Solvation Ultrafast Dynamics of Reactions. 11. Dissociation and Caging Dynamics in the Gas-to-Liquid Transition Region
In this paper we give a full account of the work presented in earlier communications [Lienau et al. Chem. Phys. Lett. 1993, 213, 289; 1994, 218, 224; J. Chim. Phys. 1995, 92, 566]. With femtosecond time resolution, studies are presented of the dynamics in real time of an elementary chemical reaction, the dissociation and recombination of iodine in supercritical rare-gas solvents, in the gas-to-liquid transition region. Through pressure variation, the properties of the solvent, helium, neon, argon, or krypton, are changed from those of an essentially ideal gas at low densities to those of a liquidlike fluid at pressures of several thousand bar. Of particular interest here are (i) the impact of solute−solvent interactions on the coherence of the wave packet nuclear motion, (ii) the collision-induced predissociation of the B state, and (iii) the geminate recombination of the atomic fragments and the subsequent vibrational energy relaxation within the A/A' states. In helium and neon, the coherence of the vibrational motion persists for many picoseconds, even at pressures of 2000 bar. For pressures between 100 and 2000 bar of helium and neon, the dephasing rate is only weakly affected by the solvent density. In all solvents, the solvent-induced predissociation rate increases nearly linearly with solvent density. In argon at 2500 bar, the predissociation rate reaches 1.05 ps^(-1). Relative geminate recombination yields for the formation of new A/A' state iodine molecules and the time scale for the geminate recombination and the subsequent A/A' state vibrational relaxation dynamics are also studied. The solvation and chemical dynamics are examined, using simple analytical models, in relation to the solvent density and polarizability. With the help of molecular dynamics, detailed in the accompanying paper, we present a microscopic picture of the elementary processes under the free and solvation conditions encompassing the different density regimes in the gas-to-liquid transition region.