LAUSR

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.