A high-resolution vacuum ultraviolet photoionization, photoelectron, and pulsed field ionization study of CS2 near the CS2+(X 2Π3/2,1/2) thresholds

Abstract

High-resolution photoionization efficiency (PIE) and pulsed field ionization photoelectron (PFI-PE) spectra for CS2 have been measured using coherent vacuum ultraviolet (VUV) laser radiation in the energy range of 81 050–82 100 cm−1. The PIE and threshold photoelectron (TPE) spectra for CS2 in the energy range of 80 850–82 750 cm−1 have also been obtained using synchrotron radiation for comparison with results of the VUV laser study. The analysis of the PIE spectra reveals three Rydberg series converging to the excited CS2+(2Π1/2) spin–orbit state. These series, with quantum defects of 1.430, 1.616, and 0.053, are associated with the [2Π1/2]npσu, [2Π1/2]npπu, and [2Π1/2]nfu configurations, respectively. The Stark shift effect on the ionization threshold of CS2 has been examined as a function of dc electric fields (F) in the range of 0.65–1071 V/cm. The observed F dependence of the Stark shift for the ionization onset of CS2 is consistent with the prediction by the classical adiabatic field ionization formula. The extrapolation of the ionization onset to zero F yields accurate values for IE[CS2+(X̃ 2Π3/2)]. This study shows that in order to determine accurate IEs and to probe autoionizing structures for molecular species by PIE measurements, it is necessary to minimize the electric field used for ion extraction. The assignment of Renner–Teller structures resolved in the VUV PFI-PE spectrum is guided by the recent nonresonant two-photon (N2P) PFI-PE and theoretical studies. The analysis of the PFI-PE spectrum also yields accurate values for IE[CS2+(X̃ 2Π3/2,1/2)]. Taking average of the IE values determined by VUV-PFI-PE, N2P-PFI-PE, and Stark field extrapolation methods, we obtain a value of 81 285.7±2.8 cm−1 for IE[CS2+(X̃ 2Π3/2)]. For IE[CS2+(2Π1/2)], we recommend a value of 81 727.1±0.5 cm−1 determined by the Rydberg series analysis. A theoretical simulation of the 2Π3/2(000) and 2Π1/2(000) VUV-PFI-PE band profiles reproduces the observed branching ratio of 1.9±0.3 for CS2+(X̃ 2Π3/2)/CS2+(2Π1/2). The relative intensities of vibronic structures observed in the VUV PFI-PE and TPE spectra are in agreement. Evidence is found, indicating that the strongly (Stark field induced) autoionizing Rydberg state, 17pσu, which is ≈10 cm−1 below the IE of CS2, has a minor contribution to the observed profile for the X̃ 2Π3/2(000) PFI-PE band.