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Angular dependence of strong field sequential double ionization for neon and acetylene simulated with time-dependent configuration interaction using CIS and CISD-IP.

Strong field ionization is fundamentally important for attosecond spectroscopy and coherence control. However, the modeling beyond the single active electron approximation is still difficult. Time-dependent configuration interaction with singly excited configurations and a complex absorbing potential (TDCIS-CAP), can be used to simulate single and double ionization by intense laser fields. When the monocation does not have degenerate states, TDCIS-CAP starting from a Hartree-Fock calculation of the cation is suitable for simulating the second ionization step. When the monocation has two or more degenerate states, the simulations should treat these degenerate states equivalently. CISD-IP (single and double excitation configuration interaction with ionization) can be used to treat degenerate states of the cation on an equal footing by representing the cation wavefunctions with ionizing single (1 hole) and double (2 holes/1 particle) excitations from the neutral molecule. Since CISD-IP includes single excitations for each of the monocation states, time dependent CISD-IP with a complex absorbing potential (TDCISDIP-CAP) can also be used to simulate ionization to the dications states. In this work, TDCIS-CAP and TDCISDIP-CAP have been used to simulate the angular dependence of ionization of the neon cation and acetylene cation. In both cases, the second electron is ionized predominantly from an orbital perpendicular to the orbital involved in the first ionization. The TDCISDIP-CAP simulations show some features involving interactions between the monocation states that are not seen in the TDCIS-CAP simulations.