Ground-state energy of quasi-free positrons in non-polar fluids

We have calculated the background energy (V-0) for positrons in noble gases with an ab initio potential and the Wigner-Seitz (WS) ansatz. In contrast to the general pseudo-potential approach, we have used accurate ab initio potentials for the positron-atom interaction. The ansatz includes an assumed form of the potential, resulting from an average over fluid atoms, and we propose four different options for this. By comparing the different options to literature data for an effective electron number (Z(eff)), we find that agreement can be obtained for light elements but fails for heavy elements. We suspect that the strong polarizability of the heavy elements makes the simple potential averaging, as assumed in the WS model, insufficient to fit the measurements without also making use of pseudo-potentials. We also raise our suspicion that the comparison of annihilation rates between ground-state calculations and experimental values is not appropriate. Furthermore, the congruence of V-0 to Z(eff) values predicted by a contact potential approximation appears to be invalidated by our results.

Automated summary

The background energy (V-0) of positrons in noble gases was calculated using an ab initio potential and the Wigner-Seitz ansatz. The study proposed four different assumed forms of the potential, aimed at comparing with literature data and finding that agreement can be obtained for light elements but fails for heavy elements. The strong polarizability of heavy elements may require the use of pseudo-potentials in addition to the simple potential averaging assumed in the WS model.