Photofragmentation of mass-selected ICl−(CO2)n cluster ions: Solvation effects on the structure and dynamics of the ionic chromophore

Photofragmentation studies at 644 nm and 740 nm of ICl−(CO2)n cluster ions (n=0–8) have been carried out in a tandem time-of-flight mass spectrometer. Photodissociation of these cluster ions at a wavelength at which bare ICl− produces only I− results in the formation of three classes of fragment ions: I−, Cl−, and ICl− based clusters. The I− based clusters correspond to the direct photoproduct in which a Cl atom has escaped the cluster ion. The ICl− and Cl− based clusters are a result of a nonadiabatic electronic transition to the ground state mediated by the solvent. The relative importance of these photofragment channels strongly depends on the cluster ion size. An ICl− caged product is first observed for ICl−(CO2)2, increasing rapidly to a maximum at n≊6 and then decreasing. This caging efficiency is dramatically different from the I−2(CO2)n cluster ions where complete caging was observed for 16 solvent molecules. The Cl− photofragment channel increases smoothly for the cluster size range studied and becomes the dominant channel for n=8. The relative yields of the ICl− and Cl− based products reflect the extent to which solvation influences the photodissociation pathways of ICl−.