The structure of exotic atoms containing positrons and positronium

Abstract

The structures of a number of exotic atoms with an attached positron or positronium atom are studied using a large-scale variational expansion in terms of a basis of explicitly correlated Gaussian functions. The binding energies and annihilation rates for seven exotic species with electronically stable ground states, namely HPs, Lie + , LiPs, Bee + , Nae + , NaPs and Mge + have been predicted. The binding energy for HPs, 0.038 1944 Hartree, is the largest attained so far. Two of the species, Lie + and Nae + , with approximate binding energies of 0.0024 and 0.0005 Hartree respectively, are seen to have structures best described as a positronium atom orbiting a residual Li + or Na + positively charged core. The Bee + atom with an approximate binding energy of 0.0028 Hartree is best characterized as a positron orbiting a polarized Be core. The binding energy of the Mge + ground state, 0.014 Hartree, is larger than that of any other positronic atom (a neutral atom with an attached positron). The LiPs and NaPs atoms, with approximate binding energies of 0.012 and 0.0072 Hartree respectively, have structures similar to HPs although the binding energies are smaller and the valence electrons and the positron are found at larger distances from the nucleus.

Stability of few-charge systems in quantum mechanics

“…Meanwhile, variational calculations have been pushed to higher accuracy. See, for instance, [150,155,160,161,162,163,164,165,166], and references therein. For instance, Zong-Chao Yan and Ho [166] obtained x ≃ 0.052.…”

Positron scattering and annihilation from hydrogenlike ions

“…This effect will be called the clustering effect and is well known in condensed-matter systems [33,34], positron-atom scattering systems [20,29,30,36], and positron-atom bound states [19,37].…”

Comment on “Semiemperical Approach to Positron Annihilation in Molecules”

“…(1) to Bee 1 , Mge 1 , LiPs, NaPs, and KPs implies pickoff annihilation rates of 10 11 s 21 or greater. LiPs [5], NaPs [5], and KPs [6] all have annihilation rates of about 2 3 10 9 s 21 , while those of Bee 1 [5] and Mge 1 [5] 7] have annihilation rates (per positron) of 2.45 3 10 9 and 2.3 3 10 9 s 21 , respectively. Pickoff annihilation contributes for both systems, but leads only to an annihilation rate slightly larger than G c .…”

“…The evidence for exotic systems that bind a positron suggests that positron annihilation is dominated by direct annihilation with the cluster electron [5,6,8]. The cluster annihilation rate G c represents a natural upper limit to the annihilation rate in an atomic or molecular environment since the positron can form a cluster with only one electron.…”

“…The cluster annihilation rate G c represents a natural upper limit to the annihilation rate in an atomic or molecular environment since the positron can form a cluster with only one electron. While pickoff annihilation can slightly increase the annihilation rate, the annihilation rate is largely determined by the extent to which a Ps cluster forms part of the total wave function, and none of these positron binding systems have an annihilation rate (per positron) larger than 2.5 3 10 9 s 21 [5][6][7][8][9].…”