Positron Binding to Lithium Excited States

In the last 15 years hundreds of papers have been devoted to the study of positron-atom or positron-molecule interaction. A large body of evidence has accumulated showing that many atoms in their ground state can bind a positron forming an electronically stable system. Studies on the possibility that a positron binds to an atomic excited state, however, are scarce. The first atom that was proved able to bind a positron in its ground state is lithium. Surprisingly, nothing is known on the possibility that a positron could bind to one of its excited states. In this Letter we study the positron attachment to the $1s^{2}2p$ ${}^{2}P^o$, $1s2s2p$ ${}^{2}P^o$ and $2p^3$ ${}^{4}S^o$ excited states of the lithium atom. While the ${}^{2}P^o$ state cannot bind a positron, and the ${}^{4}S^o$ could at most form a metastable state, a positron can attach to the ${}^{4}P^o$ state of lithium forming a bound state with a binding energy of about 0.003 hartree. This state can alternatively be considered an excited state of the system $e^{+}\mathrm{Li}$ and it could be, in principle, exploited in an experiment to detect $e^{+}\mathrm{Li}$, whose existence has been predicted theoretically but has not yet been observed experimentally.