Photoion spectra in coincidence with threshold electrons near the Xe 4d ionization limits

The yield spectra of Xe 2+ and Xe 3+ ions near the 4d ionization limits have been measured by a threshold electron-photoion coincidence method. These spectra consist of some prominent peaks corresponding to the 4d -1 np Rydberg states and two dominant peaks corresponding to 4d threshold ionization. It is proposed that threshold-electron emission from the inner-shell resonance states is due to not only shake-off (e.g. Xe 4d -1 6p to Xe 2+ 5p -2 ) but also two-step autoionization (e.g. Xe 4d -1 6p to Xe + 5s -1 5p -1 7p to Xe 2+ 5s -1 5p -1 ). The two-step autoionization process plays an important role in decays from the lower n Rydberg states. Two dominant peaks somewhat above the 4d ionization limits show the effect of post-collision interaction, namely, shifts of peak positions and asymmetric profiles. The shifts are almost the same in the Xe 2+ and Xe 3+ spectra.