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 resolut ion, studies are presented of the dynamics in real time of an elementa ry chemical reaction, the dissociation and recombination of iodine in supercritical rare-gas solvents, in the gas-to-liquid transition regio n. Through pressure variation, the properties of the solvent, helium, neon, argon, or krypton, are changed from those of an essentially idea l gas at low densities to those of a liquidlike fluid at pressures of several thousand bar. Of particular interest here are (i) the impact o f solute-solvent interactions on the coherence of the wave packet nucl ear 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 he lium and neon, the coherence of the vibrational motion persists for ma ny picoseconds, even at pressures of 2000 bar. For pressures between 1 00 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. I n argon at 2500 bar, the predissociation rate reaches 1.05 ps(-1). Rel ative geminate recombination yields for the formation of new A/A' stat e iodine molecules and the time scale for the geminate recombination a nd the subsequent A/A' state vibrational relaxation dynamics are also studied. The solvation and chemical dynamics are examined, using simpl e analytical models, in relation to the solvent density and polarizabi lity. With the help of molecular dynamics, detailed in the accompanyin g paper, we present a microscopic picture of the elementary processes under the free and solvation conditions encompassing the different den sity regimes in the gas-to-liquid transition region.