We discuss experimental and theoretical results of absorption features of the oxygen A-band transitions when synchronous detection at higher harmonics using Wavelength Modulation Spectroscopy (WMS) is performed. A key aspect of structure higher harmonic detection is discussed. It is shown that the signal magnitude and spectral locations of turning points and zero crossings of WMS signal demonstrate key signatures of collision dynamics of gaseous specie parameters and lineshape parameters. In addition, it is also shown that these salient features provide sensitive probes for any changes in the gas environment or lineshape parameters. We discuss several advantages and subtle physical effects that can be probed by higher order detection. As an example, we show resolution of several overlapping congested line spectra with highly disparate oscillator-strength. Optically thick regime of oxygen A-band transitions are probed by WMS and shown to exhibit distinctive features that are reflected in higher harmonic signals. These experimental results are the first ones to examine optical pathlength saturation by WMS. These effects greatly depend on the lineshape and gas parameters and experimental variables. The rich structure of WMS signals, especially at higher detection orders, is central to the technique\u27s advantages in resolving these subtle effects. We show greater sensitivity of turning points and zero crossings with lineshape or gas parameters. We also show that in certain situations the sensitivity could be significant especially in the wing region of the profile where the absorption signal is low.
We discuss two approaches to quantify advantages of higher harmonic detection and structure (number of zero crossings and turning points). The method is based on statistical analysis and principles of Shannon\u27s classical information theory, where the precision in Wavelength Modulation Spectroscopy (WMS) and measurements with molecular species is quantified utilizing information theory.
We show that there is an optimal harmonic detection order that yields the maximum information in presence of distortion and noise. Distortion and noise effects are treated separately. Particular cases of distortion i.e. modulation broadening, pressure broadening, pathlength saturation and Fabry-Perot fringing are discussed, and their relation to information in the measurement of lineshape parameters is outlined. It is shown that the optimal harmonic order can be understood by considerations of complexity in the signal structure rather than those of conventional Signal to Noise ratios. It is also shown that under certain experimental conditions higher detection orders (N ≥ 5) yield precise and optimal results in estimation of lineshape parameters in a given noise environment. The merit of optimal harmonic detection order is based on maximum information (in bits) that can be extracted at a particular harmonic signal in the presence of noise
