Excited, Resonant, and Unnatural Parity States of Positronic Atoms and Ions

Calculations have demonstrated that 11 neutral atoms can bind positrons. We report on configuration interaction (CI) calculations that have demonstrated the existence of a $^2$P$^o$ excited state of $e^+$Ca, which consists predominantly of a positronium (Ps) cluster orbiting the Ca$^+$ ion in the $L=1$ partial wave [1]. This raises the possibility of detecting the formation of positronic bound states by an optical transition to the ground ($^2$S$^e$) $e^+$Ca state. The $e^+$Mg system is shown to lack an equivalent excited state, however, by extracting phase shifts from CI pseudostate energy shifts, we show that the system has a low-energy $p$ shape resonance [2]. Finally, CI and stochastic variational method calculations of annihilation suppressed, unnatural parity $^{2,4}S^{o}$, states of PsH, LiPs, NaPs and KPs are reported [3,4]. The LiPs system being Borromean in nature. \\ $[1]$ M.W.J.Bromley and J.Mitroy Phys.~Rev.~Lett. \textbf{97} 183402 (2006) \\ $[2]$ J.Mitroy and M.W.J.Bromley Phys.~Rev. (under review)\\ $[3]$ J.Mitroy and M.W.J.Bromley Phys.~Rev.~Lett. (in press)\\ $[4]$ M.W.J.Bromley, J.Mitroy and K.Varga Phys.~Rev.~A (in preparation)