Optical and infrared emission spectra of electron-bombarded liquid helium reveal the presence of a number of excited singlet and triplet states of the He₂ molecule, including the a³Σᵤ⁺ metastable state. The observed liquid spectra show vibrational and unresolved rotational structure. There is also preliminary evidence that excited atomic states, including the metastable 2³S₁ state, are populated in the liquid.
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Dennis et al. (1969) studied this question.
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