Experimental and theoretical study of core-valence double photoionization of OCS

Abstract

Experimental and theoretical study of core-valence double photoionization of OCS.Physical Review A. Atomic, Molecular, and Optical Physics, 82(4) O 1s, C 1s, and S 2p core-valence double ionization electron spectra of the OCS molecule have been obtained experimentally by a time-of-flight photoelectron-photoelectron coincidence spectroscopy technique. In order to analyze and assign the spectral features observed, we present a protocol for computing core-valence ionization energies of such systems. The protocol is based on a restricted active space multiconfigurational self-consistent field (MCSCF) methodology with a freeze-relax procedure to guarantee a correct core-valence state root index without variational collapse. Corrections for extended dynamical correlation and core-core correlation, respectively, are made by multiconfigurational perturbation theory and by uncontracted basis set Møller-Plesset theory. Envisioning applications to larger molecules, a spin-restricted open-shell density functional method is also applied for the lowest core-valence energies. Furthermore, cross sections through a scheme for computing multiatom Auger transitions generating core-valence holes are presented. We find that the procedure outlined is capable of deriving the energy onset of core-valence ionization within a fraction of an eV and that assignments can be made of the most salient spectral features.

Symmetry breaking in core-valence double photoionization of SO2

“…LDA is an exception, giving double ionization energies for the O 1s case that were several eV different in comparison to the experimental results. In contrast, the S 2p core-valence states are well produced by the LDA functional, as also found in our previous OCS study [3] (see further discussion below).…”

“…The procedure is similar to that presented in one of our recent papers on core-valence double ionization of the OCS molecule [3]. The calculations were based on a two-step relaxation process of the single core-hole state, including optimization of doubly occupied orbitals with respect to the core hole, and relaxation of the core hole itself.…”

“…Triplet spin coupling is employed throughout for the core-valence states, thus neglecting the splitting into singlets caused by the core-valence exchange interaction. According to previous studies, however, these splittings are expected to be small (few tenths of an eV) [2,3]. Triplets and singlets can be assumed to be statistically populated according to the number of spin sublevels.…”

“…Following previous basis-set investigations on the CS 2 and OCS molecules [2,3], the calculations were performed utilizing augmented correlation-consistent polarized core-valence triple-ζ (aug-cc-pCVTZ) basis sets of Dunning and coworkers [26,27]. Triplet spin coupling is employed throughout for the core-valence states, thus neglecting the splitting into singlets caused by the core-valence exchange interaction.…”

“…DIPES is based upon coincidence detection of two electrons that are created in the same process, and can be obtained in an efficient way by using the time-of-flight photoelectron-photoelectron coincidence (TOF-PEPECO) spectroscopy method introduced by Eland et al [1]. In previous works, we have made concomitant considerations of theory and computation to unravel the information content of the DIPES measurement technique [2,3]. We have demonstrated the versatility of the technique to explore core-valence double-hole states, both in comparison with conventional (i.e., single ionization) valence photoelectron spectra and in using designed computational protocols as helpful instruments to assign the spectra.…”