Advanced spectroscopic diagnostics for lighting plasmas

Abstract

Summary form only given, as follows. Advanced diagnostics based on vacuum ultraviolet laser induced fluorescence and high sensitivity absorption spectroscopy for glow discharge plasmas used in lighting will be described. The resonance line of the atomic mercury ion at 194 nm is now accessible to tunable dye laser systems. Laser induced fluorescence provides excellent spatial and temporal resolution for mapping the relative density of mercury ions and other species in lighting plasmas. High sensitivity absorption spectroscopy using a synchrotron, a large (3 m) vacuum echelle spectrometer, and a CCD detector array complements laser diagnostics by providing absolute column densities. Absorption spectroscopy using detector arrays has far greater sensitivity than older experiments using a single channel, sequentially scanned spectrometer. Continuum source intensity fluctuations often limit the sensitivity of single channel, sequentially scanned absorption experiments. The simultaneous detection of all spectral channels across an absorption feature using a detector array eliminates the effect of source fluctuations. Photon statistical fluctuations are then the primary limit on the sensitivity of the absorption experiment. The array detector makes it possible to efficiently accumulate good photon statistics. Sensitivities to fractional absorptions of 10/sup -5/ are now possible at wavelengths from the vacuum ultraviolet to the near infrared. Column densities of atoms or atomic ions as small as 10/sup 8/ cm/sup -2/ are detectable using high sensitivity absorption spectroscopy. A digital subtraction technique for discriminating against the line emission from the glow discharge and detecting only the continuum emission, which has absorption features after transmission through the glow discharge, is used. These diagnostics are providing a more quantitative understanding of the negative glow region of hot cathode Hg-Ar (fluorescent lamp) discharges.