Experimental study of 4f wavefunction contraction: 4d-photoionization of low-charged ions of I, Xe, Cs and Ba

The photon-ion merged beam technique has been used to measure the ionization cross sections of I-, Cs+, Ba+ and Ba2+ ions in the energy region from 40 to 185 eV, which is dominated by photoexcitation from the 4d shell (the experimental data are available at http://www.ifa.au.dk/amo/atomphys/atomphys.htm). Within the experimental accuracy, the total oscillator strengths for these ions and for the I+, I2+, Xe+ and Xe2+ ions, that were recently studied by some of the present authors, are identical, with the contribution from the 4d → np or nf resonances becoming more important with increasing nuclear charge and ionization stage, and with the maximum value of the continuum cross section increasing as the nuclear number is enlarged. The present data support the assumption that the 4f wavefunctions contract gradually with increasing ionization, both along isonuclear and isoelectronic sequences, for ionic charges ranging from -1 to +2.