Ultraviolet (UV) Raman spectroscopy is being applied to the detection of chemical agent contamination of natural and man-made surfaces. In support of these efforts, we have measured the UV Raman signatures of the G-series nerve agents GA (tabun), GB (sarin), GD (soman), GF (cyclosarin), and the agent simulant diisopropyl methylphosphonate (DIMP) at 248 nm and 262 nm, as well as taking their UV Raman and UV absorption cross-sections. Of these chemicals, only GA exhibits any significant pre-resonance enhancement. We also show that reduction of the excitation wavelength from 262 nm to 248 nm effectively shifts the Raman spectrum away from a substantial sample fluorescence background, implying a significant improvement in detection capability.
We have examined the effect of intensity-axis correction on noise in white light spectra recorded with charge-coupled device (CCD) detectors. Measurements were made with five detectors in Raman spectrometers. Detectors were both liquid-nitrogen and thermoelectrically cooled devices and one room temperature device. Both random and pattern noise have been considered. We used cross-correlation of noise sets to provide an indicator of a fixed pattern in the spectra and an assessment of the efficacy of the correction procedure in removing this pattern. For four of the five detectors intensity-axis correction provided a significant improvement in signal-to-noise ratio. Correction was particularly important for measurements made with lower-cost CCD detectors of the kind proposed for process control instruments.
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