Theoretical study of the electronic structure of carbon dioxide: Bending potential curves and generalized oscillator strengths

The electronic spectrum of carbon dioxide is predicted by means of multireference single- and double-excitation configuration interaction (MRD–CI) calculations. Bending potential curves for a large number of electronic states are calculated and the generalized oscillator strengths (GOS) obtained for the corresponding transitions are compared with recent electron energy loss spectra (EELS) obtained by Tanaka and co-workers as well as with optical intensity data. The CO2 absorption spectrum is dominated by vertical Rydberg transitions from the linear ground state but valence excited states with bent equilibrium geometries are also found to be important. Emphasis is placed on the intense spectral features in the 11.0–11.4 eV region of the spectrum. The two most intense peaks are both found to originate from π–3pπ transitions, one of which only becomes allowed upon molecular bending. Good agreement with the EELS results is obtained for the variation of the GOS with K2 and for the associated transition energies.