Microscopic models for iodine photodissociation quantum yields in dense fluids

Abstract

Experimental measurements of the photodissociation quantum yield for iodine in a variety of solvents show that the quantum yield becomes less than unity at moderate densities (∼0.01 of liquid densities) and that as liquid densities are reached, the quantum yield drops quite dramatically with increasing density. We examine two models for the iodine photodissociation quantum yields. The first was proposed by Otto, Schroeder, and Troe [J. Chem. Phys. 81, 202 (1984)] to explain the drop in quantum yield at moderate densities. It assumes the formation of van der Waals complexes with the solvent which, when excited can fragment to produce iodine in a distribution of vibrational states in the ground electronic state. We calculate the concentrations of van der Waals complexes with ethane and confirm that there are large enough concentrations to explain the experimental results. The second model is developed to explain the steep drop in quantum yield at high densities. It is based on solvent caging with trapped pairs of iodine atoms recombining to form iodine molecules. Calculations based on this model agree well with experimental results. We discuss the implications that the van der Waals model has on the interpretation of molecular beam experiments involving iodine van der Waals complexes. At high densities both mechanisms (the moderate density mechanism and the caging mechanism) must be superimposed even though it is the caging mechanism which leads to the stronger density dependence. The realization that two distinct pathways exist for returning the excited iodine to its ground state significantly clarifies the interpretation of picosecond experiments examining iodine photochemistry in liquids.