The photochemistry of CD₃I adsorbed on the TiO₂(110) surface

The photochemistry of CD₃I adsorbed on the TiO₂(110) surface at ∼100 K has been investigated at 257, 275, and 302 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 methyl radicals by direct photolysis of those methyl iodide adsorbate molecules located at the vacuum-surface interface. The photodissociation dynamics is altered from the gas phase. Methyl radicals with velocities comparable to those observed in gas phase dissociation are detected with a narrow angular distribution consistent with a preferential alignment of the C-I bond along the surface normal. The I/I* branching ratio of these high velocity species is altered toward production of ground state iodine compared to the gas phase analogue. Collisionally slowed fragments are also observed with a much broader angular distribution. The variation in photofragment intensity with adsorbate coverage is consistent with an overlayer that grows by a statistical or stochastic process. Coverage dependent velocity distribution measurements indicate the presence of multiple adsorption phases.