The 4d-Photoabsorption of Ba, Ba+, and Ba++: A View of Shell Collapse vs. Contraction

Abstract

Ba with Z=56 is at the edge of 4f collapse; neutral ground state atoms with Z 56 have a “collapsed” 4f orbital with rav∼1a0. Since the 4d orbital is collapsed (rav∼1a0) the nature of the 4d‐absorption is expected to depend critically on whether the 4f orbital can be considered “collapsed” or not. Using a laser technique to prepare dense homogenous columns of Ba+ and Ba++ we have obtained the 4d photoabsorption spectra of Ba, Ba+ and Ba++. This technique thus allows us to observe the effects on the 4f orbitals of the increased nuclear attraction experienced in the absence of screening by the 6s electrons. It is found that exchange effects are critically important in configurations of the type 4d94f, and that progressive “contraction” rather than “collapse” is a more appropriate description under such conditions.