Two-photon Above Threshold Ionization of Helium

TL;DR

This study uses advanced quantum defect and R-matrix methods to analyze two-photon ionization of helium, revealing detailed resonance structures and decay behaviors near ionization thresholds.

Contribution

It introduces a combined MQDT and R-matrix approach to compute high-resolution spectra of helium's two-photon ionization, classifying resonances and decay channels.

Findings

Resonances classified by group theory quantum numbers.

Weak adherence to propensity rules for even parity states.

High-resolution spectra of Rydberg resonances near thresholds.

Abstract

Multiphoton ionization provides a clear window into the nature of electron correlations in the helium atom. In the present study, the final state energy range extends up to the region near the $N=2$ and $N=3$ ionization thresholds, where two-photon ionization proceeds via continuum intermediate states above the lowest threshold. Our calculations are performed using multichannel quantum defect theory (MQDT) and the streamlined R-matrix method. The sum and integration over all intermediate states in the two-photon ionization amplitude is evaluated using the inhomogeneous R-matrix method developed by Robicheaux and Gao. The seamless connection of that method with MQDT allows us to present high resolution spectra of the final state Rydberg resonances. Our analysis classifies the resonances above the $N=2$ threshold in terms of their group theory quantum numbers. Their dominant decay channels are found to obey the previously conjectured propensity rule far more weakly for these even parity states than was observed for the odd-parity states relevant to single photon ionization.