4D PHOTOIONIZATION OF SINGLY-CHARGED IONS

The relative partial photoion-yield spectra of A(2+) and A(3+) from th e 4d photoionization of A(+) (A=Xe, Pa, and Eu) have been measured by using the ion-photon merged-beam technique in the 4d-photoionization e nergy region. A broad 'giant resonance' peak was observed for all thre e target species. The width of the resonance was wider in Xe+ and narr ower in Eu+. Some apparent differences from the photoionization of neu tral atoms were observed in the spectra for Xe+ and Ba+. For the Xe+ t arget, besides the giant-resonance peak at around 100 eV, a shoulder a round 85 eV and sharp strong peaks at 55.4 and 56.1 eV were observed. By comparing the results with those from a multi-configuration Dirac-F ock (MCDF) calculation, the strong peaks were assigned as 4d --> 5p, a nd the shoulder and the main peak were assumed to be the 4d --> epsilo n f resonance and the 4d5p --> 4fnp 'shake-up' transitions, respective ly. In the case of Ba+, some prominent peaks on the rising slope of th e giant resonance were observed in the spectra. On the contrary, in th e Eu+ photoionization, the observed spectral shapes and peak positions were almost the same as those in neutral Eu case, except for the spli t peak of the giant resonance observed in the Eu2+ spectrum which show ed a 'Beutler-Fano type' profile. This difference in the ionic state e ffect on the giant-resonance profile between Xe, Ba, and Eu is conside red to be due to the difference of the depth of 'orbital collapse.' Wh en the obtained giant resonance spectra ale compared to each other, th e Z dependence of the resonance profile looks the same as that of the neutral lanthanides. Also our very recent results for the photoionizat ion of Xe2+ and Xe3+ targets show vividly how a systematic change of t he primary charge state affects the 4d giant resonance in an isonuclea r sequence. These results are discussed in terms of the 'orbital colla pse' picture.