Excimer formation in high-pressure microhollow cathode discharge plasmas in helium initiated by low-energy electron collisions

We report the observation of intense continuous vacuum ultraviolet radiation in the range 60-100 nm from microhollow cathode discharge plasmas in high-pressure He (up to 600 Torr). Two prominent emissions, a narrow, sharply peaked feature in the region of 58-65 nm and a broad emission from 65 to 95 nm are attributed to the He*₂ first- and second-excimer continuum emission, respectively. We also observed several narrow atomic emission lines in the 95-125-nm range that correspond to atomic O, N, and H emission lines. We attribute the presence of these atomic line emissions to near-resonant energy transfer processes involving the He*₂ excimers and trace concentrations of the impurities O₂, N₂, and H₂ in the discharge feed gas. The processes leading to the atomic line emissions in the present case are similar to the near-resonant energy transfer process observed previously in high-pressure microhollow cathode discharge plasmas in Ne/H₂ gas mixtures, which resulted in the emission of intense, monochromatic atomic hydrogen Lyman-α radiation (P. Kurunczi et al. J. Phys. B: At. Mol. Opt. Phys. 32 (1999) L651). He*₂ excimer formation in a microhollow cathode discharge plasma is initiated by low-energy electron collisions (excitation of the metastable He levels or ionization of the He atoms) followed by three-body collisions. The emission of He*₂ excimer radiation from a microhollow cathode discharge plasma indicates that these discharges are very efficient sources of energetic electrons, as the formation of He*₂ excimers requires a sufficiently large number of electrons with energies well above 20 eV.