine.24 On Ag(111), CH3I photolyzes to produce CH3, adsorbed I, and adsorbed C2H6.15,16 In this case, the silver

surface quenches the photodissociation for the monolayer relative to the multilayer. On MgO(100) and in condensed thick layers, CH3I was found to produce CH3, I, and I* gas-phase photofragments where I and I* represent ground state and excited state iodine atoms, respectively.18,19,23 On TiO2(110), CD3I photodecomposes to form CD3 radicals and I atoms with no significant difference in photodissocation cross sections relative to the gas phase.20,22 In our study, we focus on mapping out the photoproducts of the reaction on Al2O3(0001) and comparing these, as well as photoreaction cross sections, to the gas-phase photochemistry. Experimental Section The UHV chamber used in these experiments was pumped by tandem turbomolecular pumps with pumping speeds of 170 and 110 L/s and a titanium sublimation pump. The chamber was equipped with an ion gauge and a Hiden 301 PIC mass spectrometer. Background pressures of 5 × 10-10 Torr were maintained during the course of the investigation with the background gas being predominantly hydrogen. Single crystals of Al2O3(0001) with dimensions of 2 cm × 1.5 cm × 0.75 mm were purchased from Crystal Systems. A tantalum foil (0.0127 mm thick), for resistive heating, was sandwiched between two Al2O3 crystals using small molybdenum clips. The assembly was then mounted at the bottom of a liquid nitrogen cooled cryostat. A chromel-alumel thermocouple was attached directly to one Al2O3 crystal using a hightemperature alumina-based ceramic adhesive. The crystals were then cleaned with acetone and methanol and placed in the ultrahigh vacuum (UHV) chamber. The Al2O3(0001) surface was cleaned in vacuum by exposure to an oxygen plasma discharge with the crystal at 373 K and then heated daily to 1100 K. Auger analysis by other groups has demonstrated that this procedure produces consistently clean Al2O3 surfaces.26 A temperature range from 90 to 1100 K was attainable using liquid nitrogen cooling and resistive heating. Iodomethane was purchased from Aldrich and placed in a small stainless steel cylinder attached directly to a variable leak valve. The iodomethane underwent several cycles of freezing,

10.1021/jp9922864 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/07/1999

9718 J. Phys. Chem. B, Vol. 103, No. 44, 1999

Figure 1. Temperature-programmed desorption (TPD) traces at mass 142, for various coverages of CH3I on Al2O3(0001).

followed by pumping and thawing, in order to remove any volatile impurities. The outlet of the variable leak valve was connected to a stainless steel tube with 1/8′′ i.d. which was directed toward the front Al2O3 crystal surface. The distance between the end of the stainless steel tube and the crystal surface was approximately 1 cm. Temperature-programmed desorption (TPD) traces were acquired by linearly ramping the temperature of the crystal at a rate of 5 K/s while digitizing the ion current of the mass spectrometer. UV exposure of iodomethane was achieved using the fourth harmonic from a Continuum Minilite II Q-switched Nd:YAG laser. The beam from this source was enlarged to a diameter of 1.5 cm and directed through calcium fluoride windows toward the crystal surface. UV fluences ranging from 1 to 2 mJ/cm2 and a repetition rate of 15 Hz were used for photodissocation. Photoproducts desorbing into the gas phase were detected by digitizing the ion current of the mass spectrometer during laser exposure. Photoproducts remaining on the surface were detected by comparing TPD’s of identical coverages with and without UV exposure. In either case, multiplexing of the mass spectrometer allowed several masses to be detected simultaneously. Results Figure 1 displays temperature-programmed desorption (TPD) traces at mass 142, the parent peak, for various coverages of CH3I on Al2O3(0001). The assigned coverages are based on the adlayer sticking model described below. At low coverages, a single peak is seen in the TPD at 160 K. As the coverage is increased, a second peak grows in at about 140 K. The hightemperature peak is attributed to those CH3I molecules adsorbed directly to the surface, while the low-temperature peak is attributed to multilayer CH3I molecules. As we have seen for other systems on aluminum oxide, the multilayer peak appears before the saturation of the surface layer peak, implying that the multilayer is forming before the surface layer completely fills. Figure 2 displays TPD traces of approximately 0.3 ML initial iodomethane coverage as a function of exposure to 266 nm, 1.4 mJ pulses at 15 Hz. There is a decrease in TPD area for the iodomethane peak as a function of exposure time. There is no corresponding increase in any other mass spectral peaks during temperature-programmed desorption. We specifically looked for other products in the 1-50 amu range (adsorbed ethane and methane) and at masses 127, 268, and 155 which would

Nishimura et al.

Figure 2. TPD traces of approximately 0.3 ML initial iodomethane coverage as a function of exposure to 266 nm, 1.4 mJ pulses at 15 Hz.

Figure 3. TPD traces of approximately 4.5 ML initial iodomethane coverage as a function of exposure to 266 nm, 1.3 mJ, pulses at 15 Hz.

correspond to the highest peaks in the mass spectra of surface adsorbed iodine, diiodomethane, and diiodoethane, respectively, but could find no detectable signal. Figure 3 displays TPD traces of approximately 4.5 ML initial iodomethane coverage as a function of exposure to 266 nm, 1.3 mJ, pulses at 15 Hz. Again, we see a decrease in the TPD area for the iodomethane peak as a function of exposure time but no corresponding increase in any other mass spectral peaks during temperature-programmed desorption. Figures 4 and 5 display the mass spectral signals for methane (mass ) 16), iodomethane (mass ) 142), iodine (mass ) 127), and ethane (mass ) 30) during laser exposure of approximately 12.3 ML of iodomethane on the surface. Methane, ethane, and iodomethane are all detected by monitoring their parent peaks in the mass spectrum. Iodine is monitored at 127 amu, the only peak in an I2 mass spectrum. Some of the 127 amu signal is simply due to the cracking pattern of iodomethane. However, the iodine signal is about 40% larger than would be expected from the cracking of iodomethane, indicating the presence of iodine. Figures 6 and 7 show the integrated photochemical yields of the various photoproducts at different initial coverages. Each data point represents the integral of the mass spectrometric signal for that mass over a 30 s laser exposure. The photochemical yield increases linearly at low coverages, but then flattens out at coverages exceeding 5 ML.

Photochemistry of CH3I Adsorbed on Al2O3(0001)

J. Phys. Chem. B, Vol. 103, No. 44, 1999 9719

Figure 4. Mass spectral signals for iodomethane (mass ) 142) and iodine (mass ) 127) during UV exposure of approximately 12.3 ML of iodomethane on the surface. Figure 6. Integrated photochemical yields of iodomethane and iodine at different initial coverages. Each data point represents the integral of the mass spectrometric signal for that mass over a 30 s laser exposure. The solid line represents the results of our stochastic model. The best fit was found with the reaction depth, n, equal to 2 ML.

Figure 5. Mass spectral signals for methane (mass ) 16) and ethane (mass ) 30) during UV-exposure of approximately 12.3 ML of iodomethane on the surface.

Discussion The TPD traces shown in Figure 1 reveal a surface layer desorption peak at low coverage and the growth of a multilayer peak before the surface layer peak saturates. This implies that the multilayer begins to form before the surface layer is completely full. The relative areas of the surface layer and multilayer peaks as a function of coverage were simulated with a simple statistical sticking model employed previously.27 Briefly, the model utilized 3000 surface sites and allowed them to be randomly occupied with no surface migration following adsorption. As the sites filled, no preference was given to empty sites over occupied ones; the probability of occupying any one site was the same. By performing the calculation at different total coverages, the relative number of molecules in the surface layer and the multilayer could be calculated. In this simulation, the surface layer does not saturate until the equivalent of over 2.5 ML of total coverage is adsorbed. The multilayer begins to fill at coverages as low as 0.1 ML. The results of this model were compared to the experiment by determining the relative areas in the surface layer and multilayer peaks in the experimental TPD traces. Since these peaks overlapped substantially, a convolute-and-compare routine was employed to extract the relative areas. In this analysis, the TPD curves were simulated using zero-order (multilayer) and first-order (surface layer) desorption kinetic expressions with Gaussian broadening. The

Figure 7. Integrated photochemical yields of ethane and methane at different initial coverages. Each data point represents the integral of the mass spectrometric signal for that mass over a 30 s laser exposure. The solid line represents the results of our stochastic model. The best fit was found with the reaction depth, n, equal to 2 ML.

two curves were then summed, and the contribution of each to the overall simulated TPD spectrum was adjusted until the best fit was obtained with the experimental TPD spectrum. By carrying out this analysis for a number of TPD traces, the relative number of molecules in the surface layer and multilayer could be obtained as a function of total coverage. The data were scaled with a single parameter to fit the theoretical model. This parameter determines the reported coverages. The decrease in iodomethane TPD signal as a function of exposure time, shown in Figures 2 and 3, can be used to calculate the photoreaction cross section for surface-adsorbed CH3I. For a simple first-order process, Nt/N0 ) exp(-σNhν), where Nt is the number of iodomethane molecules at time t, N0 is the initial number of iodomethane molecules, σ is the photoreaction cross section, and Nhν is the number of photons. Using a measured pulse energy of 1.3 mJ, a pulse rate of 15 Hz, and approximating that 70% of the pulse hits the surface, a photoreaction cross section of 2.6 ( 0.2 × 10-19 cm2 is

9720 J. Phys. Chem. B, Vol. 103, No. 44, 1999 calculated. The gas-phase photodissociation cross section is 1.5 × 10-18 cm2.11 The aluminum oxide surface appears to quench the photodissociation relative to the gas phase. There are a number of possible reasons for this including orientational effects, cage effects, and electronic effects. We checked for orientational effects by measuring the photodissociation cross section at different incident angles of ultraviolet light but could observe no differences in the cross sections compared to normal incidence. Cage effects could also lower the photodissocation cross section, but the cage effect would most likely be coverage dependent. We saw no significant change in the photoreaction cross section in coverages ranging from 0.2 to 4.5 ML. The most likely reason for the decreased photoreaction cross section relative to the gas phase is electronic. Aluminum oxide surfaces have been shown to quench fluorescence due to the presence of surface states 28 and therefore may also quench the CH3I excited state and lower the photodissociation cross section. Since the TPD traces after UV exposure reveal only iodomethane on the surface, all of the photoproducts must desorb into the gas phase during photoexposure. The products we observe in the gas phase, as shown in Figures 4 and 5, are iodomethane, methane, ethane, and iodine. Because of our observed photoproducts, and because CH3I photodissociates to form methyl and iodine radicals in the gas phase, the photochemical cleavage of the surface adsorbed CH3-I bond most likely produces methyl and iodine radicals on the surface. These radicals can recombine and desorb as iodomethane, which we observe, or they can form other products. Methane can form by the abstraction of hydrogen by methyl radicals, ethane can form by the direct reaction of two methyl radicals, and iodine can form by the direct combination of two iodine radicals. Hydrogen iodide most likely forms as well, but since its cracking pattern is primarily at mass 127, we cannot distinguish it from the other photoproducts. The coverage dependence of the integrated photochemical yields, shown in Figures 6 and 7, show that the total photochemical yield increases with increasing coverage up to about 5 ML. At that point, the amount of product formed becomes constant even with increasing coverage. A similar coverage dependence has been observed for the photodissociation of CH3I on MgO(100)23 and was interpreted in terms of reaction depth. CH3I molecules trapped within the film may undergo photochemical bond cleavage, but then recombine producing no net reaction. CH3I molecules near the surface can undergo photochemical reactions and then desorb. To get an idea of the reaction depth, we modeled the data using a simple statistical model. In this model, we allowed 100 surface sites to be randomly occupied. Since we know that our adlayer does not grow in a layer-by-layer fashion, we allowed all sites to have equal sticking probabilities, including those that were already occupied. A random number generator simply picked numbers, which then meant that the site corresponding to that number was occupied. If the same number came up again, the site was doubly occupied and so on. Then, as a function of coverage, we determined the number of molecules in the top n layers, where n represents the reaction depth in monolayers. One monolayer was defined as the point where 100 molecules had adsorbed at the surface; because our sticking was random, not every site is filled at 1 ML and many sites were occupied by two or more molecules. Figures 6 and 7 show the results of the model (solid line), superimposed on our data. The best fit was found with n ) 2, implying that molecules in the first couple of layers can undergo photodissociation and eject products into

Nishimura et al. the gas phase; molecules in deeper layers most likely recombine following photochemical cleavage of the CH3I bond. Conclusions The primary photoproducts following 266 nm irradiation of CH3I adsorbed on Al2O3(0001) were gas-phase iodomethane, gas-phase iodine, gas-phase methane, and gas-phase ethane. While the overall amount of CH3I remaining on the surface decreased after irradiation, no other photoproducts remained on the surface. The approximate cross section for depletion of CH3I from the surface was (2.6 ( 0.2) × 10-19 cm2. The total photochemical yield of gas-phase photoproducts increased with increasing coverage up to a total coverage of approximately 5 ML and then remained constant with further increases in coverage, indicating that only molecules in the first few layers undergo photoreactions. A simple stochastical model with random adsorption was used to model these data and indicated a reaction depth of approximately 2 ML. Acknowledgment. This research was supported in part by the donors of the Petroleum Research Fund, administered by the American Chemical Society, and by a grant from the National Science Foundation (CHE-9510153). N.J.T. gratefully acknowledges David F. Marten for many helpful discussions. References and Notes (1) Gerisher, H.; Willig, F. In Physical and Chemical Applications of Dyestuffs; Topics in Current Chemistry, Vol. 61; Springer-Verlag: Berlin, 1976; p 31. (2) Chuang, T. J. J. Vac. Sci. Technol. 1982, 21, 798. (3) Laser-Controlled Chemical Processing of Surfaces; Johnson, A. W., Ehrlich, D. J., Schlossberg, H. R., Eds.; Elsevier: New York, 1985. (4) Domen, K.; Chuang, T. J. Phys. ReV. Lett. 1987, 59, 1484. (5) Hohman, J. R.; Fox, M. A. J. Am. Chem. Soc. 1982, 104, 401. (6) Zhou, X. L.; Zhu, X. Y.; White, J. M. Surf. Sci. Rep. 1991, 13, 73. (7) Ho, W. Surf. Sci. 1994, 299, 966. (8) Zhou, X. L.; Zhu, X. Y.; White, J. M. Acc. Chem. Res. 1990, 23, 327. (9) Legget, K.; Polanyi, J. C.; Young, P. A. J. Chem. Phys. 1990, 93, 3645. (10) Slayton, R. M.; Franklin, N. R.; Tro N. J. J. Phys. Chem. 1996, 100, 1551. (11) Calvert, D. A.; Pitts, N. A. Photochemistry; John Wiley and Sons: New York, 1966. (12) Penn, S. M.; Hayden, C. C.; Carlson Mugskens, K. J.; Crim, F. F. J. Chem. Phys. 1988, 989, 2909. (13) Ogorzalek Loo, R.; Haerri, H.-P.; Hall, G. E.; Houston, P. L. J. Chem. Phys 1989, 90, 4222. (14) Van Veen, G. N. A.; Baller, T.; DeVries, A. E. Chem. Phys. 1984, 87, 405. (15) Zhou, X.-L.; White, J. M. Surf. Sci. 1991, 241, 270. (16) Zhou, X.-L.; White, J. M. Chem. Phys. Lett. 1990, 167, 205. (17) Jo, S. K.; White, J. M. J. Am. Chem. Soc. 1993, 115, 6934. (18) McCarthy, M. I.; Gerber, R. B.; Trentelman, K. A.; Strupp, P.; Fairbrother, D. H.; Stair, P. C.; Weitz, E. J. Chem. Phys. 1992, 97, 5168. (19) Kutzner, J.; Linkeke, G.; Welge, K. H.; Feldmann, D. J. Chem. Phys. 1989, 90, 548. (20) Garret, S. J.; Holbert, V. P.; Stair, P. C.; Weitz, E. J. Chem. Phys 1994, 100, 4615. (21) Dixon-Warren, St. J.; Hegel, D. V.; Jensen, E. T.; Polanyi, J. C. J. Chem. Phys. 1993, 98, 5954. (22) Garret, S. J.; Holbert, V. P.; Stair, P. C.; Weitz, E. J. Chem. Phys. 1994, 100, 4626. (23) Howard Fairbrother, D.; Briggman,. K. A.; Stair, P. C.; Weitz, E. J. Phys. Chem. 1994, 98, 13042. (24) Solymosi, F.; Revez, K. Surf. Sci 1993, 280, 38. (25) Dixon-Warren, St. J.; Jensen, E. T.; Polanyi, J. C. J. Chem. Phys 1993, 98, 5938. (26) Poppa, H.; Moorhead, D.; Heineman, K. Thin Solid Films 1985, 128, 252. (27) Slayton, R. M.; Aubuchon, C. M.; Camis, T. L.; Noble, A. R.; Tro, N. J. J. Phys. Chem. 1995, 99, 2151. (28) Haynes, D. R.; Helwig, K. R.; Tro, N. J.; George, S. M. J. Chem. Phys. 1990, 93, 2836.