Vacuum-ultraviolet absorption spectrum of the Rb⁺ ion in a laser-generated plasma

Discrete features due to 3d photoexcitation in Rb⁺ have been observed and measured in the 100-140 eV photon energy range using the dual laser-produced plasma technique. Lines due to 3d^l04s²4p⁶-3d⁹4s²4p⁶np (n = 5-8) are identified with the aid of configuration-interaction Hartree-Fock calculations. An upper limit of 0.1 eV for the linewidths of the 3d⁹4s²4p⁶5p and 3d⁹4s²4p⁶6p levels was deduced from the data. This value is comparable to Auger widths obtained from electron spectroscopy of near neighbours atomic Kr and Rb. The Hartree-Fock calculations also predict that the np electron is largely a spectator in the decay of the 3d^-1np states. The overall appearance of the spectrum is accounted for by a relativistic time-dependent local density approximation (RTDLDA) calculation. Above the 3d threshold, the relative photoionization cross section is seen to increase monotonically reaching a peak near 200 eV. This observation is explained as a delayed onset in the 3d-εf cross section.