Photodissociation of methyl iodide adsorbed on low-temperature amorphous ice surfaces.

Photodissociation dynamics of methyl iodide (CH3I) adsorbed on both amorphous solid water (ASW) and porous amorphous solid water (PASW) has been investigated. The ejected ground-state I(2P3/2) and excited-state I(2P1/2) photofragments produced by 260- and 290-nm photons were detected using laser resonance-enhanced multiphoton ionization. In contrast to gas-phase photodissociation, (i) the I(2P3/2) photofragment is favored compared to I(2P1/2) at both wavelengths, (ii) I(2P3/2) and I(2P1/2) have velocity distributions that depend upon ice morphology, and (iii) I2 is produced on ASW. The total iodine [I(2P3/2)+I(2P1/2)+I2] yield varies with substrate morphology, with greater yield from ASW than PASW using both 260- and 290-nm photons. Temperature-programmed desorption studies demonstrate that ice porosity enhances the trapping of adsorbed CH3I, while pore-free ice likely allows monomer adsorption and the formation of two-dimensional CH3I clusters. Reactions or collisions involving these clusters, I atomic fragments, or I-containing molecular fragments at the vacuum-surface interface can result in I2 formation. [ABSTRACT FROM AUTHOR]