Example: vibrational energy transfer in collisions of N2 with He
Quantitative information about processes of vibrational energy transfer in molecular collisions is of crucial importance for kinetic modeling in combustion, aeronautics, laser physics, and plasma chemistry.
Vibrational transition from the first excited vibrational state of N2 to the ground state in collisions with He (N2(v=1)+He->N2(v=0)+He, v is vibrational quantum number) is a typical example of such process. The rate constant of the process (N2(v=1)+He->N2(v=0)+He was evaluated within the framework of the Schwartz-Slawsky-Herzfeld (SSH) theory of the VT energy exchange in collisions involving diatomic molecules. This theory is based on the assumption that the vibrational transition is induced only by coupling between the vibrational and translational degrees of freedom, and, therefore, vibrational energy is converted into translational one. The dominant role is played by the repulsive part of the interaction between the colliding molecules. The SSH theory refers only to those pairs of molecules for which the anisotropy of intermolecular interaction is small.
The necessary parameters of the interaction between N2 and He were evaluated in Kintech Lab by ab initio calculations at the DFT level with the hybrid three-parameter (B3LYP) exchange–correlation functional using the GAUSSIAN-03 program package.
It can be seen in the figure that the SSH theory with the parameters obtained from standard quantum chemical calculations provides the quantitative description of vibrational relaxation in homonuclear diatomic molecules.
- Rate constant of the process N2(v=1)+He-> N2(v=0)+He. The results of calculations are shown by the blue line; experimental data are shown by black squares (Maricq M.M. and al Chem. Phys. 75, 347 (1983)) and red circles (Whight D.R Journ. Chem. Phys. 48, 325 (1968)).
