Mid-Infrared Time-Resolved Frequency Comb Spectroscopy of Transient Free Radicals

We demonstrate time-resolved frequency comb spectroscopy (TRFCS), a new broadband absorption spectroscopy technique for the study of trace free radicals on the microsecond timescale. We apply TRFCS to study the time-resolved, mid-infrared absorption of the deuterated hydroxyformyl radical trans-DOCO, an important short-lived intermediate along the OD + CO reaction path. Directly after photolysis of the chemical precursor acrylic acid-d(1), we measure absolute trans-DOCO product concentrations with a sensitivity of 5 X 10(10) cm(-3) and observe its subsequent loss with a time resolution of 25 mu s. The multiplexed nature of TRFCS allows us to detect simultaneously the time-dependent concentration of several other photoproducts and thus unravel primary and secondary chemical reaction pathways.