Solvation Ultrafast Dynamics of Reactions. 11. Dissociation and Caging Dynamics in the Gas-to-Liquid Transition Region

Abstract

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.

Solvation Ultrafast Dynamics of Reactions. 13. Theoretical and Experimental Studies of Wave Packet Reaction Coherence and Its Density Dependence

“…4 Iodine crystals were introduced to the high-pressure cell, and the cell was evacuated to pressures below 10 -3 Torr and then refilled with the pure rare gas (He, Ne, or Ar). The rare-gas pressure was varied between 0 and 4000 bar.…”

Quantum Yields for the Photodissociation of Iodine in Compressed Liquids and Supercritical Fluids

“…According to these and later studies [14][15][16] the dynamics is characterised by a variety of competing processes summarised in Fig. 1.…”

Solvation Ultrafast Dynamics of Reactions. 12. Probing along the Reaction Coordinate and Dynamics in Supercritical Argon

“…In the preceding paper, 1 we presented studies of the reaction dynamics of iodine in rare-gas solvents: helium, neon, argon, and krypton. Of particular interest was the dynamics of bond breakage, wave packet dephasing, and changes in the potential energy surfaces as the liquid density regime was reached.…”