Photoionization of atomic oxygen from 920 to 650 Å

Abstract

Atomic oxygen was produced in a low pressure gas discharge and was equilibrated in a small chamber to eliminate electronically excited species prior to the formation of an atomic beam which was photoionized. Relative photoionization cross sections were measured from 920 to 650 Å at a wavelength resolution (FWHM) of 0.42 Å. From the ratio of threshold intensities corresponding to ionization from the 3P 2, 3P 1, and 3P 0 states of OI, the beam temperature was estimated to be 375 ± 25°K. Autoionization structure was observed corresponding to all of the transitions seen in absorption by Huffman, Larrabee, and Tanaka, including those transitions for which autoionization is forbidden on the basis of Russell-Saunders coupling. The seven lowest energy autoionizing lines were measured at a wavelength resolution (FWHM) of 0.16 Å. The distribution of intensities within a multiplet corresponded to the theoretical intensities predicted for an approximately 375°K beam in all but two cases. These notable exceptions were the two most intense autoionization peaks in the spectra, both of which were forbidden in pure Russell-Saunders coupling. The anomalous intensity distributions may be attributed to competition from emission which preferentially robs intensity from selected transitions within a multiplet, to nonstatistical autoionization probabilities, or to perturbations which alter the relative absorption intensities within a multiplet.