SOLVATION ULTRAFAST DYNAMICS OF REACTIONS .12. PROBING ALONG THE REACTION COORDINATE AND DYNAMICS IN SUPERCRITICAL ARGON

In this paper, our focus is on the influence of the solvent density on the caging, recombination dynamics, and the nature of the reaction co ordinate of iodine in supercritical argon at pressures of 0-2500 bar. Femtosecond probing with widely tunable pulses allows us to directly r esolve the geminate recombination of iodine atoms and the subsequent r elaxation processes, A nonzero recombination yield is found at argon p ressures as low as 200 bar, and this yield increases strongly with inc reasing solvent density. The mechanism involves recombination onto the A/A' states. At high pressures, a large fraction of the iodine atoms undergo an ultrafast ''in-cage'' recombination which is measured on th e subpicosecond lime scale at 2500 bar of argon. In addition, a fracti on of the iodine atoms bleak through the solvent cage and begin a diff usive motion through the rare-gas solvent. Experimental evidence is pr esented and indicates that this diffusive motion leads to reencounters and subsequent recombination of the geminate iodine pair. This diffus ive recombination occurs on a significantly longer time scale than the rapid ''in-cage'' recombination. The newly-formed iodine molecules un dergo vibrational relaxation within the A/A' state, and the dynamics o f this process and its dependence on the solvent density are revealed. A key concept here is the solvent density-induced control of the rigi dity of the first solvent shell surrounding the dissociating iodine at oms. As shown before [Liu et al, Nature 1993, 364, 437], such studies of solvation present a unique opportunity of examining the microscopic influence of the solvent structure on reaction dynamics in clusters a nd solutions.