Studies of electron correlation in the photoionization process

Electron correlation is a result of the interaction of two or more electrons confined in a region of space, and may conveniently be treated under the formalism of configuration interaction (CI). Photoionization provides a rather direct experimental method for studying CI. The types of CI involved in the photoionization process can be divided into three categories: initial-state configuration interaction (ISCI), final-ionic-state configuration interaction (FISCI), and continuum-state configuration interaction (CSCI). The photoelecton spectroscopy of Ba, Sm, Eu, and Yb was studied using both HeI (22.22 eV) and NeI (16.85 eV) radiation. Satellite structure observed in these spectra using NeI (and for Yb, HeI also) radiation could be satisfactorily explained by ISCI alone. The HeI spectra of Sm, Eu, and, in particular, Ba showed dramatic changes in the satellite population which could only be explained by autoionization, a special form of CSCI. The detailed nature of this mechanism was explored in Ba with synchrotron radiation. It was found that the autoionizing level decays preferentially via an Auger-type mechanism. Further insight into autoionization was gained in the electron impact/ejected electron spectra of Ba (5p66s2) and Yb (5p64f146s2). Autoionizing levels excited above the first (5p)-1 threshold decayed primarily in a two-step Auger process, leading to a relatively large number of doubly charged ions. For autoionizing states below the (5p)-1 limit, decay appears to go to ground and excited states of the singly charged ion. The first experimental determination of the lifetime of the XeII 5s5p62S/sub 1/2/ state yielded a value of 34.4(6) ns. Reasonable agreement with theory could only be reached by including both FISCI and relativistic effects in calculating the lifetime. 173 references, 43 figures, 10 tables