Intrinsic channel closing in strong-field single ionization of H2

The ionization of H2 in intense laser pulses is studied by numerical integration of the time-dependent Schr"odinger equation for a single-active-electron model including the vibrational motion. The electron kinetic-energy spectra in high-order above-threshold ionization are strongly dependent on the vibrational quantum number of the created H2+ ion. For certain vibrational states, the electron yield in the mid-plateau region is strongly enhanced. The effect is attributed to channel closings, which were previously observed in atoms by varying the laser intensity.