Absorption cross section of NO2 by the reflection method from ab initio calculations involving the three low lying electronic states

Abstract

The potential energy surfaces of the three low lying electronic states of NO2, namely the X̃ 2A1, Ã 2B2, and B̃ 2B1 states, and the transition dipole moment surfaces between the ground state and both excited states have been calculated at two levels of ab initio theory; complete active space self-consistent field (CASSCF) and internally contracted multireference configuration interaction (CMRCI). Only 9 points of these surfaces, located around the equilibrium geometry of X̃ 2A1 and corresponding to C2v geometries, have been found sufficient for calculating the cross section, in the 10000–45000 cm−1 energy range, by means of a 2D version of the reflection method. The agreement between the experimental low resolution data and the ab initio absorption cross section is satisfactory, mainly at the CMRCI level, at which the energy at maximum amplitude, the width, the maximum amplitude and the effective transition dipole moment describing both involved electronic transitions are predicted within 4%, 6%, 20%, and 11%, respectively. The sources of errors coming from the reflection approximation and from the level of ab initio approximation are analyzed on the basis of test calculations. The relative importance of the different contributions to the total cross section (both electronic transitions, cold and hot bands) is also discussed. In addition, quantum calculations based on Franck–Condon factors have been performed in order to improve the description of the low energy part of the cross section and to interpret the observed radiative lifetimes.