The adsorption and photochemistry of CD3I adsorbed on TiO2(110) at ∼110 K has been studied by means of temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS). Complex desorption behavior is observed in TPD suggesting the presence of several distinct coverage regimes. At submonolayer coverages there are two coexisting phases: one dominated by adsorbate–adsorbate interaction, the other dominated by adsorbate–substrate interactions. The first completed monolayer corresponds to (3.8±0.3)×1014 molecules cm−2 and shows only one desorption peak, although this is broad and extends asymmetrically to high temperature indicative of a changing desorption activation energy. With increasing coverage, a discrete, less tightly bound second layer is formed which slowly rearranges to produce three-dimensional clusters of methyl iodide, as indicated by a sharp reduction in the I (3d5/2)/Ti(2p) XPS intensity ratio. There is evidence that this rearrangement does not involve the first monolayer. Irradiation of 1 ML CD3I by 254 or 334 nm photons causes cleavage of the C–I bond and expulsion of I and C containing species into the vacuum. There is retention of ∼20% of the initial I atoms after irradiation at 254 nm. The photodissociation cross section, σ, of adsorbed CD3 I on TiO2(110) at 254 and 334 nm is calculated to be ∼1.1×10−18 cm2 and ∼1.3 ×10−20 cm2, respectively. At 254 nm, the adsorbate and gas phase σ are similar suggesting photodissociation is dominated by adsorbate excitation, but at 334 nm the adsorbate photodissociation cross section is almost an order of magnitude larger than its gas phase counterpart. This suggests that a second photoexcitation mechanism may be contributing to adsorbate photodissociation, possibly involving photogenerated substrate carriers.
The ultraviolet photodissociation and photodesorption of CD3I adsorbed on the TiO2(110) surface at ∼100 K has been investigated at 257, 275, 302, and 351 nm using modulated continuous-wave laser irradiation followed by resonantly enhanced multiphoton ionization of fragments expelled from the adsorbate layer. Photodissociation at these wavelengths produces CD3 radicals. Nonthermal photodesorption also contributes to removal of CD3I from the adsorbate layer, becoming a major mechanism at 351 nm. Similar processes are observed at both 1 and 25 monolayer (ML) coverages. The cross section for CD3I depletion from the monolayer is qualitatively similar to the gas phase CD3I absorption profile, decreasing by ∼3 orders of magnitude between 257 and 351 nm. Depletion cross sections, S(λ), for CD3I are 3±2×10−18 cm−2, 8±3×10−19 cm−2, 1±0.5×10−19 cm−2, and 3±1×10−21 cm−2 for 257, 275, 302, and 351 nm irradiation, respectively. The depletion cross section for 25 ML CD3I coverage is approximately an order of magnitude less than for 1 ML coverage with S(λ) calculated to be 3±2×10−19 cm−2, 1.5±0.7×10−19 cm−2, 1.5±0.7×10−20 cm−2, and 2±0.8×10−22 cm−2 for 257, 275, 302, and 351 nm radiation, respectively. We find no correlation between substrate absorption and the wavelength dependence of photodissociation or photodesorption suggesting that direct excitation of the adsorbate molecule is the dominant dissociation mechanism. The lack of substrate involvement may be due to poor coupling of the CD3I adsorbate and TiO2 substrate electronic structures.
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