A beam of 400-eV positrons is partially transmitted through a 40-{} C film in vacuum. Measuring the γ-ray energy spectrum with a downstream Ge(Li) detector, a new γ-ray peak is observed, the energy of which can be changed by applying a voltage to a grid behind the film. Attributing the new peak to a Doppler-shifted annihilation γ implies a mass-to-charge ratio 3.011(14)m/e, thus signaling the presence of the e^-e⁺e^- positronium negative ion, Ps^-. The Ps^- conversion efficiency of the C film is (2.8±{}0.3)×{}10^-4.
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A 1981 study studied this question.
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