IONIZATION, DISSOCIATION, AND LEVEL SHIFTS OF H-2(-FREQUENCY AC FIELD() IN A STRONG DC OR LOW)

Using a complex basis-set expansion of the electronic wave function, w e have calculated the induced shifts and widths of the first few elect ronic Sigma levels of the H-2(+) molecular ion in the presence of a de field, over a range of internuclear separations. We have also calcula ted, through order omega(2), the shifts and widths of the Is and 2p le vels (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 Ba ndrauk [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 loc alized in the higher well of the double-well electronic potential, wit h energetically nearby highly excited states that are localized in the lower well; over-the-barrier ionization from the lower well can proce ed without the impediment of backscattering of the electron from the h ump between the wells. Finally, we have calculated shifts and widths o f various vibrational levels of the electronic ground state and the th reshold intensities for dissociation of H-2(+) from some of these vibr ational levels. We compare our results with the threshold intensity me asured by Ilkov et al. [Phys. Rev. A 51, R2695 (1995)] at the CO2 lase r wavelength.