Ionization, dissociation, and level shifts of [Formula Presented] 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 Σ levels of the [Formula Presented] molecular ion in the presence of a dc field, over a range of internuclear separations. We have also calculated, through order [Formula Presented], the shifts and widths of the 1s and 2p levels (we use the field-free united-atom-limit quantum numbers) when an ac field of low frequency ω 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 [Formula Presented] 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 [Formula Presented] laser wavelength. © 1996 The American Physical Society.