Photochemistry in CH3I Adlayers on TiO2 (110) Studied with

Seong Han Kim, Peter C. Stair,* and Eric Weitz*. Department of Chemistry, Northwestern University, Evanston, Illinois 60208. Received February 24, 199...
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Langmuir 1998, 14, 4156-4161

Photochemistry in CH3I Adlayers on TiO2(110) Studied with Postirradiation Thermal Desorption Seong Han Kim, Peter C. Stair,* and Eric Weitz* Department of Chemistry, Northwestern University, Evanston, Illinois 60208 Received February 24, 1998. In Final Form: June 3, 1998 The reaction dynamics of photofragments produced by UV photolysis of neat CH3I films, adsorbed on defect-free TiO2(110), have been investigated using postirradiation temperature programmed desorption (PITPD). At low fluences, the film structure remains frozen, retaining photofragments and reaction products in the film. CH4, C2H6, I2, CH2I2, and C2H5I are detected in PITPD. CH4 is a product of translationally hot methyl radicals reacting with CH3I. Desorption of the CH4 trapped in the CH3I films is limited by diffusion in the films. The I-containing products detected in PITPD come predominantly from reactions of trapped I, CH2I, and CH3 radicals, which occur during the TPD process. The desorption of I-containing products in PITPD follows desorption-limited kinetics on the TiO2(110) surface. For C2H6 detected in PITPD, it is not clear whether the dominant reaction pathway involves hot or cold radicals. Electrons produced in the TiO2 substrate upon UV irradiation do not induce significant dissociation of CH3I adsorbed on the (110) surface and thus do not contribute significantly to fragment reactions.

I. Introduction The photochemistry of molecules adsorbed on surfaces provides a base of knowledge for understanding fundamental processes in many technologically important applications such as chemical vapor deposition, photocatalysis, photoelectrolysis, and photochemical etching. There have been a number of recent reviews that deal with the photochemistry of small molecular adsorbates.1-4 Among organic molecules, methyl iodide has been extensively investigated because the gas-phase ultraviolet (UV) photochemistry has been well studied both experimentally and theoretically.5-9 The absorption of a single photon between ∼210 and ∼350 nm promotes an electron in the iodine nonbonding orbital to the repulsive C-I antibonding orbital, resulting in C-I bond scission in less than 0.5 ps.10 The excess energy, absorbed photon energy minus bond dissociation energy, is partitioned primarily into the kinetic energy of the methyl radical and the iodine atom. Depending on whether a curve crossing occurs, dissociation produces either ground-state iodine (I) or spin-orbit excited-state iodine (I*) atoms.5-14 These primary photofragments can react with each other or with unreacted CH3I before their kinetic energy is collisionally thermal* To whom correspondence should be addressed. (1) Zhou, X.-L.; Zhu, X. Y.; White, J. M. Surf. Sci. Rep. 1991, 13, 73. (2) Polanyi, J. C.; Rieley, H. In Dynamics of Gas-Surface Interaction; Rettner, C. T., Ashfold, M. N. R., Eds; Royal Society of Chemistry: London, 1991; p 329. (3) Richter, L. J.; Cavanagh, R. R. Prog. Surf. Sci. 1992, 39, 155. (4) Zhu, X.-Y. Annu. Rev. Phys. Chem. 1994, 45, 113. (5) Gedanken, A.; Rowe, M. D. Chem. Phys. Lett. 1975, 34, 39. (6) Van Veen, G. N. A.; Baller, T.; DeVries, A. E. Chem. Phys. 1985, 97, 179. (7) Ogorzalek Loo, R.; Haerri, H.-P.; Hall, G. E.; Houston, P. L. J. Chem. Phys. 1989, 90, 4222. (8) Guo, H.; Schatz, G. C. J. Chem. Phys. 1990, 93, 393. (9) Johnson, B. R.; Kittrell, C.; Kelly, P. B.; Kinsey, J. L. J. Phys. Chem. 1996, 100, 7743. (10) Knee, J. L.; Khundkar, L. R.; Zewail, A. H. J. Chem. Phys. 1985, 83, 1996. (11) Jensen, E. T.; Polanyi, J. C. J. Phys. Chem. 1993, 97, 2257. (12) Trentelman, K. A.; Fairbrother, D. H.; Strupp, P. G.; Stair, P. C.; Weitz, E. J. Chem. Phys. 1992, 96, 9221. (13) Fairbrother, D. H.; Briggman, K. A.; Stair, P. C. Weitz, E. J. Chem. Phys. 1995, 102, 7267. (14) Holbert, V. P. Garrett. S. J.; Stair, P. C.; Weitz, E. Surf. Sci. 1996, 346, 189.

ized. The reactions of these “hot” species may differ from ordinary thermal reactions since the energy distribution of the reactive species is not equilibrated, i.e., there is a large population of hyperthermal species. In the gas phase, the formation of CH4, CH2I2, I2, C2H6, C2H4, and CHI3 was reported after photolysis of CH3I at 2537 Å.15 Formation of CH4 was attributed to the abstraction of an H atom from CH3I by hot methyl radicals.15-18 However, there was some disagreement about the formation of C2H6: whether it is formed from a hot methyl radical reaction with CH3I18 or from the recombination of cold radicals.17 Truby and Rice reported that for flash-lamp photolysis of (CH3I + 20N2) mixtures, C2H6 produced via a hot radical reaction could account for ∼5% of that produced via the recombination of thermalized radicals.19 Kinetic data for various gas-phase reactions can be found in ref 20. Reactions of photofragments in the adsorbed CH3I layers can follow different pathways than the analogous gasphase processes. Zhou and White studied the photolysis of CH3I on Ag(111) using a high-pressure Hg arc lamp.21 From postirradiation temperature programmed desorption (PITPD) experiments, they found that a small portion of the CH3 is retained at the surface and desorbs recombinatively as C2H6, with a peak desorption temperature of ∼260 K. All I atoms remain on the surface and are desorbed as atomic I at a higher temperature. Using reflection-absorption infrared spectroscopy (RAIRS), Grassian and co-workers detected the formation of CH4 and CH2I2 from the 248 nm pulsed laser irradiation of CH3I thin films on Ag(111).22 They also detected small amounts of C2H6, C2H5I, CHI3, and I2 in PITPD. Recently, time-of-flight quadrupole mass spectrometry (TOF-QMS) has been used to study the fluence depen(15) Harris, G. M.; Willard, J. E. J. Am. Chem. Soc. 1954, 76, 4678. (16) West, W.; Schlessinger, L. J. Am. Chem. Soc. 1938, 60, 961. (17) Schultz, R. D.; Tayor, H. A. J. Chem. Phys. 1950, 18, 194. (18) Souffie, R. D.; Williams Jr., R. R.; Hamill, W. H. J. Am. Chem. Soc. 1956, 78, 917. (19) Truby, F. K.; Rice, J. K. Int. J. Chem. Kinet. 1973, 5, 721. (20) Costela, A.; Figuera, J. M.; Martin, M.; Perez, J. M.; Valle, L. J. Chem. Soc., Faraday Trans. 1 1980, 76, 30. (21) Zhou, X.-L.; White, J. M. Surf. Sci. 1991, 241, 270. (22) Coon, R. S.; Myli, K. B.; Grassian, V. H. J. Phys. Chem. 1995, 99, 16416.

S0743-7463(98)00218-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/03/1998

Photochemistry in CH3I Adlayers on TiO2(110)

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dence of photoproduct yields during high fluence 257 nm irradiation of CH3I multilayers on TiO2(110).23 The reaction is initiated by photodissociation of CH3I on the surface or in the film.

CH3I + hν(257 nm) f CH3 + I(2P3/2, 2P1/2)

(1)

CH3 stands for a hot methyl radical prior to losing a significant fraction of its nascent kinetic energy. Photofragments were found to react with neighboring CH3I molecules or other photofragments to form photoproducts via the following reactions:23

CH3 + CH3I f CH4 + CH2I

(2)

CH3 + CH3I f C2H6 + I

(3)

CH2I + I f CH2I2

(4)

CH2I + CH3I f C2H5I + I

(5)

I + I f I2

(6)

It is interesting that at high fluence, in the adsorbed layer, C2H6 results predominantly from the reaction of energetic CH3 radicals with the neighboring parent molecules, a process which differs from the gas-phase reaction involving the recombination of two thermalized CH3 radicals. This paper focuses on the dynamics of the photofragment reactions taking place in CH3I films under low fluence irradiation. In contrast to the evaporation of adlayers produced by a rise in substrate temperature under high fluence irradiation, CH3I adlayers remain frozen under low fluence irradiation, and diffusion of photofragments in the adlayers is very limited. Under these conditions, photofragment reaction probabilities depend on their nascent kinetic energy distribution. Comparing the PITPD results with the above reaction mechanism, it is found that CH4 is formed during laser irradiation via the reaction of hot methyl radicals prior to thermalization to the film temperature and that I2, CH2I2, and C2H5I, observed in PITPD, are formed mainly during the TPD process, as a result of reactions of trapped photofragments and intermediates. The origin of C2H6 in PITPD cannot be established from this study alone. Subvacuum level electrons produced by UV excitation of the TiO2 substrate do not induce significant C-I bond dissociation of CH3I adsorbed on the defect-free (110) surface. II. Experimental Section The experimental setup used in this study has been described in detail elsewhere.23 Briefly, experiments were performed in an ultrahigh-vacuum (UHV) chamber equipped with an Ar ion sputtering gun, a low-energy electron diffraction/retarding-field Auger electron spectrometer (LEED/AES), a collimated gas doser, and a quadrupole mass spectrometer (QMS). The TiO2(110) single crystal was cemented to a Ni backing foil which was mounted on a UHV sample manipulator with facilities for liquid nitrogen cooling to 90 K and heating to 1000 K via electron bombardment behind the crystal. A type K thermocouple was attached to the edge of the crystal for sample temperature measurements. The surface was cleaned in a vacuum with cycles of Ar ion sputtering followed by annealing in oxygen. After repeated cycles, the crystal showed a yellowish hue, indicative of a fully oxidized bulk sample.24 The surface showed no contamination in AES and a p(1 × 1) pattern in LEED. After LEED and AES analyses, the surface was reannealed in oxygen (23) Kim, S. H.; Briggman, K. A.; Stair, P. C.; Weitz, E. J. Vac. Sci. Technol. 1996, A14, 1557. (24) Eriksen, S.; Egdell, R. G. Surf. Sci. 1987, 180, 263.

to remove any surface defects produced by the electron beam. The absence of surface defects was confirmed from the absence of a 210 K TPD peak due to CH3I from defect sites.23 Methyl iodide, CH3I (Fisher, 99.9%) and CD3I (Aldrich, 99.5+ at. % D), was purified by several freeze-pump-thaw cycles. Deuterated methyl iodide was used to avoid high background interference with methane signals. Dosing was accomplished by exposing the surface, at 90 K, to methyl iodide collimated by a microcapillary array. Nominal exposures in langmuir (1langmuir ) 1 × 10-6 Torr‚s) were calculated from the background pressure rise using an uncorrected ion gauge. From TPD measurements, a dose of 0.6 langmuir was determined to be 1 monolayer (ML).23 The absolute coverage of CH3I on TiO2(110) at 1 ML was calculated to be (2.2 ( 0.1) × 1014 molecules/cm2 from a QMS signal calibration.25 The 257 and 320 nm laser pulses were generated by frequency doubling the 514 and 640 nm output of a XeCl excimer pumped dye laser. The laser beam was directed into the UHV chamber through a LiF window with an angle of incidence of 45° to the surface normal. Quartz plates and neutral density (ND) filters were used for laser power adjustment. The laser fluence at the surface was calculated taking into account the measured transmittance of the LiF window at 257 nm. The irradiated area on the surface was determined to be 0.233 cm2 from the decrease of the CH3I thermal desorption peak area after complete depletion of one spot in a 1 ML film. The TiO2(110) surface was dosed with methyl iodide to an initial coverage and then irradiated with laser pulses. After a given photon dose, the surface was radiatively heated from the backside without energetic electron bombardment. With this procedure, electron-stimulated dissociation and thereby electroninduced reaction can be prevented. Desorbing molecules were monitored as a function of surface temperature with the QMS running in a multiplexing mode (PITPD).

III. Results After low fluence (e600 µJ/cm2) photolysis of CH3I (CD3I) multilayers on the TiO2(110) surface, the reaction products detected in PITPD are CH4 (CD4), C2H6 (C2D6), I2, CH2I2 (CD2I2), and C2H5I (C2D5I). These are the same species that were detected in the gas-phase after high fluence irradiation (g10mJ/cm2).23 Without UV irradiation, only the parent molecule, CH3I (CD3I), is detected in TPD.23 It should be noted that at a fluence of 600 µJ/cm2, only traces of CH4 (CD4) and I2 are detected in the gas phase during laser irradiation. The product yields in PITPD scaled roughly with the initial coverage of CH3I up to ∼40 ML. In PITPD of 1 ML CH3I/TiO2(110), the only detectable products were I2 and traces of CH2I2 and C2H5I. Figure 1a shows the PITPD profiles of CD4, C2D6, and undissociated CD3I after irradiation of a 20 ML coverage of CD3I on TiO2(110). TPD experiments without irradiation demonstrate that some CD4 signal results from fragmentation and secondary reactions of CD3I in the ionizer of the mass spectrometer. The intensity of the mass 20 signal due to CD3I fragmentation tracks that of CD3I. However, the desorption profiles of CD4 and C2D6 in PITPD do not follow that of CD3I. The CD4 and C2D6 desorption peaks appear to contain at least two components: a main peak and a low temperature shoulder. This indicates that CD4 and C2D6 are principally photoproducts that are desorbing from the surface, rather than products of secondary reactions taking place in the mass spectrometer. Figure 1b shows the isotope distribution for methane formed from the photolysis of a film composed of an equimolar homogeneous mixture of CH3I and CD3I. Mass 19 and 20 signal intensities resulting from CD3I ionization were determined from TPD without irradiation and (25) Holbert, V. P.; Garrett, S. J.; Bruns, J. C.; Stair, P. C.; Weitz, E. Surf. Sci. 1994, 314, 107.

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Figure 1. (a) PITPD of CD4, C2D6, and CD3I from 20 ML CD3I/ TiO2(110) after irradiation with 7.6 × 1018 photons/cm2 at 600 µJ/cm2 at 257 nm. The heating rate was 5 K/s. Mass 139 (CI+) was monitored instead of mass 145 (CD3I+) for scaling. (b) Isotope distribution in methane peaks at a heating rate of 8 K/s after the 257 nm photolysis of 33 ML (CH3I + CD3I) with 1.6 × 1018 photons/cm2 at 550 µJ/cm2. The contribution from parent molecular ionization has been subtracted.

subtracted, with scaling, from those measured in a PITPD experiment. The data implies that there is no significant kinetic isotope effect for methane formation observed in PITPD. The intensities of CD4 and CD3H signals are almost the same at the low-temperature shoulder. At the peak temperature, the amount of CD3H appears to be larger than that of CD4. However, this difference in relative amounts is more likely to be the result of errors in subtraction of the parent molecule contributions, rather than an actual difference in the yields. Comparison of parts a and b of Figure 1 indicates that the fraction of methane desorption attributable to the low temperature shoulder, at ∼130 K, is significantly decreased at a higher heating rate (8 vs 5 K/s), indicating that conversion from molecules in the low temperature shoulder to the main peak has occurred. Figure 2 shows the PITPD profiles of I-containing products at two different laser fluences, 160 and 550 µJ/ cm2. For the same number of incident photons, the I2, CH2I2, and C2H5I desorption profiles are essentially independent of laser fluence. This result indicates that the yields of I2, CH2I2, and C2H5I in PITPD, after low fluence irradiation, are first order in the photon dose. This would appear to be inconsistent with the previously proposed mechanisms based on the time-of-flight (TOF) detection of products desorbing under high fluence irradiation (g10 mJ/cm2).23 At high fluences, the C2H5I yield is first order in the photon dose (eq 5) but the I2 and CH2I2 yields are second order (eqs 4 and 6). This difference in the fluence dependence of product yields in the high and low fluence regimes will be discussed in section IV in terms of the reaction dynamics. Desorption profiles of I-containing products in PITPD vary with the total photon dose and the initial CH3I coverage. Figure 3 shows PITPD profiles of I2, CH2I2, and C2H5I from a 20 ML coverage of CH3I/TiO2(110) as a function of the 257 nm photon dose. CH2I2 and C2H5I desorb over a broad temperature range (>70 K). Except for the low-temperature peak of C2H5I, the onset of product desorption is above the desorption temperature of the undissociated CH3I (∼145 K). The peak desorption

Kim et al.

Figure 2. PITPD of I2, CH2I2, and C2H5I after the 257 nm photolysis of 15 ML CH3I/TiO2(110) at 90 K with 1.7 × 1018 photons/cm2 at (a) 160 µJ/cm2 and (b) 550 µJ/cm2.

temperatures are ∼200 K for I2, ∼200 and ∼220 K for CH2I2, and ∼150 and ∼180 K for C2H5I. The fraction of low- vs high-temperature desorption for CH2I2 and C2H5I changes with the extent of photolysis. For a constant photon dose, changes in PITPD signals of I-containing products with the initial CH3I coverage (data not shown) also show similar behavior in the temperature and shape of the desorption profiles to that described above. The desorption profiles of I-containing products are very similar to the TPD profiles recorded after direct adsorption of these molecules: (1) The thermal desorption temperature of CH2I2 dosed on TiO2(110) decreases from 250 to 205 K as the exposure increases. (2) For C2H5I on TiO2(110), the desorption temperature decreases from 210 to 175 K as the coverage increases to 1 ML and multilayers desorb at ∼145 K. (3) Dosing I2 on TiO2(110) was not successful in our apparatus due to its low vapor pressure. However, the I2 thermal desorption temperature from TiO2(110) can be estimated from experiments on MgO(100) since both surfaces show almost the same thermal desorption behavior for CH3I.23,24 The I2 desorption temperature from MgO(100) increases from 200 to 220 K with exposure.26 In the insets to Figure 3, the yields of I-containing products in PITPD are plotted as a function of total photon dose at 257 nm. Within experimental error, all three products show a linear increase in their yields with total photon dose. Since the photodissociation of CH3I is a single photon process, the data indicate that the product yields depend on the extent of photolysis in the film. This is consistent with the results of Figure 2, which show PITPD yields for the same photon dose at different fluences. The relative PITPD yields of I-containing products from multilayers of CH3I on TiO2(110), for the same photon dose, at 257 and 320 nm are compared in Figure 4. Note that the PITPD signal intensity of the 257 nm data is about 2 orders of magnitude larger than that of the 320 nm data. The relative yields of I-containing products at 257 and 320 nm are consistent with the ratio of the reported absorption cross-sections of gas-phase CH3I. The gas-phase absorption cross-section of CH3I at 320 nm is (26) Bruns, J. C. Ph.D. Thesis, Northwestern University, 1990.

Photochemistry in CH3I Adlayers on TiO2(110)

Langmuir, Vol. 14, No. 15, 1998 4159

Figure 3. PITPD of (a) I2, (b) CH2I2, and (c) C2H5I after the 257 nm photolysis of 20 ML CH3I/TiO2(110) at 600 µJ/cm2 with 9.6 × 1016, 2.0 × 1017, 3.9 × 1017, and 1.8 × 1018 photons/cm2 (from bottom to top).

atoms from neighboring CH3I molecules in the film, forming methane (eq 1). A simple kinematic model of energy transfer based on momentum conservation in binary collisions predicts that the methyl photofragments will retain energy in excess of the reaction barrier for approximately five collisions. At the same time, thermalization can take place via further collisions with CH3I molecules or the following methyl radical exchange reaction:20

CH3 + CH3I f CH3I + CH3

Figure 4. PITPD of I2, CH2I2, and C2H5I at a photon dose of 1.57 × 1018 photons/cm2. The solid lines are results from 320 nm irradiation at 690 µJ/cm2 of 17 ML CH3I/TiO2(110) and the dotted lines are results from 257 nm irradiation at 550 µJ/cm2 of 15 ML CH3I/TiO2(110). Note that, for comparison, the intensities of the dotted lines have been reduced by 2 orders of maginitude.

∼2 orders of magnitude lower (∼10-20 cm2) than that at 257 nm (∼10-18 cm2).5,6 The difference in peak shape for 257 and 320 nm irradiation is caused by the coverage dependent thermal desorption kinetics for each of the products. The PITPD signals of I2, CH2I2, and C2H5I scale with initial CH3I coverage in a similar manner at both wavelengths. For coverages less than ∼4 ML, the reaction products were not detected in PITPD for 320 nm irradiation, presumably due to their low concentration on the surface. However, for 257 nm irradiation, I2 and traces of CH2I2 and C2H5I were detected even at 1 ML. IV. Discussion A. Photofragment Reaction Dynamics. (a) Methane. Since the average translational energy of nascent CH3 photofragments (∼39 kcal/mol)14 is above the barrier for hydrogen abstraction from CH3I (13 kcal/mol),27 these hot methyl radicals can efficiently abstract hydrogen

(7)

Thermalized methyl radicals in the film cannot abstract hydrogen atoms from neighboring molecules. In Figure 1, no significant kinetic isotope effect is found for the formation of CD4 and CD3H from the photolysis of an equimolar homogeneous mixture of CH3I and CD3I, even though hydrogen abstraction by methyl radicals is an activated process. For the pyrolysis of an equimolar mixture of CH3I and CD3I at 633.5 K, the ratio of initial rates, R(CD3H)/R(CD4), was reported to be about 3.4, which is in agreement with the value expected from the zero point energy difference of C-H(D) bond.28 For a methyl radical collisionally thermalized at the film temperature, the ratio of yields for hydrogen abstraction from a methyl iodide molecule, CD3H/CD4, is calculated to be ∼1300 at 90 K and ∼75 at 150 K. Furthermore, the rate constant for a hydrogen abstraction reaction with a preexponential factor of 1012 cm3 molecule-1 s-1 and an activation energy of 13 kcal/mol is extremely small at these low temperatures.27 In Figure 1, two desorption channels for methane can be distinguished from the effect of the heating rate on the PITPD profiles. The low-temperature (130 K) desorption shoulder in methane PITPD is attributed to a diffusionlimited escape of methane through the methyl iodide film. The attractive forces between the polar methyl iodide and the nonpolar methane are expected to be weaker than between methyl iodide molecules. Annealing the CH3I multilayer film, adsorbed at 90 K, shows that the film starts to melt at ∼120 K.29 If the film structure becomes soft during heating, methane trapped in the film can (27) Kerr, A. J.; Moss, S. J. CRC Handbook of Bimolecular and Termolecular Gas Reactions; CRC Press: Boca Raton, FL, 1982; p 194. (28) Kodama, S.; Ooi, Y. Bull. Chem. Soc. Jpn. 1990, 63, 877. (29) Kim, S. H.; Stair, P. C.; Weitz, E. Manuscript in preparation.

4160 Langmuir, Vol. 14, No. 15, 1998

overcome the barrier to diffusion and migrate to the film surface. For a low heating rate, a greater fraction of trapped methane can desorb via this process before the whole film starts to evaporate. The high temperature (∼145 K) desorption peak corresponds to the desorption of trapped methane which accompanies the desorption of undissociated methyl iodide multilayer. (b) I-Containing Products: I2, CH2I2, and C2H5I. The fluence dependence of the I-containing product yields can provide information about the origins of these products detected by PITPD. If these products were produced during laser irradiation and then trapped in the film, it is expected that, for constant photon dose, the yields of I2 and CH2I2 would be higher at higher fluence, because these products are produced via bimolecular reactions involving two photofragments (eqs 4 and 6), while the yield of C2H5I, produced via a pseudo-first-order reaction, eq 5, should be constant regardless of fluence. However, the results in Figure 2 show constant yields for all the I-containing products at different laser fluences. In addition, the linear increase of the I2, CH2I2, and C2H5I yields with photon dose at constant laser fluence (Figure 3) indicates that the yields of these products in PITPD depend only on the total amount of photofragments (CH3 and I) and intermediate radicals (CH2I) produced on the surface. These results lead to the conclusion that CH3, I, and CH2I are trapped in the frozen film and react with each other as the film melts during the TPD process. Differences in the kinetics of product formation for low fluence PITPD vs high fluence TOF-QMS experiments result from differences in the reaction conditions. High fluence irradiation induces a temperature rise to above the thermal desorption temperature of the CH3I multilayer.30 Therefore, during the high fluence laser pulse, the CH3I film evaporates promptly from the surface. During evaporation, the photofragments are mixed and can react with each other. The product yields in the high fluence regime depend on the concentration of photofragments in the irradiated area during the laser pulse and exhibit a laser fluence dependence. In contrast, during the low fluence laser pulse, the CH3I film remains frozen. Under these conditions, the reaction probabilities for the bimolecular reactions between radicals during irradiation will be small due to limited diffusion in a frozen medium. Thus, CH3(thermalized), I, and CH2I will be trapped in the film. As photolysis of CH3I in the film continues, these trapped radicals accumulate and reactions of these thermal radicals occur during the TPD process. If all radicals in the film react, the yield of products will depend only on the total photon dose and not the laser fluence. It is interesting that the desorption temperatures of the I-containing products are higher than the desorption temperature of undissociated CH3I, and their desorption profiles are consistent with desorption-limited kinetics. This suggests that the radical reactions forming CH2I2, C2H5I, and I2 take place in the temperature range where the CH3I multilayer desorbs. At this temperature, the frozen film melts and the trapped radicals can mix and react with each other. After complete desorption of the CH3I multilayers, these products desorb from the surface at higher temperatures, following desorption-limited kinetics. Thus, we conclude that the CH2I2, C2H5I, and I2 detected in PITPD are produced mainly via the following reactions during desorption of the undissociated CH3I film: (30) Kim, S. H.; Stair, P. C.; Weitz, E. J. Chem. Phys. 1998, 108, 5080.

Kim et al.

CH2I(t) + I(t) f CH2I2(a)

(4′)

CH2I(t) + CH3I f C2H5I(a,g) + I(a)

(5′′)

CH2I(t) + CH3(t) f C2H5I(a,g)

(5′)

I(t) + I(t) f I2(a)

(6′)

Here t, a, and g in parentheses stand for the trapped state in the CH3I film, the adsorbed state on TiO2(110), and the gas phase, respectively. It is interesting to note that eq 5′′ is the reaction of a radical (CH2I) with a closed shell molecule (CH3I), while eqs 4′, 5′, and 6′ are association reactions of radicals. In the gas phase, a reaction analogous to eq 5′′ involves formation of a complex with an overall activation energy that is low or slightly negative.31-34 However, little is known about the energy barrier for product formation from a complex of this type. Though eq 5′ is expected to contribute to the formation of C2H5I, we cannot rule out a contribution from eq 5′′. (c) Ethane. The ethane detected in PITPD can result from both methyl abstraction by hot methyl radicals (eq 3) and the recombination of cold methyl radicals (eq 3′).

CH3(t) + CH3(t) f C2H6

(3′)

Methyl radicals trapped in the film can readily combine to form ethane during the TPD procedure because there is no activation energy for radical recombination. Direct attempts to detect trapped methyl radicals in PITPD, by setting the electron energy of the mass spectrometer ionizer between the ionization potential of methyl radical (9.83 eV)35 and the appearance potential of CH3+ from CH3I (14.5 eV), were not successful. However, this result does not imply that CH3 radicals are not trapped in the CH3I film. First, the trapped CH3 radicals react to form C2H5I and C2H6 during PITPD. Second, the detection sensitivity of the QMS is quite low at low electron impact energy. Thus, we are unable to unambiguously determine the predominant mechanism for ethane formation in the CH3I film. B. Contribution from the TiO2 Substrate. The bulk band gap of the rutile TiO2 has been estimated to be 3.05 eV (410 nm),36,37 and the threshold for photoemission from TiO2(110) occurs at the work function, Φ, of 5.4 eV (225 nm).38,39 Therefore, 257 and 320 nm irradiation of TiO2(110) will produce subvacuum-level electrons in the conduction band and holes in the valence band. These photogenerated electrons can potentially interact with electronegative adsorbate molecules to produce chemistry. The wavelength dependence of the flux of photogenerated electrons at the TiO2(110) surface has been calculated using the reflectivity and absorption coefficient of the (31) Furuyama, S.; Golden, D. M.; Benson, S. W. Int. J. Chem. Kinet. 1969, 1, 283. (32) Furuyama, S.; Golden, D. M.; Benson, S. W. J. Am. Chem. Soc. 1969, 91, 7564. (33) Timonen, R. S.; Seetula, J. A.; Niiranen, J.; Gutman, D. J. Phys. Chem. 1991, 95, 4009. (34) Seetula, J. A.; Gutman, D.; Lightfoot, P. D.; Rayes, M. T.; Senken, S. M. J. Phys. Chem. 1991, 95, 10688. (35) Paisner, J. A.; Solarz, R. W. In Laser Spectroscopy and Its Applications; Radziemski, L. J., Solarz, R. W., Paisner, J. A., Eds; Marcel Dekker: New York, 1987; p 182. (36) Henrich, V. E. Prog. Surf. Sci. 1979, 9, 143; Prog. Surf. Sci. 1983, 14, 175. (37) Zhong, Q.; Vohs, J. M.; Bonnell, D. A. Surf. Sci. 1992, 274, 35. (38) Henrich, V. E.; Dresselhaus, G.; Zeiger, H. J. Phys. Rev. Lett. 1976, 36, 1335. (39) Onishi, H.; Aruga, T.; Egawa, C.; Iwasawa, Y. Surf. Sci. 1988, 199, 54.

Photochemistry in CH3I Adlayers on TiO2(110)

substrate reported in ref 40. The flux of energetic electrons at the surface, generated within 1 nm of the surface, decreases by ∼40% from 257 to 320 nm. However, the relative yields of I-containing products at 257 and 320 nm (Figure 4) differ by 2 orders of magnitude and are consistent with the ratio of the reported absorption cross sections for gas-phase CH3I. A previous study, using resonance-enhanced multiphoton ionization to detect methyl radicals, has also shown that the dissociation of CH3I on TiO2(110) is dominated by the direct optical excitation of CH3I.40 Thus, we conclude that photoexcited electrons in the TiO2(110) substrate produced by UV irradiation do not significantly induce dissociation of CH3I. Substrate excitation by UV irradiation has been reported to induce dissociation of methyl halides and their analogues adsorbed on metal (Ni, Pt, Ag) or semiconductor (GaAs) surfaces.41-50 Thus, the absence of a significant dissociation channel for CH3I on defect-free TiO2(110) via attachment of subvacuum-level electrons originating in the substrate is an unexpected result. In the previous study for CH3Cl on TiO2(110) with 257 and 320 nm irradiation, we observed no dissociation or desorption of CH3Cl on our defect-free TiO2(110) sample, which was attributed to the absence of a direct electronic interaction between the photoexcited TiO2(110) and the electronic states of CH3Cl.30 Yates and co-workers also found that, without molecular oxygen chemisorbed on defect sites, there was no dissociation of CH3Cl on TiO2(110) from UV irradiation of the substrate at wavelengths that can produce subvacuum-level electrons.51,52 The oxygen chemisorbed on defect sites appears to open up a new dissociation channel for CH3Cl on TiO2(110). In the case of CH3I on TiO2(110), however, CH3I does interact directly with photogenerated electrons in TiO2(110). The previous study using a 1 ML film of CH3I has shown that resonant interactions between the electronic states of CH3I and subvacuum-level electrons in TiO2(110), photogenerated upon 257 and 320 nm irradiation, induce desorption of intact CH3I.30 For CH3I on TiO2(110), substrate-mediated desorption appears to be more efficient than substratemediated dissociation during 257 and 320 nm irradiation. These results indicate that the interactions between CH3I and TiO2(110) are different from the adsorbate-substrate interactions observed in studies of other methyl halides on Ni, Pt, Ag, and GaAs.41-50 (40) Garrett, S. J.; Holbert, V. P.; Stair, P. C.; Weitz, E. J. Chem. Phys. 1994, 100, 4626. (41) Gilton, T. L.; Pehnbostel, C. P.; Cowin, J. P. J. Chem. Phys. 1989, 91, 1937. (42) Marsh, E. P.; Tabares, F. L.; Schneider, M. R.; Gilton, T. L.; Meier, W.; Cowin, J. P. J. Chem. Phys. 1990, 92, 2004. (43) Jo, S. K.; White, J. M. Surf. Sci. 1991, 255, 321. (44) Sun, Z.-J.; Schwaner, A. L.; White, J. M. J. Chem. Phys. 1995, 103, 4279. (45) Dixon-Warren, St. J.; Jensen, E. T.; Polanyi, J. C. Phys. Rev. Lett. 1991, 67, 2395. (46) Jensen, E. T.; Polanyi, J. C. J. Phys. Chem. 1993, 97, 2257. (47) Dixon-Warren, St. J.; Jensen, E. T.; Polanyi, J. C. J. Chem. Phys. 1993, 98, 5938. (48) Ukraintsev, V. A.; Long, T. J.; Harrison, I. J. Chem. Phys. 1992, 96, 3957. (49) Yang, Q.; Schwarz, W. N.; Osgood, R. M., Jr. J. Chem. Phys. 1993, 98, 10085. (50) Yang, Q.; Schwarz, W. N.; Lasky, P. J.; Hood, S. C.; Loo, N. L.; Osgood, R. M., Jr. Phys. Rev. Lett. 1994, 72, 3068. (51) Wong, J. C. S.; Linsebigler, A.; Lu, G.; Fan, J.; Yates, J. T., Jr. J. Phys. Chem. 1995, 99, 335. (52) Lu, G.; Linsebigler, A.; Yates, J. T., Jr. J. Phys. Chem. 1995, 99, 7626.

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We speculate that the absence of TiO2-mediated CH3I dissociation may be related to the nature of the vertical electron affinity level of CH3I and weak interactions of atomic iodine with TiO2(110). In the gas phase, the vertical electron affinity level of CH3I in the Franck-Condon region is less repulsive than those of CH3Br and CH3Cl.53-55 Thus, the time scale for dissociation of CH3I- would be slower than CH3Br- and CH3Cl-, increasing the probability of quenching by the substrate or neighboring molecules.56 In addition, the energetics for dissociation is less favorable for TiO2(110) compared to the metal and GaAs surfaces due to a much weaker halide-surface interaction. On metallic surfaces, halogen atoms are chemisorbed so strongly that they desorb as metal halides (Cl and Br) or atomically (I) at very high temperatures (>800 K).21,42,48 On the GaAs surface, halogen atoms (Cl and Br) form halide compounds with the surface gallium atoms and desorb as gallium halides at a temperature of ∼600 K (surface etching).49,57 By comparison, the iodine atoms on TiO2(110) are bound very weakly and desorb as I2 at ∼200 K. Thus, after electron attachment to CH3I, the driving force for a transiently anionic CH3I species to dissociate the C-I bond will be much smaller on TiO2(110) than on metal or GaAs surfaces. V. Conclusions PITPD was used to study the formation and desorption of reaction products in a CH3I film adsorbed on TiO2(110) after low fluence UV laser irradiation. The isotope (H/D) distribution for methane detected in PITPD clearly shows that the methane is a reaction product of hot methyl radicals. This reaction occurs before the CH3 photofragment loses the excess kinetic energy obtained in the photodissociation process. Thermal desorption of methane from the film depends on the heating rate, indicative of a diffusion-limited process. The yields of all the Icontaining products in PITPD depend on the total concentration of photofragments and intermediates trapped in the film, implying that the I2, CH2I2, and C2H5I detected in PITPD are produced predominantly via reactions of trapped radicals. These reactions occur upon heating of the film. This is in contrast to the reaction dynamics occurring after high fluence irradiation of a CH3I film where reactions take place during or immediately following the laser pulse. Under low fluence conditions, thermal desorption of the I-containing products is desorption-limited and not reaction-limited. Whether ethane, observed in PITPD, is mainly the result of hot methyl radical reactions or the recombination of cold radicals could not be determined from the present data. The wavelength dependence of product yields from the CH3I adlayers reveals that substrate excitation does not significantly induce dissociation of the C-I bond in CH3I on the defectfree TiO2(110) surface. Acknowledgment. The authors gratefully acknowledge support of this work by the National Science Foundation under Grant No. CHE 92-92523. LA980218N (53) Moutinho, A. M.; Aten, J. A.; Los, J. Chem. Phys. 1974, 5, 84. (54) Tang, S. Y.; Mathur, B. P.; Rothe, E. W.; Reck, G. P. J. Chem. Phys. 1976, 64, 1270. (55) Burrow, P. D.; Modelli, A.; Chiu, N. S.; Jordan, K. D. J. Chem. Phys. 1982, 77, 2699. (56) Roop, B.; Lloyd, K. G.; Costello, S. A.; Campion, A.; White, J. M. J. Chem. Phys. 1989, 91, 5103. (57) Cha, C. Y.; Weaver, J. H. J. Vac. Sci. Technol. 1996, B14, 3559.