High Resolution Pulsed Field Ionization−Photoelectron Bands for CS₂⁺(<i>Ã</i>²Π_u):  An Experimental and Theoretical Study

Abstract

The vacuum ultraviolet pulsed field ionization−photoelectron (PFI−PE) spectra for CS 2 were measured in the full energy range of 12.6−13.5 eV, revealing complex vibronic band structures for the CS 2 + ( Ã 2 Π u ) state. Three-dimensional potential energy functions (PEFs) for CS 2 + ( Ã 2 Π u ) were also generated theoretically using the complete active space self-consistent field and internally contracted multireference configuration interaction methods. Using these PEFs, the harmonic frequencies, anharmonic constants, and Renner−Teller rovibronic energy levels for CS 2 + ( Ã 2 Π u ) were calculated variationally. Many Fermi polyads have been identified in the rovibronic states of CS 2 + ( Ã 2 Π u ). Using present theoretical and available optical data, we assigned most of the PFI−PE rovibronic bands due to excitation of the ν 1 + (symmetric stretching), ν 2 + (bending), and ν 3 + (asymmetric stretching) modes for CS 2 + ( Ã 2 Π u ). The simulation of rotational contours resolved in the high-resolution PFI−PE bands for CS 2 + ( Ã 2 Π u ); ν 1 + = 0−3, ν 2 + = 0,2, ν 3 + = 0) provided accurate ionization energies for the formation of these states from CS 2 (X̃ 1 Σ g + ).