Gateway Modes in the Collisional Energy Transfer from Highly Vibrationally Excited CS2

Abstract

The role of different vibrational modes in the energy transfer from highly vibrationally excited CS2 is investigated in model classical trajectory calculations. The primary tool for this work involves determining the dependence of the average energy transfer on the vibrational frequency of each mode. These calculations show that the energy transfer is highly sensitive to the frequency of the bend and the symmetric stretch mode. By increasing or decreasing the bend frequency the efficiency of the energy transfer decreases or increases, respectively. Variation of the symmetric stretch frequency causes a comparable effect. Variation of the asymmetric stretch mode frequency can lead to activation of energy transfer only if the frequency of the other two modes is made so high that the energy transfer through those modes becomes negligible. If the vibrational frequencies of highly excited CS2 are set to their “natural” values, the bending mode is the conduit through which the energy “leaks out” of the molecule, similarly to the slightly excited molecule. Bending is the gateway mode not only because it is the lowest frequency mode but also because of the shape of the molecule: a sideways attack, which is much more probable than an axial attack, always interacts with the bend mode but only to a small extent with the asymmetric stretch mode, independently of the actual frequencies. The activity of a mode in energy transfer is determined mostly by its frequency, but the nature of the motion of the atoms involved in the mode also play a role.