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

Abstract

In this paper of the series, which gives the full account of an earlier communication [Liu et al. J. Chem. Phys. 1996, 105, 5294], the reaction wave packet coherence of iodine in rare-gas fluids is studied, both experimentally and theoretically, in the gas-to-liquid transition region. The phase coherence dynamics, which exhibit striking density behavior, are resolved in real time and across a broad density range (0-35 nm -3 ) for three different rare gases: helium, neon, and argon. In the solvent, the wave packet reaction coherence is observed on time scales longer than that of the solute vibrational motion, and the loss of coherence is due to solvent collisioninduced predissociation and collision-induced dephasing. By means of molecular dynamics simulations, the experimental results are reproduced by classical and semiclassical treatments. The centrifugal force, by vibration-rotation coupling, is found to be dominant at low and intermediate densities. A motion narrowing effect causes the contribution of vibration-rotation coupling to dephasing to decrease at high densities. Direct contributions from the solute-solvent forces increase with the solvent density throughout the whole dynamics range. The combining influence of the two forces gives rise to the drastically different behaviors observed in three different density regions.

On the role of coherence in the transition from kinetics to dynamics: Theory and application to femtosecond unimolecular reactions

“…Ref. 34. In a bound potential the decay of peak heights in the autocorrelation function, within the first few vibrational periods, is a signature of the wave packet spreading on this time scale.…”

From structure to structural dynamics: Ahmed Zewail's legacy

“…Contrary to the expectation of ultrafast energy randomization, coherent nuclear motion can be maintained in molecular systems in solution. A fact that was repeatedly confirmed for molecules in solutions or in crystals 89–95 and for biological chromophores in proteins 96,97 …”

“…The interplay between these time scales (coherent versus diffusive) was first addressed by him using femtosecond spectroscopy. He thus investigated various reactions ranging from barrier crossing events, 86 over rotation of molecules 88 to bond breaking events 89 . Contrary to the expectation of ultrafast energy randomization, coherent nuclear motion can be maintained in molecular systems in solution.…”

Ultrafast excited states relaxation dynamics in solution investigated by stimulated emission from a styryl dye

“…Among others, the ultrafast intramolecular relaxation and solvation could impact on the fate of excited states of these styryl dyes [9], and subsequently, the emission maximum, linewidth and other basic parameters of dye lasers relying on them. Because of this significant importance in both experimental [10] and theoretical [11] themes, a great deal of interest have been focused on the ultrafast dynamics of excited dye molecules in solution [12].…”