Threshold electron impact excitation of hydrogen chloride

A high-resolution threshold-electron spectrometer was used to study excitation of HCl molecules O-10 meV above the thresholds for electronically excited states. The spectrum is obtained in an energy region from 4.2-14 eV corresponding to the excitation of lower valence states and Rydberg states below and just above the 2 Pi 3/2, 1/2 , ionisation threshold. This first study of threshold excitation of HCl molecules reveals a considerable difference in relative intensities of some of the states compared with the case of higher-energy collisions. The transition into the lowest 3 Pi valence state is reported for the first time. The threshold ionisation region is discussed in terms of the superposition of the correlation-dominated direct ionisation process and the excitation function of an autoionising level decaying by releasing a zero-energy electron.