Fine Structure in Energy-Distribution-Difference Ionization-Efficiency Curves

Refinements in the experimental application of the energy‐distribution‐difference (EDD) method for resolving fine structure in ionization‐efficiency curves are presented. These include data acquisition and computer‐processing techniques by which first‐differential EDD curves can be obtained with a good signal‐to‐noise ratio. Using these techniques, a study has been made of the fine structure observed in the first‐differential EDD ionization‐efficiency curves of C₂H₂⁺ and Xe⁺ near their thresholds. Distinct “steps” are resolved in the first‐differential ionization‐efficiency curve of C₂H₂⁺. The energy separations between these steps agree quite well with energy separations between vibrational levels of the C₂H₂⁺ ion determined by photoionization techniques. However, the relative cross sections for these processes determined by electron impact are drastically different from those reported for photoionization. Possible explanations for these differences are discussed. The electron‐impact results for xenon indicate that a large portion of the cross section between the ${}^2{P}_{\text{3\hspace{0.17em}/\hspace{0.17em}2}}$ and ${}^2{P}_{\text{1\hspace{0.17em}/\hspace{0.17em}2}}$ states of Xe⁺ is due to autoionization from known Rydberg levels.