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Muonium in the $2S$ state has been produced by the beam-foil method with a μ⁺ beam of 10 MeV/c. The metastable $2S$ state of muonium has been detected by a static electric field quenching method, and the transition 2S1/2, F=1→2P1/2, $F=1$ induced by a radio-frequency electric field has been observed with an event rate of about 4/h.
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Badertscher et al. (1984) studied this question.
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