Microsecond-resolved electro-optic dual-comb spectroscopy in the 10~12.5 $\mu$m fingerprint region for radical kinetics

Abstract

Dual-comb spectroscopy enables broadband, high-resolution measurements with microsecond temporal resolution, but extending this capability to the 10~12.5 $\mu$m 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 crystal. Operation near a turning-point quasi-phase-matching condition at approximately 140 $^\circ$C reduces the wavelength sensitivity of the nonlinear conversion, enabling robust tuning of the idler comb over 83 cm$^{-1}$, corresponding to approximately 1.2 $\mu$m near 12 $\mu$m, by adjusting only the signal-comb center wavelength while keeping the pump wavelength and crystal temperature fixed. As a demonstration, we perform high-resolution, microsecond-resolved spectroscopy of transient chlorine monoxide (ClO) near 12 $\mu$m. Time-resolved dual-comb spectra capture the temporal evolution of ClO produced by the Cl + O$_3$ reaction with a temporal resolution of 1.5 $\mu$s, enabling quantitative determination of the ClO formation rate coefficient. These results establish this dual-comb platform as a promising tool for quantitative, microsecond-resolved studies of short-lived radicals, particularly atmospherically relevant halogen oxides.