Vacuum ultraviolet spectroscopy of microhollow cathode discharge plasmas

Hollow cathode discharge devices with hole dimensions in the range from 0.1 - 0.5 mm (microhollow cathode discharges or MHCDs) can be operated at high pressure (up to and exceeding atmospheric pressure). MHCDs are known to be efficient sources of non-coherent ultraviolet (UV) and vacuum ultraviolet (VUV) radiation when operated in rare gases, rare gas - halide mixtures, and gas mixtures containing rare gases and trace amounts of gases such as H ₂, O ₂, and N ₂. Highest internal efficiencies in direct current MHCD excimer sources of close to 10 % were obtained in xenon at a pressure of 400 Torr. By applying nanosecond electrical pulses to the dc discharge the efficiency could be increased to approximately 20%. The radiative emittance which for dc discharges in xenon was measured as 1.4 W/cm ² could be increased to over 15 W/cm ² through pulsed operation. In addition to rare gas and rare-gas halide excimer emission, intense, monochromatic atomic line emissions have been reported from high-pressure MHCD plasmas in pure rare gases and in rare gases admixed with trace amounts (less than 1%) of H ₂, O ₂, and N ₂. The atomic line emission is the result of a near-resonant energy transfer process involving the excimers and the diatomic molecules. For instance, Ne ₂* excimers in the bound ³Σ _u state have enough energy to dissociate H ₂ and excite one of the H atoms to the n = 2 state. The subsequent decay of the excited H atom results in the emission of the 121.6 nm H Lyman-α line. We discuss the results of dc and time-resolved emission spectroscopy in the UV and VUV to elucidate the microscopic mechanisms of the rare gas excimer formation and emission processes, the properties of the MHCD plasma, and microscopic details of the near-resonant energy transfer processes that lead to the emission of the intense atomic line radiation in the range 100 - 130 nm.