Ionization, dissociation, and level shifts of H-2(+) in a strong dc or low-frequency ac field

Using a complex basis-set expansion of the electronic wave function, we have calculated the induced shifts and widths of the first few electronic Sigma levels of the H-2(+) molecular ion in the presence of a de field, over a range of internuclear separations. We have also calculated, through order omega(2), the shifts and widths of the Is and 2p levels (we use the field-free united-atom-limit quantum numbers) when an ac field of low frequency omega is present. In accord with Zuo and Bandrauk [Phys. Rev. A 52, R2511 (1995)], we find that the 2p ionization width exhibits peaks as the internuclear separation is increased. We attribute these resonances to the mixing of the 2p state, which is localized in the higher well of the double-well electronic potential, with energetically nearby highly excited states that are localized in the lower well; over-the-barrier ionization from the lower well can proceed without the impediment of backscattering of the electron from the hump between the wells. Finally, we have calculated shifts and widths of various vibrational levels of the electronic ground state and the threshold intensities for dissociation of H-2(+) from some of these vibrational levels. We compare our results with the threshold intensity measured by Ilkov et al. [Phys. Rev. A 51, R2695 (1995)] at the CO2 laser wavelength.