De-excitation of Electronically Excited Sodium by Nitrogen

Abstract

A semiquantitative calculation is made of the cross section for the quenching of Na(32P) by molecular nitrogen, as a function of initial kinetic energy and of final vibrational quantum number, υf, of the nitrogen molecule. The large observed cross section, which is of gas-kinetic order, can be explained in terms of an intermediate ionic state, involving Na+ and N2− (υ = υ−). This state is unstable at infinite separation of Na and N2, but because of the Coulomb attraction it becomes stable at collision distances below about 3 Å. As a result of the vibrational structure of both the intermediate and final states, we treat the reaction in terms of a diffusion of the probability flux through a two-dimensional network of potential-energy curves parametrized by both the electronic state and also the vibrational quantum numbers υ− and υf. At each potential-energy curve crossing we compute the transition matrix element for insertion into a Landau–Zener type of transition probability. The transition matrix element is represented as the product of an electronic interaction function (obtained from a correlation, due to Hasted and Chong, of results obtained from charge-transfer processes involving multiply charged ions) and a vibrational overlap integral or Franck–Condon factor. Results are also presented on the quenching of Na(42P) by N2, and on the quenching of Na(32P) by CO. All the results have the same general character: The total cross section is of gas-kinetic order and depends only weakly on kinetic energy. The partial cross sections for excitation of the different final vibrational levels vf show a rather broad distribution, with somewhat more than half the energy of electronic excitation ending up as vibrational excitation.