ORIENTATION DEPENDENCE OF FIELD-IONIZATION OF THE HYDROGEN MOLECULAR ION

We present calculations of quasienergies of the lower states of the hy drogen molecular ion in a strong external electric field. Arbitrary or ientations 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 disso ciative ionization fragments. The ionization rates of the lowest state s are, in general, very sensitive to orientation: for certain orientat ions 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 internu clear distances rather than large critical bond lengths. For low-frequ ency intense laser fields this would imply that the critical region fo r nonclassical Coulomb explosions occurs very close to the neutral bon ding distance. The dipole moment is found to be easily decoupled from the molecular axis for these field strengths giving rise to a broad an gular distribution of photofragments. Comparisons with recent compleme ntary calculations for a parallel orientation confirm the accuracy of our results.