Mid-Infrared Time-Resolved Frequency Comb Spectroscopy of Transient Free Radicals
Adam J. Fleisher, Bryce Bjork, Thinh Q. Bui, Kevin C. Cossel, Mitchio Okumura, Jun Ye
Quantitative measurements in chemical kinetics require unambiguous determinations of reactant, intermediate, and product concentrations on time scales faster than the reaction rate. Direct absorption spectroscopy in the mid-infrared (mid-IR) can fulfill the quantitative requirement, often with high detection sensitivities, thanks to strongly absorbing fundamental molecular vibrations. Here, we demonstrate a new broadband transient absorption technique, time-resolved frequency comb spectroscopy (TRFCS), for the study of chemical kinetics on the μs timescale. We use TRFCS to study the time-resolved, mid-IR absorption of the deuterated hydroxyformyl radical trans-DOCO, an important short-lived intermediate along the OH + CO reaction path. Directly after photolysis of a chemical precursor, we measure an absolute trans-DOCO concentration of 3.10(5) x 1012 molecule cm-3 and observe its subsequent reaction rate with a time resolution of 25 us.