Molecular beam photoionization study of carbon disulfide, carbon disulfide dimer and clusters

Abstract

A new molecular beam photoionization mass spectrometer which is significantly superior to previous similar instruments in both wavelength resolution and ion intensity is described. Photoionization efficiency (PIE) data for CS2+ near the threshold (1210–1232 and 1075–1130 Å) were obtained with a resolution of 0.14 Å (FWHM). Rydberg series I, n=16–24, which converges to the spin–orbit excited state, 2Π1/2, of CS2+ was resolved on the first vibrational step in the PIE data. Observation of the autoionization resonance, series I (n=16), indicates that the value for the first ionization energy (I.E.) for CS2 obtained in previous absorption experiments (Ref. 23) is too high. The I.E.’s for the 2Π3/2 and 2Π1/2 states of CS2 were determined to be 10.0685±0.0020 and 10.1230±0.0020 eV, respectively. PIE data for (CS2)n+ with n ranging from 2 to 5 have been obtained in the wavelength region from 650 to 1350 Å with this apparatus. The observed I.E.’s for (CS2)2, (CS2)3, (CS2)4, and (CS2)5 are 9.36±0.02, 9.22±0.02, 9.10±0.02, and 9.04±0.02 eV, respectively. From the measured ionization energy, and the estimated dissociation energy (0.05 eV) of (CS2)2, the binding energy for CS2+⋅CS2 is deduced to be 17.5±1 kcal/mol. This is in agreement with the value obtained previously in a high pressure mass spectrometry experiment. The Rydberg series for CS2, which are manifested as autoionization lines in the PIE data for (CS2)2+ in the region 850–960 Å, are red shifted in energy by ∼0.075 eV for Rydberg series III and ∼0.14 eV for Rydberg series IV. Adapting the most recent assignment of Rydberg series III and IV by Larzilliere et al. symmetry arguments support a linear or near linear geometry for the carbon disulfide dimer.