Photo-Ionization Efficiency Curves. Measurement of Ionization Potentials and Interpretation of Fine Structure

A method is described for measuring photo-ionization efficiency (PE) curves over the energy range 9–14 eV. The method is experimentally easier than measuring the photo-ionization cross section and gives essentially the same result. PE curves are given for Xe, O2, and NO and first ionization potentials are determined; Xe, 12.129±0.002 eV; O2, 12.065±0.003 eV; and NO, 9.250±0.005 eV. It has been established that steps in the PE curve of NO give a quantitative measure of the energies of the vibrational levels in the NO+ ion and that peaks in the curves of all three gases correspond to neutral levels that auto-ionize. The probability laws for ionization and auto-ionization are discussed.