Study of OCS by electron energy loss spectroscopy

A high resolution study of the OCS molecule is presented using low-energy electron energy loss spectroscopy. Measurements have been carried out at various incident electron energies and scattering angles, covering the whole range from 5 to about 17 eV. The high energy resolution (∼30 meV) has allowed the detailed analysis of the vibrational progressions and Rydberg series, and some identifications are supported by the plots of their differential cross sections. For the first time, the two overlapping progressions of the (10σ) 1Π state, around 7.4 eV, are resolved by electron impact. A new triplet state is observed at 7.59 eV, assigned to the (11σ) 3Π known state, whose 1Π term would correspond to a known progression lying under the strong 1Σ+ band. Above the first ionization limit, the spectrum presents a striking similarity with the photoionization efficiency curves, with the strong autoionizing features which form Rydberg series converging to the ? 2Σ+ state of OCS+. Even the weak series VIII is present, appearing as a small dip. Further measurements carried out at low residual electron energy (?5 eV) reveal that such features then become peaks of strength comparable with the other features. Typical triplet behavior are pointed out for some of them. Two new series (X and XI) are thus proposed, while another feature is assigned to the n = 3 term of series VII converging to the ? 2Σ+ state. For all of the series observed in this region, the quantum defects are determined and orbital assignments are proposed.