Mass-Spectrometric Study of Photoionization VI O_2, CO_2, COS, and CS_2†

Abstract

Photoionization-efficiency curves are obtained for the molecule and fragment ions of the subject molecules in the wavelength region extending from onset of ionization to 600 Å. The initial onset of O2+ is observed at 12.072 eV. Autoionization in the continuum is correlated with various progressions of Rydberg series. Curves for both O+ and O− ions formed by the ion–pair process from O2 are obtained and 1.48 eV is derived for the electron affinity of oxygen atoms. The shape of the CO2+ curve including structure ascribed to autoionization is discussed. The unresolved 2Пg-doublet threshold is observed at 13.77 eV with the first vibrational level at 13.93 eV. The onset of the O+-fragment ion indicates about 0.04 eV excess energy in the dissociation process. It is suggested that dissociative ionization occurs from the autoionizing Rydberg level just above the calculated threshold value. Partially resolved doublet components of COS+ are observed at 11.18 and 11.22 eV, respectively. Intense autoionization is observed. Various thermochemical values are calculated from the fragment-ion thresholds. The doublet components of the ion ground state of CS2+ are observed at 10.059 and 10.112 eV, respectively. Intense autoionization is observed at wavelengths which are in excellent agreement with known Rydberg levels for the molecule. Although the S+ ion is formed with excess energy, the CS+ ion gives a value of 11.71 eV for the ionization energy of the CS radical. This is in agreement with but more precise than a directly measured electron-impact value.