257 nm photoinduced chemistry of methyl iodide adsorbed on MgO(100)

The 257 nm photochemistry of CD₃I adsorbed on MgO(100) has been investigated. The photofragments ejected along the surface normal were probed using resonantly enhanced multiphoton ionization spectroscopy coupled with time-of-flight mass spectrometry. Methyl radicals and atomic iodine fragments in both the ground I(²P_3/2)≡I and spin-orbit excited I(²P_1/2)≡I* states were observed as direct photofragments produced at the surface. Laser-induced desorption of methyl iodide competes with dissociation at the surface. The total cross section for removal of adsorbed CD₃I from the surface at 257 nm is comparable to the gas phase cross section. A significant portion of the atomic iodine fragments remain trapped at the surface with preferential trapping of I*. Additionally, nonresonant I⁺ and I₂⁺ signals were detected and initially observed to increase as a function of the irradiation time and then decrease with further irradiation. This behavior is shown to be indicative of photoinduced production and photoinduced removal of a surface intermediate. A surface reaction mechanism and the existence of a common precursor resulting from the interaction of adsorbed atomic iodine with adsorbed methyl iodide or methyl iodide clusters is postulated.