ON THE VIBRATIONAL DEPENDENCE OF ELECTRON-IMPACT IONIZATION OF DIATOMIC-MOLECULES

The dependence of the Na-2 electron impact ionization rate is measured as a function of Vibrational excitation in a crossed molecule-electro n beam arrangement at collision energies E(coll) less than or equal to 3 eV above the ionization threshold. Specific vibrational distributio ns in the X(1) Sigma(g)(+) state with average vibrational energies of 0.17 eV, 0.276 eV, and 0.349 eV, are prepared via Franck-Condon pumpin g using a narrow-band cw laser. Enhancement of the ionization rate is observed only at impact energies near the ionization threshold where t he ionization rate increases linearly as a function of vibrational exc itation. Analysis of the experimental data is based on three model cal culations. The first of these calculations equates vibrational energy with kinetic energy and agrees well with the experimental data. A seco nd, more refined model allows for differences in state-to-state ioniza tion rates and uses Franck-Condon factors to estimate transition proba bilities, but leads to a less favorable agreement. The third one emplo ys a semi-classical formulation of the Franck-Condon principle. It pro vides the best agreement with the experimental data. In contrast with an earlier study of electron impact ionization of diatomic molecules [ 20], we find no evidence of dynamical modification of the ionization r ate, due to vibrational motion of the nuclei, at the present level of accuracy of our data and analysis.