WAVELENGTH DEPENDENCE OF THE PHOTODISSOCIATION AND PHOTODESORPTION OF CD3I ADSORBED ON THE TIO2(110) SURFACE

The ultraviolet photodissociation and photodesorption of CD3I adsorbed on the TiO2(110) surface at similar to 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 similar to 3 orders of magnitude between 257 and 351 nm. Depletion cross sections, S(lambda), for CD3I are 3+/-2X10(-18) cm(-2), 8+/-3X10(-19) cm(-2), 1+/-0.5X10(-19) cm(-2), and 3+/-1X10(-21) cm(-2) for 257, 275, 302, and cm 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(lambda) calculated to be 3+/-2X10(-19) cm(-2), 1.5+/-0.7X10(-19) cm(-2), 1.5+/-0.7X10(-20) cm(-2), and 2+/-0.8X10(-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.