Positron and positronium interactions with Cu

Abstract

The configuration-interaction (CI) method is used to investigate the interactions of positrons and positronium with copper at low energies. The calculations were performed within the framework of the fixed-core approximation with semiempirical polarization potentials used to model dynamical interactions between the active particles and the $(1s\ensuremath{-}3d)$ core. Initially, calculations upon the ${e}^{+}\mathrm{Li}$ system were used to refine the numerical procedures and highlighted the extreme difficulties of using an orthodox CI calculation to describe the ${e}^{+}$ Li system. The positron binding energy of ${e}^{+}\mathrm{Cu}$ derived from a CI calculation which included electron and positron orbitals with $\mathcal{l}<~18$ was 0.005 12 hartree while the spin-averaged annihilation rate was $0.507\ifmmode\times\else\texttimes\fi{}{10}^{9}{\mathrm{s}}^{\ensuremath{-}1}.$ The configuration basis used for the bound-state calculation was also used as a part of the trial wave function for a Kohn variational calculation of positron-copper scattering. The positron-copper system has a scattering length of about ${13.1a}_{0}$ and the annihilation parameter ${Z}_{\mathrm{eff}}$ at threshold was 72.9. The dipole polarizability of the neutral copper ground state was computed and found to be ${41.6a}_{0}^{3}.$ The structure of CuPs was also studied with the CI method and it was found to have a binding energy of 0.0143 hartree and an annihilation rate of $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{9}{\mathrm{s}}^{\ensuremath{-}1}.$