THE ADSORPTION AND PHOTOCHEMISTRY OF CD3I ON TIO2(110)

The adsorption and photochemistry of CD3I adsorbed on TiO2(110) at sim ilar to 110 K has been studied by means of temperature programmed deso rption (TPD) and x-ray photoelectron spectroscopy (XPS). Complex desor ption behavior is observed in TPD suggesting the presence of several d istinct coverage regimes. At submonolayer coverages there are two coex isting phases: one dominated by adsorbate-adsorbate interaction, the o ther dominated by adsorbate-substrate interactions. The first complete d monolayer corresponds to (3.8+/-0.3) X 10(14) molecules cm(-2) and s hows only one desorption peak, although this is dread and extends asym metrically to high temperature indicative of a changing desorption act ivation energy. With increasing coverage, a discrete, less tightly bou nd second layer is formed which slowly rearranges to produce three-dim ensional 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 o f 1 ML CD3I by 254 or 334 nm photons causes cleavage of the C-I bond a nd expulsion of I and C containing species into the vacuum. There is r etention 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 photodis sociation is dominated by adsorbate excitation, but at 334 nm the adso rbate photodissociation cross section is almost an order of magnitude larger than its gas phase counterpart. This suggests that a second pho toexcitation mechanism may be contributing to adsorbate photodissociat ion, possibly involving photogenerated substrate carriers.