Characterization of lineshape structure by wavelength modulation spectroscopy

Abstract

Wavelength Modulation Spectroscopy (WMS) is performed in the oxygen A-band at different harmonics for different modulation indices and optical pathlengths. The experimental data is compared to two models with different absorption profiles: one assuming a Lorentzian lineshape function and the other with a Voigt lineshape function. We show that the larger amount of structure in higher harmonic signals provides greater sensitivity to the type of lineshape profile utilized for modeling. A useful new feature explored in this work is optical path length saturation in WMS that was described for the first time in a recent paper from our group. We discuss the role of such saturation and how it can be utilized as a diagnostic to probe lineshape. We also address the effect of the ever present modulation broadening. Results of experiments in which sets of nine scans (direct absorption, 1f, 2f,…,8f; i.e., detection harmonic N=0,1,2,3,…,8) were made simultaneously are described and discussed. Finally, the role that increased structure plays—as a result of increasing order of detection, N, as well as from the modification of the signal profile with increasing optical thickness—is outlined from the perspective of classical information theory.