Photodissociation of carbon dioxide in singlet valence electronic states. II. Five state absorption spectrum and vibronic assignment

TL;DR

This study uses quantum mechanical calculations to analyze the absorption spectrum of CO₂ in the 120-160 nm range, accurately reproducing experimental features and assigning vibronic contributions, revealing insights into molecular dynamics and electronic state interactions.

Contribution

The paper presents a detailed vibronic analysis of CO₂'s absorption spectrum using five coupled electronic states and transition dipole moments, advancing understanding of vibronic interactions and spectral assignments.

Findings

Accurate reproduction of experimental spectral envelope and features.

Identification of vibronic contributions from Renner-Teller coupling, conical intersections, and Herzberg-Teller effects.

Progressions in high energy band linked to pseudorotational motion along CI seam.

Abstract

The absorption spectrum of CO$_2$ in the wavelength range 120\,nm --- 160\,nm is analyzed by means of quantum mechanical calculations performed using vibronically coupled PESs of five singlet valence electronic states and the coordinate dependent transition dipole moment vectors. The thermally averaged spectrum, calculated for T=190\,K via Boltzmann averaging of optical transitions from many initial rotational states, accurtely reproduces the experimental spectral envelope, consisting of a low and a high energy band, the positions of the absorption maxima, their FWHMs, peak intensities, and frequencies of diffuse structures in each band. Contributions of the vibronic interactions due to Renner-Teller coupling, conical intersections, and the Herzberg-Teller effect are isolated and the calculated bands are assigned in terms of adiabatic electronic states. Finally, diffuse structures in the calculated bands are vibronically assigned using wave functions of the underlying resonance states. It is demonstrated that the main progressions in the high energy band correspond to consecutive excitations of the pseudorotational motion along the closed loop of the CI seam, and progressions differ in the number of nodes along the radial mode perpendicular to the closed seam. Irregularity of the diffuse peaks in the low energy band is interpreted as a manifestation of the carbene-type \'cyclic' OCO minimum.