WAVELENGTH DEPENDENCE OF THE PHOTODISSOCIATION AND PHOTODESORPTION OFCD3I 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 irrad iation followed by resonantly enhanced multiphoton ionization of fragm ents expelled from the adsorbate layer. Photodissociation at these wav elengths produces CD3 radicals. Nonthermal photodesorption also contri butes to removal of CD3I from the adsorbate layer, becoming a major me chanism at 351 nm. Similar processes are observed at both 1 and 25 mon olayer (ML) coverages. The cross section for CD3I depletion from the m onolayer is qualitatively similar to the gas phase CD3I absorption pro file, decreasing by similar to 3 orders of magnitude between 257 and 3 51 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(-2 1) cm(-2) for 257, 275, 302, and cm 351 nm irradiation, respectively. The depletion cross section for 25 ML CD3I coverage is approximately a n order of magnitude less than for 1 ML coverage with S(lambda) calcul ated to be 3+/-2X10(-19) cm(-2), 1.5+/-0.7X10(-19) cm(-2), 1.5+/-0.7X1 0(-20) cm(-2), and 2+/-0.8X10(-22) cm(-2) for 257, 275, 302, and 351 n m radiation, respectively. We find no correlation between substrate ab sorption and the wavelength dependence of photodissociation or photode sorption suggesting that direct excitation of the adsorbate molecule i s the dominant dissociation mechanism. The lack of substrate involveme nt may be due to poor coupling of the CD3I adsorbate and TiO2 substrat e electronic structures.