Theoretical core-level excitation spectra of N₂and CO by a new polarisation propagator method

Abstract

A recently derived approximation scheme for the polarisation propagator has been applied in a study of discrete K-shell excitations in N2 and CO. The new scheme referred to as second-order algebraic diagrammatic construction (ADC(2)) provides a direct approach to excitation energies and transition moments and gives a consistent second-order and first-order treatment for transitions to singly and doubly excited states, respectively. The essential computational requisite is a Hermitean eigenvalue problem in the space of single and double excitations on the Hartree-Fock ground state. Spin-free decoupled ADC(2) working equations for the singlet-singlet and singlet-triplet transitions have been formulated and employed. As the only additional approximation, the mixing between configurations with a different number of excited core-level electrons has been neglected. The calculated excitation energies of both the core-valence and core-Rydberg transitions are in very good agreement with the experimental data and are distinctly improved with respect to previous theoretical work, including extended configuration interaction treatments. The authors emphasise the accuracy achieved for the oscillator strengths which yield a very satisfactory description for the intensity ratios of the dipole-allowed transitions. The absolute dipole oscillator strengths are in excellent accord with the experimental values of Kay et al. (1977).