Microsecond-resolved electro-optic dual-comb spectroscopy in the 10–12.5 μ m fingerprint region for radical kinetics

Dual-comb spectroscopy enables broadband, high-resolution measurements with microsecond temporal resolution, but extending this capability to the 10–12.5 μm molecular fingerprint region remains technically challenging, particularly for transient radical kinetics. Here, we demonstrate microsecond-resolved dual-comb spectroscopy in this spectral range using electro-optic combs and difference-frequency generation in an orientation-patterned gallium phosphide (OP-GaP) crystal. Operation near a turning-point quasi-phase-matching condition at ∼140 °C reduces the wavelength sensitivity of the nonlinear conversion, enabling robust tuning of the idler comb over 83 cm−1, corresponding to ∼1.2 μm near 12 μm, by adjusting only the signal-comb center wavelength while keeping the pump wavelength and crystal temperature fixed. We further apply this platform to high-resolution, microsecond-resolved spectroscopy of transient chlorine monoxide (ClO) near 12 μm. Time-resolved dual-comb spectra capture the temporal evolution of ClO produced by the Cl + O3 reaction with a temporal resolution of 1.5 μs, enabling quantitative determination of the ClO formation rate coefficient. These results establish turning-point OP-GaP quasi-phase matching as an effective strategy for extending electro-optic dual-comb spectroscopy into the long-wavelength fingerprint region and validate this platform as a promising tool for quantitative, microsecond-resolved studies of short-lived radicals, particularly atmospherically relevant halogen oxides.