State specific, angle velocity resolved measurements of photodissociation in clusters: I atom escape from (CH3I)n (n = 1,2,3) Original Research Article

The photodissociation of molecules in clusters affords the opportunity to study collisional relaxation of translational and rovibrational energy under solvent-like conditions. We have measured the angle velocity distribution of I(2P32) and I(2P12) from photodissociation of (CH3I)n clusters at 266 and 304 nm. For monomer (n = 1), the angular distributions are very anisotropic and the translational energy is sharply peaked at high energy. For clusters (predominantly n = 2,3), the dissociated I atom undergoes collisions, which substantially relax the translational energy and anisotropy. Three observations are reported: (1) A residual anisotropy exists in cluster photodissociation that increases with increasing translational energy. This observation is believed to reflect the correlation of anisotropy and energy loss due to the impact parameter and number of collisions of the photofragment in the cluster. Anisotropy versus kinetic energy plots maybe interpreted to give collisional relaxation rates in clusters. (2) The I atom 2P32 to 2P12 branching ratio increases significantly in clusters relative to the monomer. Possible explanations are discussed including increased adiabaticity through the curve-crossing region in dissociation due to cage effects in analogy with reported surface results. (3) TOF spectra were observed for I atoms originating from photodissociation of the cluster photoproduct 12. The recorded spectra are consistent with an 12 energy content that is much less than the initially excited (CH3I)n cluster.