NeMu* chemiluminescence: Radiolysis effects in gases

Chemiluminescent emission from NeMu*, the ultralight isotopic analogue of the Rydberg molecule NeH, has been observed in the near-infrared region. This is the first spectroscopic detection of a muonium-containing molecule. Histograms of photon events versus time were measured in Ne, Ar, and Ne/Ar gas mixtures. Three temporally distinct light-producing regions were observed: First, a large sharp peak at time zero, observed in all gases (Ne, He, N2, Ar), is assigned to scintillation light from Ne+* and Ne*, excited during muon thermalization. The ability of SF6 to quench this light suggests that much of the excitation occurs from electrons freed during the slowing down process. Second, a low-intensity broad region observed in all gases is attributed to e+ (or their associated bremsstrahlung) from muon decay. Finally, NeMu in 1-6 atm Ne with 1-2 torr Ar appeared as a high intensity emissive peak occurring late in time, up to several μs after the muon entered the vessel. The width and intensity of this peak depended linearly on the amount of Ar present. Its wavelength spectrum from 680-960 nm was measured. Although questions remain as to how NeMu* is formed, the precursor is likely Neμ+, the product of muon thermalization in Ne. Possible electron donors include metastable Ar+ (3P2 or 3P0) and long-lived free (spur) electrons