Wavelength dependence of the photodissociation and photodesorption of CD3I adsorbed on the TiO2(110) surface

Abstract

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.

A surface science perspective on TiO2 photocatalysis

“…This author observed the ejection of methyl radicals from the surface of R TiO 2 (110) during UHV photodecomposition of adsorbed acetone [701,752,753]. Similar observations have been made by Stair and coworkers for photodissociation of methyl iodide on this surface [680,754,755]. The methyl radicals emitted from adsorbed acetone were shown to further decompose on the chamber walls to yield formaldehyde.…”

“…An example of this is seen in the photochemistry of alkyl halides on R TiO 2 (110). Stair, Weitz and coworkers [680][681][682][683] have shown that bandgap excitation of TiO 2 leads to electron attachment to these adsorbed molecules, but does not result appreciable photodecomposition (as seen in the gas phase). Instead, rapid back-electron transfer to the surface occurs followed by photodesorption of the alkyl halide via the so-called Antoniewicz mechanism [684].…”

“…Although discussed in detail below, O 2 photodesorption is believed to result from the interaction of a VB hole with an adsorbed O δ− 2 species, resulting in a neutral O 2 molecule that readily desorbs from the surface. Other examples of molecular photodesorption from TiO 2 surfaces include alkyl halides [680][681][682]754,755,757], CO [783], N 2 O [705] and triethylamine [784]. The details of these events differ, but in each case the photodesorption event involves a charge carrier causing a charge change in the adsorbate-surface relationship that results in the adsorbate transitioning to a different potential energy surface (e.g., ion to neutral) that is more amenable to desorption.…”

“…The most extensive work involving photochemical processes of brominated or iodinated hydrocarbons on a single crystal TiO 2 surface was done by the Stair, Weitz and coworkers [680][681][682][683]754,755]. These authors examined the photochemistry of methyl iodide and methyl bromide on R TiO 2 (110) using 257 and 320 nm laser light.…”

The Adsorption, Thermal Desorption and Photochemistry of Methyl Iodide on an Ag‐Covered TiO2(110) Surface

“…CH 3 I is chosen for thermal and photochemistry studies at adsorbate/surface interfaces due to the weak C-I bond and its high photosensitivity. The adsorption and reaction of methyl iodide on the TiO 2 (110) surface have been extensively studied by Kim et al [11][12][13][14] and Garrett et al 15,16 The complex desorption TPD spectra suggest the coexisting of several distinct coverage regimes. The dissociatively (< 5%) adsorbed I atoms remain chemisorbed at 220 K. For photochemistry, it was found that the photodesorption of CD 3 I follows the substrate mediated mechanism in the monolayer regime.…”