PHOTOELECTRON SPECTROSCOPY OF AUTOIONIZATION PEAKS

A vacuum ultraviolet monochromator has been combined with a retarding-field electron-energy analyzer to study the distribution of photoelectrons from autoionizing states of hydrogen and nitrogen. In most cases involving hydrogen, the photoelectrons produced are monoenergetic and correspond to autoionizing transitions in which Δv is a minimum, i.e., the ejected electrons have the minimum kinetic energy. There is one possible exception to this generalization. Different mechanisms appear to be operative in the nitrogen autoionizations. In general, several bundles of electrons, corresponding to the formation of several ionic states, are observed. Whereas vibrational relaxation of the core appears to explain the results with hydrogen, an alternative mechanism involving configuration interaction has been examined in an attempt to rationalize the nitrogen results. This proposed mechanism implies that relative transition probabilities in the autoionization process are dominated by Franck-Condon factors connecting the quasidiscrete state and the ionic state. The ratio of Franck-Condon factors is very sensitive to the choice of internuclear distance for the quasidiscrete state. In many cases, it is possible to find a plausible internuclear distance for which the ratio of Franck-Condon factors agrees with experiment. If the Franck-Condon factors are indeed the major determinant of relative transition probabilities, photoelectron spectroscopy offers a method for determining the internuclear distances of the quasidiscrete states. One autoionizing transition in nitrogen, as yet unassigned, does not seem to fit this mechanism.