Pulsed field ionization-photoelectron bands for CS2+ in the energy range of 13.2–17.6 eV: An experimental and theoretical study

Abstract

Vacuum ultraviolet pulsed field ionization-photoelectron (PFI-PE) spectra for CS2 have been obtained in the energy range of 13.2–17.6 eV, revealing complex vibronic structures for the CS2+(B̃ 2Σu+ and C̃ 2Σg+) states. The PFI-PE spectra for CS2+(B̃ 2Σu+ and C̃ 2Σg+) are dominated by the ν1+ (symmetric stretching) mode. However, PFI-PE bands due to the ν2+ (bending) and ν3+ (antisymmetric stretching) modes with both even and odd quanta are clearly resolved. The simulation of rotational contours resolved in the origin PFI-PE bands yields accurate ionization energies of 14.4742±0.0005 eV (116 742±4 cm−1) and 16.1883±0.0005 eV (130 567±4 cm−1) for the formation of CS2+(B̃ 2Σu+ and C̃ 2Σg+) states from CS2(X̃ 1Σg+), respectively. The PFI-PE bands for Σu+2(302) and Σg+2(303) at 14.805 and 14.965 eV, which are in near energy resonance with the 0 K dissociation thresholds for the formation of S+(4S)+CS(X 1Σ+; ν″=0 and 1) from CS2, respectively, are found to be enhanced. These enhancements are rationalized by the prompt dissociation of excited CS2 in high-n (n⩾100) Rydberg states prior to PFI. Three-dimensional potential energy functions (PEFs) for CS2+(2 2Πu, B̃ 2Σu+, and C̃ 2Σg+) states have been generated theoretically using the complete active space self-consistent field and internally contracted multireference configuration interaction methods. On the basis of these PEFs, the harmonic frequencies for CS2+(2 2Πu, B̃ 2Σu+, and C̃ 2Σg+) and vibronic energy levels for CS2+(B̃ 2Σu+) have been calculated variationally. These theoretical predictions have made possible a satisfactory assignment of the vibronic bands resolved in the PFI-PE spectra for CS2+(B̃ 2Σu+ and C̃ 2Σg+). Using the theoretical predictions obtained here, we have also assigned vibronic bands for CS2+(2 2Πu) observed in the HeI photoelectron spectrum [Baltzer et al., Chem. Phys. 202, 185 (1996)].