DYNAMICS OF PHOTODISSOCIATION OF CH3I/AG(111) AT 248-NM

The photodissociation of CH3I/Ag(111) at 248 nm has been studied using time-of-flight mass spectrometry to detect the CH3 photofragments. At submonolayer CH3I coverage, the CH3I photodissociation was quenched by interactions with the metal substrate; at higher coverages (I < theta < 20 ML) fast CH3 photofragments were observed. The total yield of these CH3 photofragments varied as a function of coverage from zero (<1 ML), rising to a substantial value between 1 and 3 ML, and then remaining constant for higher coverages. In addition to direct neutral photolysis of CH3I/Ag(111), charge-transfer photodissociation of CH3I by photoelectrons from the substrate was observed. The photodissociation dynamics of CH3I/Ag(111) for direct photodissociation was found to be perturbed from that of gas-phase CH3I due to interactions between the dissociating chromophore and adjacent CH3I molecules. The branching ratio I*/(I+I*) was approximately 0.40 for films greater-than-or-equal-to 10 ML thick, as determined from the bimodal translational energy distribution of the CH3 fragment.