Abstract
We investigate the process of two-photon double ionization (TPDI) of the metastable helium ${}^{1,3}S$ $1s2s$ states. The process has been simulated within a fully $ab$ $initio$ numerical framework, solving the time-dependent Schr\"odinger equation in full dimensionality for the two interacting electrons, in a $B$-spline-based methodology. The presence of doubly excited (autoionizing) states in the direct TPDI regime causes resonance-enhanced multiphoton ionization, and we demonstrate this effect by accessing the ${}^{1,3}P$ $2s2p$ doubly excited states. Fully converged theoretical calculations are presented, and a well-defined cross section is extracted for the direct TPDI process and compared in the context of its analogous process in the helium ground state. In addition, the resonance-enhanced two-photon double-ionization mechanism is explored, and we discuss how this process differs from both direct and sequential ionization processes.
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