Alkyl halide photochemistry on Ag(111)

The surface photochemistry of methyl iodide (CH3I) on Ag(111) has been studied using X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). While adsorbed CH3I does not dissociate thermally at 100 K., it is easily photodissociated. Upon the CI bond cleavage, all I and a small portion of ch3 are retained at the surface; most CH3 desorbs during UV irradiation. There is no photodesorption of molecular CH3I. The photodissociation threshold is ∼ 2.6 eV for chemisorbed monolayer CH3I and ∼ 3.5 eV for physisorbed multilayer CH3I. Substrate quenching of the photodissociation is observed for the chemisorbed ch3i. Comparisons with the photochemistry of C2H5Cl and CH3Br on Ag(111) are made.