This abstract was not submitted electronically.
[G8.02] Mechanisms Contributing to Vibrational Relaxation in Polyatomic Solvents.
Branka M. Ladanyi (Dept. of Chemistry, Colorado State University), Richard M. Stratt (Dept. of Chemistry, Brown University)
Vibrational energy relaxation in polar and nonpolar solvents is studied by calculating via molecular dynamics and analyzing the time correlation function of the fluctuating force on the vibrational coordinate of a diatomic solute. The contributions of electrostatic and nonelectrostatic solute-solvent interactions are studied for dipolar and quadrupolar diatoms in acetonitrile and carbon dioxide liquids. Instantaneous normal mode and time- domain(B.M. Ladanyi and S. Klein, J. Chem. Phys. \textbf105), 1552 (1996). methods are used to determine the contributions of different molecular mechanisms, such as rotation vs translation and collective vs single-particle dynamics, to vibrational relaxation.
[G8.03] Vibrational Dephasing in Liquids with a Phonon Based Approach
Roger Loring (Department of Chemistry, Cornell University, Ithaca, New York, 14853)
A traditional approach to describing molecular vibrational relaxation in a dense fluid generalizes the notion of a binary molecular collision, appropriate to a dilute gas. This presentation employs an alternative viewpoint, in which the liquid is modelled on short time-scales as an amorphous harmonic network. Green's function methods for treating the coupling of an impurity vibration to a harmonic bath may then be applied, obviating the need to define a collision in a dense fluid. The result is an analytical method for calculating the classical-mechanical vibrational absorption spectrum of a solvated molecule from a knowledge of intramolecular and intermolecular potential energies. Results are shown to provide semiquantitative agreement with full molecular dynamics simulations. Applications include the role of long-ranged Coulombic interactions in promoting vibrational relaxation, and the relative importance of homogeneous and inhomogeneous line broadening.
[G8.04] The Elementary Events in Vibrational Relaxation.
R. M. Stratt (Department of Chemistry, Brown University)
Despite the relatively long vibrational population relaxation times commonly measured in liquids, understanding the behavior of the vibrational friction for times no longer than the few hundred fs often suffices to predict these lifetimes. This observation suggests that the instantaneous normal modes of the liquid may be of help in understanding of the key solvent events promoting the relaxation process. Indeed, a rigorous derivation does lead to a short-time expression for the relevant friction in terms of a weighted spectrum of these modes -- the vibrational influence spectrum. However,when this spectrum is examined in detail, a number of surprises appear. The average spectrum turns out to have precisely the density dependence expected from a simple independent-binary-collision theory. Moreover, when examined on a single-liquid-configuration-by-single-liquid-configuration basis, the significant motion ends up involving no more than 1 or 2 key atoms in each important liquid mode. We discuss the implications of these findings for identifying the true "elementary events" in vibrational population relaxation.
[G8.05] A Two--Body Description of Vibrational Population Relaxation and Solvation in Dense Fluids
Ross E. Larsen, Edwin F. David, Grant Goodyear, Richard M. Stratt (Brown University)
The dynamics of vibrational population relaxation and of solvation in dense liquids can be predicted for short times by using the liquid's instantaneous normal modes. These instantaneous modes provide a description of the dynamics of the solute and the solvent that is collective yet microscopic, so that the specific motions responsible for solute relaxation can be identified. We derive a particularly simple subset of the instantaneous normal modes for atomic liquids, a subset in which only two atoms vibrate in an otherwise frozen liquid background. For models of vibrational population relaxation and solvation involving short--ranged, sharply--varying forces, these special modes, consisting of motions by just the solute and a single solvent atom, are shown to account for the bulk of an atomic solvent's affect on the solute. We conclude that even in very dense liquids both vibrational population relaxation and solvation dynamics result to a large extent from a single set of simple two--body motions.