a Molecular Dynamics Investigation of Collision Induced Dipole Moments and Infrared Absorption Lineshapes in Simple Liquid Mixtures.

The main purpose of this work was to understand the dependence of collision induced absorption (CIA) lineshapes upon the impurity atom-to-host atom mass ratio. The dynamics of the impurity atom and the relationship of those dynamics to the dynamics of the host liquid being probed by CIA were sought. An associated question was whether or not there exists an impurity atom concentration dependence in the CIA process. To investigate these questions, the method of molecular dynamics was used. Collision induced dipole moments and the associated infrared absorption lineshapes were computed in Lennard-Jones liquids. Dipole moment and velocity autocorrelation functions, computed for impurity -to-host mass ratios (m*) of 0.5, 1.0, 2.1, and 6.25, were found to vary significantly with m*. An interesting pattern was observed in the time variation of the DACF and the VACF. To explain the time variation pattern, a conceptual model of an impurity atom in motion within moving surroundings was proposed. It was shown, using collision sequences evolving from a representative range of initial configurations, that the time interval patterns could be explained. Use was made, in explaining the collision sequences, of the momentum transfer analyses of Gaskell and Mason, and Balucani et al. The ability of heavy impurity atoms to penetrate their local surroundings was related to the absence of second zero crossovers in the DACF and VACF. It was also shown that the estimated times for second zero crossovers exceeded estimates ot the structural relaxation time for the host liquid. The reduced lineshapes were computed and the peak frequencies shown to be closely related to the DACF zero crossovers. A shift in the peak of the reduced lineshape to lower frequency and an increase in the intensity of the peak was observed in going from a one-impurity atom system to a two impurity atom system suggesting the existence of a concentration dependence in CIA data.