Population trapping in bound states during IR-assisted ultra-fast photoionization of Ne$^+$

TL;DR

This study explores how Ne$^+$ ions undergo ultra-fast photoionization when exposed to combined IR and harmonic laser pulses, revealing population trapping in Rydberg states and the importance of multi-threshold calculations.

Contribution

It demonstrates the role of Rydberg state population trapping in IR-assisted photoionization and highlights the necessity of multi-threshold models for accurate predictions.

Findings

Ionization yield modulations depend on time delay.

Population trapping occurs in low Rydberg states.

Single-threshold models are insufficient.

Abstract

We have investigated photoionization of Ne$^+$ in the combined field of a short infra-red laser pulse and a delayed ultra-short pulse of the infra-red laser's 23$^r$$^d$ harmonic. We observe an ionization yield compatible with a picture in which one electron gets excited into Rydberg states by the harmonic laser field and is subsequently removed by the infra-red laser field. Modulations are seen in the ionization yield as a function of time delay. These modulations originate from the trapping of population in low members of the Rydberg series with different states being populated at different ranges of delay times. The calculations further demonstrate that single-threshold calculations cannot reproduce the Ne$^+$ photoionization yields obtained in multi-threshold calculations.