Photodetachments of the metastable nsnp^{2 4}P states of Be^-, Mg^-, and Ca^- ions

Close-coupling calculations are performed on the photodetachments of the metastable nsnp^{2 4}P states of the negative beryllium (n=2), magnesium (n=3), and calcium (n=4) ions by using the R-matrix method. The calculations are carried out with the consideration of excitations of one electron from core. The results show that this effect is much more important for Ca^- than for Be^- and Mg^-. For Be^-, the total and partial cross sections are obtained from the threshold to a photon energy of 0.5 Ry. The calculated total cross section exhibits two ⁴P^o Feshbach resonances and one ⁴D^o shape resonance. Close agreement is found with the available theoretical calculations and the only experimental datum of Pegg and co-workers [Phys. Rev. A 50, 3861 (1994)]. For the Ca^- and Mg^- ions, the total and partial cross sections are calculated from the threshold to a photon energy of 0.25 Ry. For the Ca^- ion, the resonances are found for both ⁴P^o and ⁴D^o symmetries just below the ³D threshold. For the Mg^- ion, only a ⁴P^o resonance is predicted. No rapid increase associated with the opening of the 4s5s ³S channel is found in the total cross section of Ca^-. The rapid increase of the photodetachment cross section of Ca^- in the experiment by Hanstorp et al. [D. Hanstorp et al., Phys. Rev. Lett. 63, 368 (1989)] should not be attributed to the Ca^-(4s4p²⁴P)+hν⃗Ca(4s5s³S)+kp ionization process. This result is consistent with the conclusion of Petrunin and co-workers [Phys. Rev. Lett. 76, 744 (1996)] that there was no Ca^- (4s4p²⁴P) ions observed by Hanstorp et al.