Infrared emission spectra from cryogenic proton-irradiated helium gas

Spectra from proton-beam irradiated helium gas, near 4.2 K, have been examined in the near infrared spectral region using a sensitive photomultiplier-grating spectrometer combination (750-1000 nm) and a Fourier transform infrared spectrometer with an InGaAs detector (800-1800 nm). Most of the observed features can be assigned to known He, transitions, and two bands, not previously observed (d (3) Sigma(u)(+) (nu = 4) --> c (3) Sigma(g)(+) (nu = 3, 4)) have been identified and analyzed. Some of the unidentified spectra have qualitative similarity to a set of visible emission lines tentatively assigned to an excimer-dimer (i.e., He-4, two He-2 excimers weakly bound together). These features, unlike the normal He-2 spectra, demonstrate pronounced dependence on both the pressure and temperature of the sample. A comparison of the intensities of the singlet and triplet D(d) --> C(c) transitions to the singlet and triplet C(c) --> A(a) transitions is made. (C) 1995 American Institute of Physics.