Photodetachments of the metastablensnp24Pstates ofBe−,Mg−,andCa−ions

Abstract

Close-coupling calculations are performed on the photodetachments of the metastable ${\mathrm{nsnp}}^{2}$ ${}^{4}P$ 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 ${\mathrm{Ca}}^{\ensuremath{-}}$ than for ${\mathrm{Be}}^{\ensuremath{-}}$ and ${\mathrm{Mg}}^{\ensuremath{-}}.$ For ${\mathrm{Be}}^{\ensuremath{-}},$ 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 ${}^{4}{P}^{o}$ Feshbach resonances and one ${}^{4}{D}^{o}$ 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 ${\mathrm{Ca}}^{\ensuremath{-}}$ and ${\mathrm{Mg}}^{\ensuremath{-}}$ ions, the total and partial cross sections are calculated from the threshold to a photon energy of 0.25 Ry. For the ${\mathrm{Ca}}^{\ensuremath{-}}$ ion, the resonances are found for both ${}^{4}{P}^{o}$ and ${}^{4}{D}^{o}$ symmetries just below the ${}^{3}D$ threshold. For the ${\mathrm{Mg}}^{\ensuremath{-}}$ ion, only a ${}^{4}{P}^{o}$ resonance is predicted. No rapid increase associated with the opening of the $4s5s{ }^{3}S$ channel is found in the total cross section of ${\mathrm{Ca}}^{\ensuremath{-}}.$ The rapid increase of the photodetachment cross section of ${\mathrm{Ca}}^{\ensuremath{-}}$ in the experiment by Hanstorp et al. [D. Hanstorp et al., Phys. Rev. Lett. 63, 368 (1989)] should not be attributed to the ${\mathrm{Ca}}^{\ensuremath{-}}{(4s4p}^{2}{}^{4}P)+h\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\nu}}\mathrm{Ca}(4s5s{}^{3}S)+kp$ ionization process. This result is consistent with the conclusion of Petrunin and co-workers [Phys. Rev. Lett. 76, 744 (1996)] that there was no ${\mathrm{Ca}}^{\ensuremath{-}}$ ${(4s4p}^{2}{}^{4}P)$ ions observed by Hanstorp et al.