HeI α photoelectron modulation spectroscopy of H2 and D2

Abstract

A new method for the determination of photoionization cross sections is presented. The technique is used to obtain cross sections of H2 and D2 relative to those of H and D atoms from accurate HeI α photoelectron intensity measurements. With the virtually exact atomic cross section calculations available for H and D atoms our experiments provide absolute H2 and D2 photoionization cross sections. Ultraviolet photoelectron (UV PE) spectra of vibrationally excited hydrogen (v″=1,2) and deuterium (v″=1,2,3), free from overlapping bands due to vibrational ground state molecules, are presented for the first time. Positions and intensities of all observed features are excellently reproduced using well-known molecular and ionic constants and accurate ab initio potential energy curves. Combination of the cross section determinations of H2 and D2 with intensity measurements of vibrationally excited H2 and D2 reveals some qualitative information on H*2 and D*2 cross sections and β parameters.