Core-level electronic spectra in ADC(2) approximation for polarization propagator: Carbon monoxide and nitrogen molecules

An effective ab initio approach to core-level electronic spectral studies is discussed. The approach uses polarization propagator theory in a second-order algebraic diagram construction ADC(2) approximation for calculating the characteristics of electron transitions; it also uses the linear vibronic model LVM for investigating the vibrational structure of transitions. The core excitation specialization of ADC(2) is achieved by introducing the core valence separation (CVS) approximation. K-excitation spectra of CO and N2 molecules are calculated to examine the potential of the approach. The calculated spectra and the available experimental data are analyzed to characterize the method. A number of additional facts of methodological and practical value are found, and new transitions are predicted. It is concluded that ADC(2)/CVS/LVM is a promising approach to problem solving in core level spectroscopy, which requires qualitatively reliable theoretical estimations.