Effects of Electronic Exchange on the Efficiency of Vibrational Excitation by Molecular Collisions. Part I. Interaction Potential

Abstract

A theoretical study has been made of the mechanism for the transfer of molecule translational energy into the internal energy of vibration by molecular collisions. In particular, the research considered the interaction between molecules which can combine chemically to form a covalent bond. Previous theoretical treatments using a Lennard‐Jones type of interaction potential resulted in calculated efficiencies which were low by several orders of magnitude for the vibrational excitation process. A stable covalent bond is formed by the sharing of electrons, i.e., electronic exchange, between the various atoms in a molecule. The concept of electronic exchange was used in determining a more realistic interaction potential then the Lennard‐Jones for a typical case, namely, the collision of a proton and a diatomic hydrogen molecule which can combine to form a stable molecule ion. The resulting interaction potential was entirely of negative energy as opposed to the Lennard—Jones which is of positive energy and strongly repulsive for small separation distances between the colliding particles. Furthermore, the depth of the potential well with electronic exchange was about 2 eV compared to Lennard‐Jones well depths of about 0.01 eV. In addition, the equilibrium separation between the nuclei of the hydrogen molecule increased to 2½ times the normal value as the proton approached, and the spring constant of the molecule as a vibrator decreased to less than ⅕ the normal value. The above results all tend to increase the ease of vibrational excitation and should result in higher excitation efficiencies.