Vibrational energy transfer in collisions of He atoms with para-difluorobenzene

Abstract

Vibrational energy transfer in the three-dimensional collisions of He atoms with para-difluorobenzene (pDFB) has been studied theoretically. A quantum-mechanical scattering method has been employed which uses the close-coupling method for the molecular vibrations and the infinite-order-sudden approximation for rotational motion. Both V–T and V–V processes are studied. The normal mode coefficients for pDFB were obtained from an SCF gradient program. The computations showed a strong propensity for excitation and relaxation of the v30 vibrational mode, which is the mode of lowest frequency. This finding is in agreement with molecular beam experiments of Gentry and co-workers, and laser-fluorescence measurements of Parmenter and co-workers. When the different vibrational modes were given the same frequency in the calculations, modes involving atomic displacements out of the molecular plane had distinctly larger vibrational excitation cross sections than those for in-plane modes.