A metastable state of helium with an attached positron is predicted to exist with a total energy of . The state is stable against dissociation into with a binding energy of . This state is formed by the attachment of a positron to the metastable He 1s2s state. The He ( state has a decay rate of . The state can only decay by a process with an approximate rate of . The structure of these states are best characterized as a positronium atom orbiting a 1s core at large distances.
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Ryzhikh et al. (1998) studied this question.
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