Determination of Partial Photoionization Cross Sections by Photoelectron Spectroscopy

Abstract

A photoelectron energy sepectrometer has been used to determine partial photoionization cross sections of molecular oxygen, nitrogen, and water vapor for monochromatic incident radiation in the wavelength range 584–900 Å. For each gas, cross sections have been determined for transitions to particular ionic states which have been identified by measuring the corresponding photoelectron energy. For molecular oxygen, cross sections have been determined for transitions to the X 2Πg, (a 4Πu+A 2Πu), b 4Σg−, and 2Σg− states of O2+; and at incident wavelengths longer than 680 Å a process of fluorescent autoionization involving quasi-stable states of the neutral oxygen molecule has also been observed. Molecular nitrogen cross sections corresponding to transitions to the X 2Σg+, A 2Πu, and B 2Σu+ states of N2+ have been determined. For water vapor, cross sections corresponding to transitions to the 2B1 and 2A1 states of H2O+ have been observed in addition to processes of dissociative ionization. In all cases the measurements illustrate the competition between various modes of decay which are energetically possible for incident radiation of a given wavelength.