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Break-down of the single-active-electron approximation for one-photon ionization of the B $^1Σ_u^+$ state of H$_2$ exposed to intense laser fields

Ionization, excitation, and de-excitation to the ground state is studied theoretically for the first excited singlet state B $^1Σ_u^+$ of H$_2$ exposed to intense laser fields with photon energies in between about 3 eV and 13 eV. A parallel orientation of a linear polarized laser and the molecular axis is considered. Within the dipole and the fixed-nuclei approximations the time-dependent Schrödinger equation describing the electronic motion is solved in full dimensionality and compared to simpler models. A dramatic break-down of the single-active-electron approximation is found and explained to be due to the inadequate description of the final continuum states.