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

摘要

The absorption spectrum of CO$_2$ in the wavelength range 120\,nm --- 160\,nmis analyzed by means of quantum mechanical calculations performed usingvibronically coupled PESs of five singlet valence electronic states and thecoordinate dependent transition dipole moment vectors. The thermally averagedspectrum, calculated for T=190\,K via Boltzmann averaging of opticaltransitions from many initial rotational states, accurtely reproduces theexperimental spectral envelope, consisting of a low and a high energy band, thepositions of the absorption maxima, their FWHMs, peak intensities, andfrequencies of diffuse structures in each band. Contributions of the vibronicinteractions due to Renner-Teller coupling, conical intersections, and theHerzberg-Teller effect are isolated and the calculated bands are assigned interms of adiabatic electronic states. Finally, diffuse structures in thecalculated bands are vibronically assigned using wave functions of theunderlying resonance states. It is demonstrated that the main progressions inthe high energy band correspond to consecutive excitations of thepseudorotational motion along the closed loop of the CI seam, and progressionsdiffer in the number of nodes along the radial mode perpendicular to the closedseam. Irregularity of the diffuse peaks in the low energy band is interpretedas a manifestation of the carbene-type \'cyclic' OCO minimum.