Core-valence double photoionization of the CS2 molecule

Abstract

Double photoionization spectra of the CS 2 molecule have been recorded using the TOF-PEPECO technique in combination with synchrotron radiation at the photon energies h = 220, 230, 240, 243, and 362.7 eV. The spectra were recorded in the S 2p and C 1s inner-shell ionization regions and reflect dicationic states formed out of one inner-shell vacancy and one vacancy in the valence region. MCSCF calculations were performed to model the energies of the dicationic states. The spectra associated with a S 2p vacancy are well structured and have been interpreted in some detail by comparison to conventional S 2p and valence photoelectron spectra. The lowest inner-shell-valence dicationic state is observed at the vertical double ionization energy 188.45 eV and is associated with a ͑2p 3/2 ͒ −1 ͑2 g ͒ −1 double vacancy. The spectrum connected to the C 1s vacancy shows a distinct line at 310.8 eV, accompanied by additional broad features at higher double ionization energies. This line is associated with a ͑C 1s͒ −1 ͑2 g ͒ −1 double vacancy.

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

“…In contrast, the excited states (O 1s −1 8σ −1 and O 1s −1 2π −1 ) show a variety of energies depending on the calculation method used. The small singlet-triplet splittings originate from small exchange integrals and weak correlation between the core and valence holes, as expected from our previous study on CS 2 [6]. The splittings in the calculations are smallest for the lowest states O 1s −1 3π −1 and O 1s −1 9σ −1 .…”

“…As discussed in our previous work [6], there are some particularities associated with core-valence double hole states which have implications for their calculation and choice of computational procedure. Without restrictions a MCSCF wave function will collapse to a low-lying double hole state by orbital rotation and by doubly occupying the original core orbital.…”

“…The open-shell DFT part was carried out by a local unreleased version of DALTON. Following previous basis set investigations on the CS 2 molecule [6], the calculations were performed utilizing the augmented correlation-consistent polarized corevalence triple zeta (aug-cc-pCVTZ) [41,42] basis set. Figure 1 shows the O 1s −1 v −1 DIPES, measured using the photon energy of 620 eV, along with the energies obtained from the calculations A-D. As can be seen, the O 1s −1 (v) −1 spectrum has an onset at around 560 eV, which is about 20 eV above the O 1s single ionization energy at 540.3 eV (cf.…”

“…One example of this assertion is given by double ionization photoelectron spectroscopy (DIPES) that provides information about the energy distribution of dicationic states of atoms and molecules and the probability for emission of two electrons on the absorption of a single incident photon (see, e.g., Refs. [1][2][3][4][5][6] and references therein). In the strong-field regime, two-photon DIPES can be used in double core-hole spectroscopy, theoretically studied by Cederbaum and coworkers [7][8][9] and byÅgren and Jensen [10].…”

“…In one of our recent works, we reported a study of corevalence double photoionization of the CS 2 molecule [6] where we applied a self-consistent field procedure for correlating complete or restricted active spaces. The optimization was based on a norm extended optimization in a reduced variational space where the core electron occupation was restricted to single occupation (restricted active space) and where the corresponding core orbital is frozen.…”