THE ADSORPTION AND PHOTOCHEMISTRY OF CD3I ON TIO2(110)

The adsorption and photochemistry of CD3I adsorbed on TiO2(110) at similar to 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) X 10(14) molecules cm(-2) and shows only one desorption peak, although this is dread 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 (3d(5/2))/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 similar to 20% of the initial I atoms after irradiation at 254 nm. The photodissociation cross section, sigma, of adsorbed CD3 I on TiO2(110) at 254 and 334 nm is calculated to be similar to 1.1X10(-18) cm(2) and similar to 1.3 X 10(-20) cm(2), respectively. At 254 nm, the adsorbate and gas phase sigma are similar suggesting cm 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.