Emission spectra of helium hydride at 4.2 K

Emission in the visible has been observed from dense, cold helium gas in contact with solid hydrogen when irradiated by a 15-MeV proton beam. All four stable isotopic combinations have been studied, and the spectrum of one of them, ${}_{}{}^4{}_{}{}^{}\mathrm{HeD}$, appears highly perturbed. Spectral identification of rotational features and a plausible explanation of the perturbation have been provided by rovibrational energy-level calculations.