On multiple component detection in molecular plasmas using cw external-cavity quantum cascade infrared lasers

Several cw external cavity quantum cascade lasers (EC-QCLs) have been tested as radiation sources for an absorption spectrometer focused on the analysis of molecular plasmas. Based on the wide spectral tunability of EC-QCLs multiple species detection is demonstrated in low pressure Ar/N$_{2}$ MW plasmas containing CH$_{4}$ as hydrocarbon precursor. Using the direct absorption technique the evolution of the concentrations of CH$_{4}$, C$_{2}$H$_{2}$, HCN and H$_{2}$O has been monitored depending on the discharge conditions (p= 0.5 mbar, f= 2.45 GHz) in a planar MW plasma reactor. The concentrations were found to be in the range of 10 $^{11}$ -- 10 $^{14}$ molecules cm$^{-3}$. Based on the profiles of absorption lines the gas temperature T$_{g}$ has been calculated in dependence on the discharge power. Changing the discharge power from 0.2 kW to 1 kW leads to an increase of T$_{g}$ from 400 to 700 K. The typical spectral line width of the EC-QCLs under the study was about 30 MHz. Varying the power values of an EC-QCL for direct absorption measurements at low pressure conditions no saturation effects in determining the concentrations of CH$_{4}$ and C$_{2}$H$_{2}$ could be found under the used conditions.