Role of adsorbate structure in the dynamics of surface photodissociation

Photofragments produced following 257 nm irradiation of physiosorbed methyl iodide on MgO(100) and TiO2(110) have been studied using resonantly enhanced multiphoton ionization coupled with time-of-flight mass spectrometry. The observed signal is associated with photofragment production from methyl iodide chromophores present in the near surface region. The dependence of the velocity distribution of methyl photofragments on ejection angle and polarization of the photolysis beam indicates that the multilayer film is composed primarily of an anti-parallel arrangement of methyl iodide molecules whose C-I bonds are aligned close to the surface normal. At submonolayer coverage on TiO2 methyl iodide is predominantly in a parallel orientation with the iodine close to the surface and the methyl group pointing away from the surface.