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 1s22p 2Po, 1s2s2p 2Po and 2p3 4So excited states of the lithium atom. While the 2Po state cannot bind a positron, and the 4So could at most form a metastable state, a positron can attach to the 4Po 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+Li and it could be, in principle, exploited in an experiment to detect e+Li, whose existence has been predicted theoretically but has not yet been observed experimentally.