Complete double photoionisation spectra of small molecules from TOF-PEPECO measurements

Spectra of the dications derived by photoionisation from HBr, N-2, CO, NO, O-2, H2S, CO2, OCS, CS2, N2O and SO2 in the valence energy range are presented. The spectra are taken by a method which accepts all photoelectrons irrespective of energy or emission angle, and offers a resolution a few tens of meV, depending on the energy. The spectra seem to be relatively unaffected by molecular autoionisation. The full electron-electron energy distributions in double photoionisation of these molecules show autoionisation of atomic fragments in several cases. Brief descriptions of the double photoionisation spectra of the polyatomic molecules C2H2, CH4, NH3, C2H4, C4H6, C6H6 and C6F6 are also given. An approximate relationship between excitation energies of dications and orbital energies taken from photoelectron spectra using Koopmans' theorem is proposed. It is suggested that relative intensities of double photoionisation bands for different final states may be connected to details of the electron-electron correlations in the parent molecules. (C) 2003 Elsevier B.V. All rights reserved.