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.

中文翻译：

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氦气的两光子阈值以上电离

多光子电离为氦原子中电子相关性的性质提供了清晰的窗口。在本研究中，最终状态的能量范围一直延伸到接近能量的区域。$ñ=\mathrm{2个}$ 和 $ñ=3$电离阈值，其中双光子电离通过最低阈值之上的连续中间状态进行。我们的计算是使用多通道量子缺陷理论（MQDT）进行的，并且经过简化$\mathrm{[R}$-矩阵方法。使用非均质性评估双光子电离幅度中所有中间状态的总和和积分$\mathrm{[R}$-矩阵方法由Robicheaux和Gao开发。该方法与MQDT的无缝连接使我们能够呈现最终状态Rydberg共振的高分辨率光谱。我们的分析将共振点分类为$ñ=\mathrm{2个}$阈值在他们的群论量子数方面。发现它们的主要衰变通道比这些与单光子电离相关的奇数奇偶态要遵守先前推测的倾向规则，对于这些奇偶性态要弱得多。