SOLVATION ULTRAFAST DYNAMICS OF REACTIONS .13. THEORETICAL AND EXPERIMENTAL STUDIES OF WAVE-PACKET REACTION COHERENCE AND ITS DENSITY-DEPENDENCE

In this paper of the series, which gives the full account of an earlie r communication [Liu et al. J. Chem. Phys. 1996, 105, 5294], the react ion wave packet coherence of iodine in rare-gas fluids is studied, bot h experimentally and theoretically, in the gas-to-liquid transition re gion. The phase coherence dynamics, which exhibit striking density beh avior, are resolved in real time and across a broad density range (0-3 5 nm(-3)) for three different rare gases: helium, neon, and argon. In the solvent, the wave packet reaction coherence is observed on time sc ales longer than that of the solute vibrational motion, and the loss o f coherence is due to solvent collision-induced predissociation and co llision-induced dephasing. By means of molecular dynamic simulations, the experimental results are reproduced by classical and semiclassical treatments. The centrifugal force, by vibration-rotation coupling, is found robe dominant at low and intermediate densities. A motion narro wing effect causes the contribution of vibration-rotation coupling to dephasing to decrease at high densities. Direct contributions from the solute-solvent fords increase with the solvent density throughout the whole dynamics The combining influence of the two forces gives rise t o the drastically different behaviors observed in three different dens ity regions.