Absolute photoionization cross section of K+ ions from the 3p to the 3s threshold

The absolute photoionization cross section of ground state K+ ions has been measured from the 3p threshold to above the 3s threshold (31-49 eV) by VUV radiation using a merged ion-photon beam set-up. The cross section, which reaches a maximum value of 32 Mb +/- 10% at 31.8 eV and then decreases with energy, is in overall good agreement with absolute measurements by Peart and Lyon, but the present investigation also reveals the 3s3p(6)np P-2 (n = 4-6) resonances and eliminates a previously unassigned structure at 35.5 eV. The energies, widths and shape parameters of the autoionizing P-2 resonances are determined and compared with recent data from dual-laser plasma measurements and with theoretical predictions. The present experimental set-up will allow absolute photoionization cross sections less than or equal to 0.1 Mb to be measured, making it possible to measure the absolute photoionization cross sections of ions of astrophysical importance.