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)