Ca negative-ion binding energy

Abstract

We report our recent results of the measurement of the binding energy of the ground state of the calcium negative ion using electric field dissociation and accelerator mass spectrometry. The electron affinity of Ca was measured to be 17.5 -] meV for a p-wave electron emission, which is in good agreement with recent photodetachment experiments performed by Walter and Peterson [Phys. Rev. Lett. 68, 2281 (1992)]. PACS number(s): 35.10.Hn We report in this Rapid Communication our recent results of the measurement of the binding energy of the ground state of the calcium negative ion using electric field dissociation and accelerator mass spectrometry (AMS). The value of the electron affinity of Ca has recently become controversial, having been measured using laser photodetachment methods by two different groups leading to two irreconcilable results. Pegg et al. [1] have observed Eb =43+'7 meV while Walter and Peterson [2] obtained Eg =18.4+ 2.5 meV. Also a measurement of the black-body-radiation-induced decay of Ca adds information concerning the possible metastable states of the calcium negative ion [3] and suggests that the lower value of the electron affinity might be more likely.While these measurements were in progress [2,3], we were studying the negative ion of calcium. Combining the fact that weakly bound negative ions are destroyed in high electric fields with the sensitivity of AMS, we were able to track the destruction of Ca over 4 orders of magnitudes of intensity. Much to our surprise, Ca proved to be destroyed at a much lower electric field than expected from the earlier photodetachment measurement (i.e. , 43+'7 meV) and actually below the designed operating voltage range of the apparatus [4]. The value of the electron affinity of calcium deduced from our measurements is 17.5-+2 meV which is consistent with the later photodetachment measurement [2]. Even though the existence of stable or metastable calcium negative ions has been established for more than 20 years [5], the actual stability of the negative ions of atoms with closed subshells such as Ca, Sr, Ba, and Ra have been questioned for a long time. Theoretical calculations using multiconfiguration Hartree-Fock theory have shown that the extra electron of the ground state should fill an np orbital instead of the expected (n -1)d orbital [6]. The calcium electron a%nity has also been used to calibrate the theoretical predictions regarding the binding energy of negative ions of this series which has been extended to other atoms with closed subshells like Yb ([Xe]4f ' 6s ) [7]. This makes the accurate determination of the calcium binding energy even more important. According to the calculations mentioned above, the ground state of Ca is dominated by the 4s 4p P configuration. The spin-orbit splitting between the lower state P]y2 and the excited state P3y2 has been calculated by Vosko, Chevary, and Mayer [7] and by Dzuba et al. [8]. Their calculations indicated a fine-structure splitting of 5.4 and 6.9 meV, respectively. The theory of electric dissocia...

The interaction of metastable helium atoms with alkaline earth atoms: He*(23 S,21 S)+Mg, Ca, Sr and Ba

“…The excitation energies are reproduced with relative errors smaller than 0.7% with a single exception for Ca>(3d) where it amounts to 1.5%. The calculated electron affinity matches the recent experimental values [45,46] within the experimental error; for a comparison with previous theoretical results, see [47]. The calculated dipole polarizability quoted in Table 3 is probably more reliable than the experimental value.…”

Positronic complexes with unnatural parity

“…The critical polarizability for the alkali sequence is somewhere between 142 and 173 a 3 0 . The po-larizability of calcium, which just binds an electron with an electron affinity of ≈ 7 × 10 −4 hartree [42,43,44], is about 160 a 3 0 [38,45]. B. PsH…”

Observation of the negative ions:Ra−,Pa−Zhao1, Nadeau2, Garwan3 et al. 1993Phys. Rev. ASelf Cite12190View full textAdd to dashboardCite

“…The results of other theoretical calculations published more recently [3 -5] have not only confirmed the stability of Ca, but also predicted that Sr, Ba, and Ra with similar configurations (ns np) should all be stable. The electron affinity of Ca has now been measured again to high precision by two different methods [6,7], and the negative ions of Sr and Ba have been observed [g]. The experimental study of Ra possesses a challenge due to the fact that no stable radium isotope exists.…”