Line shape analysis of Doppler broadened frequency-modulated line spectra

We present a method for recovering Doppler broadened absorption line shapes from frequency modulated (FM) line spectra. The method of analysis is calibrated and demonstrated with thermalized CN radicals produced by photodissociation of cyanogen (NCCN), probed on the A–X system near 800 nm with a frequency modulated Ti: sapphire ring laser. Nonthermal, Doppler broadened lines from translationally nascent photofragments can also be recovered by direct transformations of experimental FM line profiles acquired with a time resolution exceeding 100 ns. The superior signal-to-noise afforded by FM spectroscopy, relative to other direct absorption methods, should encourage the application of transient FM spectroscopy to problems in photoinitiated reaction dynamics.