APPLICATIONS OF WAVELENGTH-MODULATION SPECTROSCOPY IN RESOLUTION OF PRESSURE AND MODULATION BROADENED SPECTRA

Theoretical and experimental results for modulation absorption spectro scopy (with simultaneous amplitude and wavelength modulations) are giv en, with particular emphasis on the effects of pressure and modulation broadening. As expected, such broadening effects result in a loss of resolution of nearly overlapping lines, whether these are from one or more absorbing species. The effects of phase-sensitive detection at ha rmonics greater than the commonly used second are investigated. It is shown that when detection is performed at higher harmonics, one is abl e to counter the loss of resolution due to pressure and modulation bro adening. Applications in measurements of congested spectra, and in the simultaneous measurement of more than one species, are discussed. Oth er general characteristics of signals obtained by using higher-harmoni c detection are also discussed, together with corresponding applicatio ns.