First spectroscopic evidence for a muonium-containing molecule: NeMu* chemiluminescence

Abstract

Evidence for the formation of NeMu*, an isotopic analog of the Rydberg molecule NeH*, has been obtained from the observation of chemiluminescent emission in the near-infrared region. This is the first spectroscopic detection of a muonium-containing molecule. NeMu* was formed by stopping a 4 MeV muon (μ+) beam in a target vessel containing 1–6 atm of Ne and ∼1 Torr Ar. The wavelength spectrum of the emission, from ∼680–1000 nm, was measured using a variable-wavelength filter, with a resolution of ±12.5 nm. Lower resolution spectra were also taken with a series of long pass filters. A complete histogram of photon events vs time was collected for each wavelength. Two strong transitions are observed, centered at 818 and 943 nm. Identification of NeMu* was made by a comparison of the experimental spectrum with a simulated spectrum based on detailed ab initio calculations, extended to higher excitation levels than had heretofore been reported. Both experimental and theoretical results are reported here. Although the mechanism by which the emitting states in NeMu* are formed remains unclear, radiolysis effects appear to play a dominant role, indicating that NeMu+ (the product of muon thermalization in Ne) undergoes charge exchange with metastable Ar* and/or is neutralized by a spur electron, both species produced during the slowing down of the high energy muon.