Orientation dependence of field ionization of the hydrogen molecular ion

We present calculations of quasienergies of the lower states of the hydrogen molecular ion in a strong external electric field. Arbitrary orientations of the molecular axis are considered for the first time. We study the variation of ionization rate with internuclear separation. This work is relevant to the angular and energy distributions of dissociative ionization fragments. The ionization rates of the lowest states are, in general, very sensitive to orientation: for certain orientations the excited state is more stable than the ground state. The rates for the excited states rise very sharply as the field increases. For very intense fields ionization arises predominantly from small internuclear distances rather than large critical bond lengths. For low-frequency intense laser fields this would imply that the critical region for nonclassical Coulomb explosions occurs very close to the neutral bonding distance. The dipole moment is found to be easily decoupled from the molecular axis for these field strengths giving rise to a broad angular distribution of photofragments. Comparisons with recent complementary calculations for a parallel orientation confirm the accuracy of our results.