  The accuracy of quantitative absorption spectroscopy depends on correctly
distinguishing molecular absorption signatures in a measured transmission
spectrum from the varying intensity or "baseline" of the light source. Baseline
correction becomes particularly difficult when the measurement involves
complex, broadly absorbing molecules or non-ideal transmission effects such as
etalons. We demonstrate a technique that eliminates the need to account for the
laser intensity in absorption spectroscopy by converting the measured
transmission spectrum of a gas sample to a modified form of the time-domain
molecular free induction decay (m-FID) using a cepstral analysis technique
developed for audio signal processing. Much of the m-FID signal is temporally
separated from and independent of the source intensity, and this portion can be
fit directly with a model to determine sample gas properties without correcting
for the light source intensity. We validate the new approach in several complex
absorption spectroscopy scenarios and discuss its limitations. The technique is
applicable to spectra obtained with any absorption spectrometer and provides a
fast and accurate approach for analyzing complex spectra.
