Studies of double photoionisation using a fast pulsed VUV source

Abstract

A lamp giving 5 ns pulses of 10–50 eV photons at 10 kHz repetition rate has been developed and used with a magnetic bottle electron time-of-flight (TOF) spectrometer to study double photoionisation of atoms and molecules. In addition to the spectra of the dications and complete electron pair energy distributions, quantum yields of double ionisation, as defined in this paper, can be determined. Electron pair energy distributions in double photoionisation of Hg, N 2 and CS 2 are presented as representative of results on many other atoms and molecules. Double photoionisation is found to be mainly indirect, dominated by autoionisation of intermediate monocations, for complex atoms, but mainly direct for small molecules in the valence energy range. The difference arises because superexcited states of molecular monocations decay by dissociation to singly charged products.