Vibrational relaxation in liquid chloroform following ultrafast excitation of the CH stretch fundamental

Abstract

Vibrational energy flow in liquid chloroform that follows the ultrafast excitation of the CH stretch fundamental is modeled using semiclassical methods. Relaxation rates are calculated using Landau-Teller theory and a time-dependent method both of which consider a quantum mechanical CHCl3 solute molecule coupled to a classical bath of CHCl3 solvent molecules. Probability flow is examined for several potentials to determine the sensitivity of calculated relaxation rates to the parameters that describe the model potentials. Three stages of relaxation are obtained. Probability is calculated to decay initially to a single acceptor state, a combination state of the solute molecule with two quanta of excitation in the CH bend and one in the CCl stretch, in 13–23 ps depending on the potential model employed. This is followed by rapid and complex intramolecular energy flow into the remaining vibrational degrees of freedom. During this second stage the lowest frequency Cl–C–Cl bend is found to serve as a conduit for energy loss to the solvent. The bottleneck for relaxation back to the ground state is predicted to be the slow 100–200 ps relaxation of the CH bend and CCl stretch fundamentals. Several aspects of the incoherent anti-Stokes scattering that follows strong infrared excitation of the CH fundamental as observed by Graener, Zürl, and Hoffman [J. Phys. Chem. B 101, 1745 (1997)] are elucidated in the present study.

Vibrational relaxation in the condensed phase

“…7 Interestingly, for chloroform the energy-level fluctuations do not lead to a strong mixing of the CH stretch vibration and its accepting modes, because the fluctuations are strongly correlated. 6 Hence, for this vibration the nonperturbative approach and the conventional Landau-Teller expression gave similar results. The method of numerically integrating the timedependent Schrödinger equation has as a clear advantage over the perturbative Landau-Teller approach that it can describe vibrational relaxation in the limit that the interacting levels become strongly mixed.…”

“…For small detuning, the relaxing and accepting vibrational states will become strongly mixed, and the relaxation process has to be calculated with a nonperturbative approach, for instance, by numerical integration of the time-dependent Schrödinger equation. 6,7,9,10 Recently, this approach has been used to calculate the effects of energy-level fluctuations on the relaxation of the OH stretch vibration of liquid methanol 7 and the CH stretch vibration of liquid chloroform. 6 For liquid methanol, it was found that the energy-level fluctuations can indeed lead to strong mixing and avoided crossings of the interacting vibrational levels, thus leading to state-to-state relaxation rates that strongly differ from the rates predicted by the conventional Landau-Teller equation.…”

“…6,7,9,10 Recently, this approach has been used to calculate the effects of energy-level fluctuations on the relaxation of the OH stretch vibration of liquid methanol 7 and the CH stretch vibration of liquid chloroform. 6 For liquid methanol, it was found that the energy-level fluctuations can indeed lead to strong mixing and avoided crossings of the interacting vibrational levels, thus leading to state-to-state relaxation rates that strongly differ from the rates predicted by the conventional Landau-Teller equation. 7 Interestingly, for chloroform the energy-level fluctuations do not lead to a strong mixing of the CH stretch vibration and its accepting modes, because the fluctuations are strongly correlated.…”

“…Recently, the effects of the fluctuations of the energy levels have been calculated for the CH stretch vibration of liquid chloroform 6 and the OH stretch vibration of liquid methanol. 7 In these studies, the relaxation rate was calculated in a nonperturbative approach by numerical integration of the time-dependent Schrödinger equation.…”

Solvation Dynamics in Liquid Water. 1. Ultrafast Energy Fluxes

“…Comparison of the associated power spectrum with the known spectroscopic features of the solvent has sometimes served to qualitatively identify the solvent motions involved; the situation here is analogous to that for tcf studies of vibrational energy transfer, where similar comparisons implicate e.g. which solvent modes are energy receptors without directly observing the solvent molecules receiving the energy [42][43][44][45] . It is to be noted that, besides its qualitative character, this tcf approach suffers from additional limitations.…”