Photoionization of 4d-electrons in singly charged Xe ions

Using a photon-ion merged-beam technique, we measured the relative yield spectra of Xe2+ and Xe3+ ions created by photoionization of Xe+ ions as a function of the photon energy in the 4d ionization region. The Xe3+ ion production, i.e. the double-electron ionization process is found to be dominant throughout the energy range investigated; A broad structure of 4d excitation ionization around 100 eV photon energy was observed in the Xe3+ yield spectrum. A spectral shoulder was observed for Xe3+ at aro;nd 85 eV, which was absent in the photoionization of neutral Xe. Some discrete lines were observed for both the Xe2+- and Xe3+-yield spectra below 75 eV. The experimental spectra were analysed by a multiconfiguration Dirac-Fock calculation. The largest structure at around 100 eV is attributable to 4d(10)5s(2)5p(5) --> 4d(9)4f5s(2)5p(4)np (n = 6, 7) two-electron transitions. The discrete lines were found to be due to 4d --> np, nf transitions. A serious 4f-orbital collapse is suspected in the 4fnp two-electron excited states, whereas the collapse is moderate in the singly excited 4f-orbitals.