Cage effects and steric hindrance in van der Waals solids, with application to alkyl iodide photolysis in rare gas hosts

Abstract

Molecular luminescence in the near ir is produced by 259–290 nm pulsed ’’photodissociation’’ of CH3I, CD3I, and perfluoroalkyl iodide guests in rare gas hosts at 4.2 °K. For CD3I and CH3I in Ne, the 15 member progression observed is assigned as fluorescence into high lying (near v″=37) vibrational states of the methyl–iodide stretch in the ground electronic state. Photoselection studies confirm the transition dipole lies along the methyl–iodine bond. The photodissociation cage effect is near complete, with only a minor quantum yield of permanent dissociation. Detailed analysis of the spectra suggests that the 0+ ’’repulsive’’ excited state is actually slightly (<2000 cm−1) chemically bound, and that the ground state potential is negligibly perturbed from its free molecule shape. A simple semiquantitative theory of the cage effect is proposed. In the impulsive limit, the excited state dynamics reflect a strong cage effect. However, in the adiabatic limit, the absorption and emission spectra reflect only a weak, van der Waals cage effect. The theory is applied to known examples of photodissociation in solids, and to rearrangements of larger molecules. A cage potential experiment of Schnepp and Dressler is reinterpreted.