Absorption Spectroscopy Studies in Low Pressure Non Equilibrium Molecular Plasmas Using Tunable Infrared Diode Lasers

Tunable infrared diode laser absorption spectroscopy (TDLAS) has been applied to investigate the chemical kinetics in reactive discharges. It was used to detect the methyl radical and nine stable molecules, CH4, CH3OH, C2H2, C2H4, C2H6, NH3, HCN, CH2O and C2N2, in H2-Ar-N2 microwave plasmas containing up to 7.2 % of methane or methanol, under both flowing and static conditions. The degree of dissociation of the hydrocarbon precursor molecules varied between 20 and 97 %. The methyl radical concentration was found to be in the range 1012 to 1013 molecules cm-3. By analysing the temporal development of molecular concentrations under static conditions it was found that HCN and NH3 are the final products of plasma chemical conversion. The fragmentation rates of methane and methanol and the respective conversion rates to methane, hydrogen cyanide and ammonia have been determined for different hydrogen to nitrogen concentration ratios. An extensive model of the chemical reactions involved in the H2-N2-Ar-CH4 plasma has been developed. Model calculations were performed by including 22 species, 145 chemical reactions and appropriate electron impact dissociation rate coefficients. The results of the model calculations showed satisfactory agreement between calculated and measured concentrations. The most likely main chemical pathways involved in these plasmas are discussed and an appropriate reaction scheme is proposed. Based on the model calculations the concentrations of non-measured species like CH2 or NH2 have been predicted.In addition, spectroscopic investigations of P- and R-branch lines of the fundamental bands of 12C14N and 13C14N in their ground electronic state have been performed at high resolution by tunable diode laser absorption spectroscopy. The radicals were generated in microwave plasmas containing methane with varying proportions of N2 and H2. From a fit to the spectra the origins of the fundamental bands of the two isotopomers were determined to be 2042.42104(84) cm-1 and 2000.08470(30) cm-1. The main product detected in the plasma was HCN. It showed concentrations which are about three orders of magnitude higher than that of CN.Moreover, the time and spatial dependence of the chemical conversion of CO2 to CO were studied in a closed glow discharge reactor (p = 50 Pa, I = 2 and 30 mA) consisting of a small plasma zone and an extended stationary afterglow. Tunable infrared diode laser absorption spectroscopy has been applied to determine the absolute ground state concentrations of CO and CO2. After a certain discharge time the concentrations of both species were observed to come into equilibrium. The spatial dependence of the equilibrium CO concentration in the afterglow was found to vary by less than 10 %. The feed gas was converted to CO more predominantly between 45 % and 60 % with increasing discharge current. The formation time of the stable gas composition decreased with increasing current too. For currents higher than 10 mA the conversion rate of CO2 to CO was estimated to be 1 x 1013 molecules J-1. Based on the experimental results a model of the CO2 conversion chemistry has also been established for this type of discharge. The calculated and measured temporal developments of species concentrations showed a satisfactory agreement for various discharge currents.Lastly, infrared tunable diode laser absorption spectroscopy has been used to analyse the fragmentation of TiCl4 into HCl in pulsed H2-Ar-N2 dc plasmas (p= 2 mbar). At small TiCl4 admixtures (0.04-0.31 %) HCl concentrations of 2-5 x 1014 molecules cm-3 were measured (current density: 0.6-1.15 mA cm2). A nearly complete conversion of Cl into HCl was found at TiCl4 admixtures below 0.2 %.