Experimental and theoretical study of generalized oscillator strengths for C 1s and O 1s excitations in CO2

Electron-energy-loss spectra of CO2 in the region of C 1s and O 1s excitations have been recorded over a wide range of momentum transfer (K), (2 a.u.^-22<70 a.u.^-2). The dipole-forbidden transition to the (C 1s σ^-1g, σ*g) 1Σ⁺g state in CO2 is detected for the first time, to our knowledge. A detailed analysis, with careful consideration of minimization of systematic experimental errors, has been used to convert the measured relative cross sections to absolute, momentum-transfer-dependent, generalized oscillator strength (GOS) profiles for all resolved C 1s and O 1s transitions of CO2. Theoretical results for the GOS, computed within the 1s first Born approximation, were obtained with ab initia configuration interaction wave functions for the C 1s transitions and with ab initio generalized multistructural wave functions for the O 1s transitions. These wave functions include relaxation, correlation, and hole localization effects. Theory predicts large quadrupole contributions to the GOS for O 1s excitations. In addition the computed GOS for O 1s → nsσ and npσ Rydberg states clearly show oscillations arising from interference between localized core excitations. Overall there is good agreement between the experimental and theoretical results, indicating that the first Born approximation holds to a surprisingly large momentum transfer for the core excitations studied.