Pulsed field ionization-photoelectorn bands for CS_2~+ in the energy range of 13.2-17.6 eV: An experimental and theoretical study

摘要

Vacuum ultraviolet pulsed field ionization-photoelectron (PFI-PE) spectra for CS_2 have been obtained in the energy range of 13.2-17.6 eV, revealing complex vibronic structures for the CS_2~+ (B ~2(sum)_u~+ and C ~2(sum)_g~+) states. The PFI-PE spectra for CS_2~+ (B ~2(sum)_u~+ and C ~2(sum)_g~+) are dominated by the v_1~+ (symmetric stretching) mode. However, PFI-PE bands due to the v_2~+ (bending) and v_3~+ (antisymmetric stetching) modes with both even and odd quanta are clearly resoled. The simulation f 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 CS_2~+ (B ~2(sum)_u~+ and C ~2(sum)_g~+) sates from Cs_2(X ~1(sum)_g~+), respectively. The PFI-PE bands for ~2(sum)_u~+ (3_0~2) and ~2(sum)_g~+ (3_0~3) 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 ~1(sum)~+; v'' = 0 and 1) from CS_2, respectively, are found to be enhanced. These enhancements are rationalized by the prompt dissociation of excited CS_2 in high-n (n>=100) Rydberg states prior to PFI. Three-dimensional potential energy functions (PEFs) for CS_2~+(2 ~2#PI#_u, B ~2(sum)_u~+, and C ~2(sum)_g~+) 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 CS_2~+(2 ~2#PI#_u, B ~2(sum)_u~+, and C ~2(sum)_g~+) and vibronic energy levels for CS_2~+(B ~2(sum)_u~+) have been calculated variationally. These theoretical predictions have made possible a satisfactory assignment of the vibraonic bands resolved in the PFI-PE spectra for CS_2~+(B ~2(sum)_u~+ and C ~2(sum)_g~+). Using the theoretical predictions obtained here, we have also assigned vibraonic bands for CS_2~+(2 ~2#PI#_u) observed in the HeI photoelectron spectrum [Baltzer et al., Chem. Phys. 202, 185 (1996)].