Vibrational structure of the O 1s ionization spectrum of CO2

The dynamical localization of the O 1s core holes in the CO2 molecule is st udied ab initio beyond the linear vibronic coupling model. The vibrational structure of the O 1s x-ray photoelectron spectrum (XPS) has been calculate d employing accurate potential energy surfaces of the ground and ionized di abetic electronic states obtained at the self-consistent field and singles and doubles configuration-interaction (SDCI) methods. A very good agreement of the SDCI computed vibrational structure of the O 1s XPS with the respec tive experimental high-resolution spectrum has been obtained. The anharmoni city of the ionic state potential-energy surface induced mostly by the grou nd-state surface anharmonicity plays a crucial role in the proper descripti on of the vibrational line intensities. The linear model is discussed in de tail. The linear model based on the vibronic coupling constants obtained fr om the geometry change in the ionized state relative to the ground-state re ference configuration has been shown to contain a hidden renormalization of the coupling constants. This improves the agreement of the calculated inte nsities of the vibrational lines in the spectrum with the experiment, compa red to the case of the strict linear model. The limitations of this renorma lized linear model are discussed. Electron relaxation and correlation consi derably affect the vibrational line intensities. The popular equivalent cor e approximation is tested for CO2. The calculation of the O 1s XPS performe d at the SDCI level with and without this approximation is shown to provide almost identical results on the vibrational intensities. [S1050-2947(99)00 309-1].