Rise and fall of the 4d¹⁰→4d⁹4f resonance in the Xe isoelectronic sequence

The extreme ultraviolet photoabsorption spectrum of a laser-produced lanthanum plasma has been recorded and found to contain a number of discrete features in the 130-eV region. These have been analyzed as 4d10→4d9nf,np transitions in La3+. We show that the 4f transition, which is expected to be the strongest, is not in evidence. The reason is that this resonance, after the collapse of the 4f wave function, has a large autoionization width. We conclude that the 4f orbital in Ba2+ is only partially collapsed, which settles a long-standing discussion of this point