Time-resolved study of a pulsed dc discharge using quantum cascade laser absorption spectroscopy : NO and gas temperature kinetics

In a pulsed dc discharge of an Ar–N2 mixture containing 0.91% of NO the kinetics of the destruction of NO has been studied under static and flowing conditions, i.e. in a closed and open discharge tube (p = 266 Pa). For this purpose quantum cascade laser absorption spectroscopy (QCLAS) in the infrared spectral range has been applied as a new approach for fast in situ plasma diagnostics which is capable of achieving a time resolution below 100 ns. The time decay of the NO concentration was measured in single discharge pulses of 1 ms duration. Additionally, the temporal behaviour of the electric field and the applied power was followed during the pulse. The comparison of the time evolution of the NO concentration under static and flowing conditions and simplified model calculations enabled an analysis of the dynamics of the plasma heating to be made. The temperature increase during the pulse is below 40 K, but has a strong influence on the line strength of the NO absorption line. The apparent decrease in the NO concentration in a single pulse of about 20% is due to the heating of the gas which in turn makes the line strength vary while the concentration remains constant for several successive pulses. Therefore the QCLAS measurements combined with model calculations are a powerful non-invasive temperature probe with a remarkable time resolution approaching the sub-microsecond time scale