Line shapes and line positions in PFI-ZEKE photoelectron and MATI spectra of positively charged ions

Abstract

A model developed to calculate the positions and shapes of the lines observed in pulsed-field-ionisation zero-kinetic-energy photoelectron (PFI-ZEKE-PE) and mass-analysed threshold ionisation (MATI) spectra of neutral molecules [Hollenstein et al., J. Chem. Phys. 115, 5461 (2001)] is generalised to positively charged systems of arbitrary charge. The model assumes that the pulsed field ionisation of the high Rydberg states detected experimentally is diabatic and that their ionisation rates in the presence of electric fields can be calculated with the asymptotic expressions of Damburg and Kolosov [J. Phys. B 12, 2637 (1979)]. We use this model to calculate the shifts of the lines with respect to the field-free ionisation thresholds and their widths and find that, to an excellent approximation, both the line shifts and line widths scale with . We also derive simple analytical expressions that can be used to estimate the line shifts and line widths for typical electric-field pulse sequences used in PFI-ZEKE-PE and MATI spectroscopy of positively charged cations.