Comparative study of the low-lying valence electronic states of carbon dioxide by high-resolution inelastic x-ray and electron scattering

We report a comparative study of low-lying valence electronic states of carbon dioxide by high-resolution inelastic x-ray and electron scattering. Momentum-transfer-dependent inelastic squared form factors for the two states (1)Sigma(+)(u) u and (1)Pi(u) and generalized oscillator strength for the 9 eV feature from the ground state X-1 Sigma(+)(g) g have been derived from the inelastic x-ray scattering method at an impact photon energy around 10 keV, and the electron energy-loss spectra measured at an incident electron energy of 1500 eV. It is found from the comparison between the present results and the previous outcomes that the recent calculations taking the vibronic effects into consideration satisfactorily reproduce the inelastic squared form-factor profile for the (1)Sigma(+)(u) u transition and the generalized oscillator strength profile for the 9 eV feature. However, the vibronic effects seem to play no role in the (1)Pi(u) transition. The difference existing between the inelastic x-ray scattering and electron energy-loss spectroscopy results in the larger momentum-transfer squared region may be attributed to the increasing role of the higher-order Born terms. Furthermore, the controversy concerning the designations of electronic states around 11 eV is solved by assigning the two peaks centered at 10.98 and 11.05 eV to the vibrational progression (1) Sigma(+')(u) and (1) Sigma(+)(u), and the peak centered at 11.16 eV to the forbidden transition 2(1)Delta(u) based on the present results.