Photoelectron spectroscopy with Zr Mζ (151 eV) radiation. A study of the variation of relative photoionization cross sections of molecules containing first row atoms (C, N, O) with exciting radiation from HeI to Mg Kα limits

Abstract

Relative perpendicular photoionization intensities for the noble gases Ne and Ar and the molecules CH₄, NH₃, H₂O, CO, N₂, and CO₂ have been measured with He^I (21.2 eV), He^II (α, 40.8 eV and β, 48.4 eV) resonance lines and Zr M_ζ (151 eV) and Mg K_α (1253.6 eV) soft x rays using the same instrument for all measurements. New valence shell spectra of NH₃, CO, and CO₂ obtained with Zr M_ζ radiation are illustrated. The experimental values are compared to values calculated for each radiation by the method of Ellison [J. Chem. Phys. 61, 507 (1974)]. Reasonable success was achieved by the theory which appears to falter most seriously with the inner valence orbitals of dominant 2s atomic character. This failure is most likely due to the improper treatment of the Cooper minimum. Attempts to improve the agreement between observed and calculated relative intensities by inclusion of multiple electron processes (’’shake up’’ transitions) are discussed.