Kinetic energy analysis of (HI)n cluster photofragments

We have employed a time-of-flight analysis to determine the velocity and spatial distribution of photofragments resulting from excitation of (HI)n cluster species at approximately 240 nm. The formation of clusters in the supersonic expansion broadens the I+ flight-time distribution and destroys the spatial anisotropy characteristic of HI monomer photodissociation, indicating that these fragments experience strong solvent cage effects when exiting the cluster environment. In addition, a high velocity component with an anisotropic distribution appears and is due to the photodissociation of I2, a product of cluster chemistry. Even with extensive clustering, the main features of the H+ flight-time spectrum do not change and cage effects are not as pervasive as for the I atom fragments. However, there is also a broad H+ component peaked near zero velocity and exhibiting an isotropic spatial distribution. Inelastic H atom collisions involving excitation of internal modes in HI cage species likely play a role in dissipating the excess photolysis energy and producing the low velocity distribution. Other pathways involving the reactive nature of H+HI collisions may be more important. Wavelength resolved spectra of the I+ flight-time features provide further insight into the origins of the observed behavior.