Alignment-dependent ionization of H2+: From multiphoton ionization to tunneling ionization

Abstract

We investigate the strong-field ionization for the ground state of H${{}_{2}}^{+}$ by numerically solving the three-dimensional (3D) time-dependent Schr\odinger equation (TDSE), and comparisons have been made among the TDSE, the different versions of molecular strong-field approximation (MO-SFA) and the molecular Ammosov-Delone-Krainov (MO-ADK) approximation. The study shows that, for the TDSE results, the ratio of ionization rates between perpendicular and parallel alignments displays a step-like structure against the Keldysh parameter $\ensuremath{\gamma}$. For small internuclear distances, the transition between the steps are found to be around $\ensuremath{\gamma}\ensuremath{\approx}1$ and is recognized as the competition between the multiphoton ionization (MPI) and tunneling ionization (TI). The ionization is more isotropic in the MPI regime. For large internuclear distances, the transition position shifts to larger $\ensuremath{\gamma}$ due to the charge-resonance-enhanced ionization (CREI). Different versions of strong-field ionization theories are compared against the TDSE results.