|
Conventional vacuum ultraviolet line sources often consist of electrodeless rf or microwave discharges of flowing or sealed inert gases at 1-10 torr. The inert buffer, typically helium or argon, is seeded with traces of parent species of the desired transitions (e.g., 02 or N2 for OI or NI transitions, respectively). The sensitivity of resonance sources as absorption or fluorescence diagnostics depends critically upon the effective line width of the source resonance radiation. This property is determined primarily by source self-absorption, and by Doppler broadening of the source radiation, which is itself a function of the translational-energy distribution of the radiating species. Self-absorption is easily minimized or characterized experimentally. However, Doppler broadening is a complex function of the lamp excitation processes and should be characterized for each type of resonance lamp. The major competing excitation mechanisms for transitions such as 01 (130 nm) or NI (120 nm, 149 nm, 175 nm) in such line sources are electron impact processes, where the excess kinetic energy of the collision is retained with the electrons, and energy transfer or dissociative excitation by rare gas metastables (Ar(3P2,0) at 11.5 eV or He(23,1S) at , 20 eV), where a significant fraction of the energy defect may appear as excess translational energy in the radiating species. The kinetics of these processes as they relate to various VUV atomic line sources are reviewed. In addition, preliminary experimental data from absorption measurements on atomic nitrogen metastables, N(2D) and N(2P), produced in a discharge-flow apparatus, are presented which show markedly different behavior between microwave-excited He/N2 and Ar/N2 lamps. The implications of these effects for design application of resonance absorption/ fluorescence diagnostics are illustrated.
|
