Time-resolved in-situ quantum cascade laser absorption spectroscopy in dielectric barrier discharges

Modern mid-infrared laser sources, known as quantum cascade lasers (QCLs), provide a means for highly time-resolved absorption spectroscopy in the molecular ``fingerprint'' region. Pulsed distributed feedback QCLs are especially suited for diagnostic studies on transient plasmas as the time-resolution can be as good as a few tens of nanoseconds. Dielectric barrier discharges in CO$_{2}$ operated in the mid-frequency (kHz) range were studied by means of in-situ QCL absorption spectroscopy in single and multiple-pass configuration. Different synchronisation schemes were applied to achieve phase-resolved measurements during individual AC cycles as well as to monitor molecular absorption signals during pulsed operation. Mixing ratios of CO in its electronic and vibrational ground state were of the order of a few percent and thus confirmed ex-situ studies of the effluent. Interestingly, the concentrations levels were changing only slowly in time, i.e. of the order of the residence time. A direct CO$_{2}$-to-CO dissociation through electron impact appears very unlikely under these conditions. The kinetics of low-lying ro-vibrational states of CO$_{2}$ along with the evolution of the CO concentration were measured on a sub-millisecond time-scale to establish the (rotational) gas temperatures.