Vibronic effects on the low-lying electronic excitations in CO₂ induced by electron impact

We report an angle-resolved electron energy loss spectroscopy (EELS) study on the valence-shell electronic excitations of CO₂. Experimentally, momentum-transfer-dependent generalized oscillator strengths (GOSs) or GOS profiles for low-lying electronic excitations have been derived from EELS spectra measured at incident electron energy of 3 keV. Theoretically, we have calculated GOS profiles using wave functions at the equation-of-motion coupled-cluster singles and doubles level. In the calculation, vibronic effects are taken into account by evaluating the electronic transition amplitudes along the individual normal coordinates. The calculation satisfactorily reproduces the experiment for the ¹Π_g and ¹Δ _u transitions and reveals prominent roles of strong coupling between the associated mixed-Rydberg-valence and valence excited states through the bending vibration of CO₂. Vibronic effects on GOS profiles have also been examined for the ¹Σ_u⁺, ¹Π_u, and 2¹Π_g transitions.