Energy and phase relaxation accompanying impulsive reactions in liquids

Abstract

Experiments made possible by new laser methods explore ultrafast features of solution phase reaction dynamics. The results from two examples are given. Photodissociation of small molecules such as and in solution yields highly excited vibrational state distributions and narrow bond length distributions. The dynamics in the transition state region and the ensuing dynamics can be followed by femtosecond laser experiments that probe the phase and energy loss processes. The mercuric iodide experiments show that the phase relaxation is dominated by coherence transfer processes which have a strong analogy to classical motions. The population relaxation dynamics gives results similar to those from classical molecular dynamics simulations which suggest, notwithstanding the significant polarity of the molecules and solvents, that the relaxations are dominated by Lennard - Jones repulsion at sufficiently low frequencies.

Angular momentum dependent friction slows down rotational relaxation under nonequilibrium conditions

“…The Massey parameter (12) tells us that the molecules possessing relatively small angular momentum experience rotationally inelastic collisions, while those possessing high enough angular momentum experience j-conserving collisions. This means that the collision rates must be j-dependent, and this is explicitly assumed in various "fitting laws", which are available in the literature for describing j-resolved cross-sections.…”

“…Traditionally, polarization photodissociation experiments (both in the steady state and in the time domain) have been performed in the gas phase under collision-free conditions. 1,2 Recently, a number of "real time" measurements 3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22 and computer simulations 23,24,25,26,27 has been reported, where the anisotropy decay of diatomic photoproducts was studied in the condensed phase. Both classical 28,29 and quantum 30 models of the anisotropy decay in the dissipative ensemble of photofragments have also been developed.…”

Photodissociation of gas phase I3− using femtosecond photoelectron spectroscopy

“…Thus far, only one comparison of this type has been made for a reaction yielding molecular products, namely, the photodissociation reaction HgI 2˜H gIϩI, which was studied in the gas phase and in ethanol by Zewail and co-workers [3][4][5][6] and by Hochstrasser and co-workers, [7][8][9][10] respectively. During the last few years, a series of femtosecond time-resolved experiments on the photodissociation of I 3 Ϫ in ethanol solution have been performed in the laboratories of Ruhman and co-workers [11][12][13][14][15][16] and, more recently, by Vohringer and co-workers [17][18][19] that have yielded the vibrational distribution of the I 2…”

Manifestation of nonequilibrium initial conditions in molecular rotation: The generalized J-diffusion model

“…[27][28][29][30] In our previous paper, 26 we have demonstrated how the angular-momentum dependent friction can be incorporated into the rotational Fokker-Planck equation. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Third, we demonstrate how the use of standard "equilibrium" approaches may lead to quantitatively and even qualitatively wrong results, if they are applied to describe nonequilibrium molecular rotation. First, we investigate how the angular momentum dependent rates manifest themselves in relaxations of initial nonequilibrium distributions and in behaviors of rotational and orientational correlation functions ͑CFs͒.…”