Lineshape asymmetries in Faraday modulation spectroscopy

Abstract

Faraday modulation/rotation spectroscopy (FAMOS/FRS) is a spectroscopic technique for detection of paramagnetic species in gas phase. Although the prevailing theoretical description predicts fully symmetric lineshapes, experiments do not in general provide such. This work shows that asymmetries in FAMOS can have at least two origins; (1) a frequency-dependent laser intensity and (2) polarization imperfections, which both are scrutinized here. A general analytical description for the latter, derived under the assumption that both the polarization imperfections and the relative absorption are small, is presented, conveniently expressed in terms of 1st Fourier coefficients of modulated dispersion and absorption lineshape functions. The resulting expression, which is thus an extension to the conventional FAMOS expression, can thereby be swiftly evaluated and allows for online fitting to measured asymmetric FAMOS signals. Curve fits to experimentally obtained data from nitric oxide measured both in the ultraviolet and the mid-infrared regions demonstrate the applicability of the methodology.