SENSITIVE DENSITY-FLUCTUATION MEASUREMENTS USING WAVELENGTH-MODULATION SPECTROSCOPY WITH HIGH-ORDER-HARMONIC DETECTION

We discuss experimental and theoretical results which show that when w avelength-modulation spectroscopy is used to monitor concentration flu ctuations of gaseous species, greater sensitivity may be obtained if o ne uses high-order detection. We also show that, depending on the ambi ent concentration being monitored, there are regions in which the comm only used second derivative would show a negligible variation of signa l magnitude with concentration fluctuations, whereas measurement with a higher harmonic would result in a much improved signal. Theoretical results for the measurements of any transition that can be described b y the Voigt profile are given. The technique discussed is illustrated by presenting the results of measurements of wavelength-modulation spe ctroscopy of lines in the oxygen A band. Different detection harmonic orders are suitable for different ambient concentrations, and a relate d criterion that helps in the determination of a suitable detection ha rmonic order is given.