Photoion spectra in coincidence with threshold electrons near the Ar 2p ionisation limits

Abstract

The yield spectra of Ar2+ and Ar3+ ions in coincidence with threshold electrons near the 2p ionisation limits have been measured using a time-of-flight mass spectrometer and synchrotron radiation. Below the 2p ionisation limits, the Ar2+ yield spectrum exhibits a prominent peak corresponding to the the 2p-14d, 5d and 6d resonance states. It is proposed that these resonance states can decay by emitting threshold electrons through two-step autoionisation, such as Ar 2p-15d to Ar+ 3p-2(1D)6d to Ar2+ 3p-2(3P). Above the 2p ionisation limits, the Ar2+ and Ar3+ yield spectra exhibit two dominant peaks. These peaks show the effect of post-collision interaction, namely shifts of peak positions and asymmetric profiles. The shifts are almost the same in the Ar2+ and Ar3+ yield spectra.