Franck–Condon factors in studies of dynamics of chemical reactions. IV. Nonadiabatic collisions

Abstract

We investigate the application of the Franck–Condon approach to nonadiabatic molecular scattering processes. Computationally simple, analytic formulas are developed to describe the energy dependence of quenching of electronically excited atoms by atoms and molecules. These formulas include the dependence of the Franck–Condon factors on the translational wavefunctions as well as the wavefunctions for the internal degrees of freedom. We use these formulas to evaluate the translational energy dependence of the fine structure transition cross sections for F(2P3/2)+X→F(2P1/2)+X, where X= Xe, H+, and H2. The cross sections generally increase as the initial translational energy increases. Our results agree semiquantiatively (or better) with those obtained from other theoretical techniques. In the case of F+H+ we find that the absolute cross section is sensitive to the analytic form used for the nonadiabatic coupling but our model gives the correct energy dependence. At the energies of our calculations we find only a small amount of vibrational excitation of H2. Finally, we use our expressions to interpret some trends of available experimental results on the quenching of Hg (3P2→3P1) by several molecules. We find that collisional excitation of the internal modes of the molecule becomes more important as the initial translational energy increases. However, these modes do not contribute to the quenching cross section in a statistical fashion.