Solvation ultrafast dynamics of reactions: V dissociation and atom recombination of iodine in the gas-to-liquid transition region

The femtosecond solvation dynamics of an elementary chemical reaction in the gas-to-liquid transition region is examined. The reaction is that of an iodine molecule dissociating and recombining in compressed supercritical rare gases, helium, neon, argon and krypton at pressures between 0 and 2500 bar. This study focuses on four elementary steps in the dissociation and recombination of iodine in solution: coherence of В state vibrational wave packet motion, solvent-induced В state predissociation, geminate recombination onto excited electronic states and subsequent vibrational relaxation. In helium and neon, vibrational coherence persists for more than 3 ps even at the highest pressures which indicates the importance of vibrational relaxation for the dephasing process. The В state predissociation rate increases with solvent density as suggested by Isolated Binary Collision models and reaches a value of (0.9 ps)-1 in argon at 2500 bar. Geminate recombination in argon at 1600 bar occurs within 2 ps, similar to liquid solvents, while vibrational relaxation into a thermalized A/A' state distribution is slower, occurring at the rate of (13 ps)-1. The geminate recombination yields are strongly dependent on the size of the solvent molecules and the overall pressure dependence of the yields is in agreement with predictions from a diffusionbased theoretical model. The present study exemplifies how variation of the solvent pressure affects the dynamics of an elementary chemical reaction and presents a unique way of studying the effect of the solvent structure on reaction dynamics in solution. We present a simple model which accounts for these phenomena in different phases: supercritical fluids, liquids, clusters and matrices