Mechanisms of Franck-Condon breakdown over a broad energy range in the valence photoionization of N2 and CO

The molecular photoionization of N-2 leading to the 3 sigma(-1)(g) 2 sigma(-1)(u) ion states and CO leading to the valence isoelectronic 5 sigma(-1), 4 sigma(-1) ion states has been studied using both theory and experiment. Vibrational branching ratios have been obtained in the 15-200 eV photoelectron energy range. The analysis of the branching ratios for these processes shows a breakdown in the Franck-Condon approximation in the range of energies studied. Some of the deviations at lower energies are well documented as due to shape resonances, and in such cases we found good agreement between the present work and previous experimental and theoretical investigations of these photoionization channels. For both N-2 and CO ionization, we also found that the partial wave cross sections have an interference pattern similar to a Young-type interference, which are related to molecular Cooper minima. Such features were also seen to induce non-Franck-Condon effects in the vibrational branching ratios at higher energies. The comparison of theory and experiment was facilitated by the introduction of an electronic factor (F) that is the logarithmic derivative of the cross section with respect to bond length and which could be directly related to the branching ratios. (C) 2012 Elsevier B.V. All rights reserved.