S Stefan Welzel

Applications of quantum cascade lasers in plasma diagnostics: a review

Over the past few years mid-infrared absorption spectroscopy based on quantum cascade lasers operating over the region from 3 to 12 µm and called quantum cascade laser absorption spectroscopy or QCLAS has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, nitrogen oxides and organo-silicon compounds has led to further applications of QCLAS because most of these compounds and their decomposition products are infrared active. QCLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species at time resolutions below a microsecond, which is of particular importance for the investigation of reaction kinetics and dynamics. Information about gas temperature and population densities can also be derived from QCLAS measurements. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of QCLAS techniques to industrial requirements including the development of new diagnostic equipment. The recent availability of external cavity (EC) QCLs offers a further new option for multi-component detection. The aim of this paper is fourfold: (i) to briefly review spectroscopic issues arising from applying pulsed QCLs, (ii) to report on recent achievements in our understanding of molecular phenomena in plasmas and at surfaces, (iii) to describe the current status of industrial process monitoring in the mid-infrared and (iv) to discuss the potential of advanced instrumentation based on EC-QCLs for plasma diagnostics.

CO and byproduct formation during CO2 reduction in dielectric barrier discharges

The dissociation of CO2 and the formation of CO, O3, and O2 were studied in a dielectric barrier discharge (DBD) at atmospheric pressure by means of ex-situ infrared absorption spectroscopy. CO mixing ratios of 0.1%–4.4% were determined for specific injected energies between 0.1 and 20 eV per molecule (0.3–70 kJ/l). A lower limit of the gas temperature of 320–480 K was estimated from the wall temperature of the quartz reactor as measured with an infrared camera. The formation of CO in the DBD could be described as function of the total number of transferred charges during the residence time of the gas in the active plasma zone. An almost stoichiometric CO:O2 ratio of 2:1 was observed along with a strongly temperature dependent O3 production up to 0.075%. Although the ideal range for an efficient CO2 dissociation in plasmas of 1 eV per molecule for the specific injected energy was covered, the energy efficiency remained below 5% for all conditions. The present results indicate a reaction mechanism which is...

Trace gas measurements using optically resonant cavities and quantum cascade lasers operating at room temperature

Achieving the high sensitivity necessary for trace gas detection in the midinfrared molecular fingerprint region generally requires long absorption path lengths. In addition, for wider application, especially for field measurements, compact and cryogen free spectrometers are definitely preferable. An alternative approach to conventional linear absorption spectroscopy employing multiple pass cells for achieving high sensitivity is to combine a high finesse cavity with thermoelectrically (TE) cooled quantum cascade lasers (QCLs) and detectors. We have investigated the sensitivity limits of an entirely TE cooled system equipped with an ∼0.5 m long cavity having a small sample volume of 0.3 l. With this spectrometer cavity enhanced absorption spectroscopy employing a continuous wave QCL emitting at 7.66 μm yielded path lengths of 1080 m and a noise equivalent absorption of 2×10−7 cm−1 Hz−1/2. The molecular concentration detection limit with a 20 s integration time was found to be 6×108 molecules/cm3 for N2O a...

Quantum Cascade Laser Absorption Spectroscopy as a Plasma Diagnostic Tool: An Overview

The recent availability of thermoelectrically cooled pulsed and continuous wave quantum and inter-band cascade lasers in the mid-infrared spectral region has led to significant improvements and new developments in chemical sensing techniques using in-situ laser absorption spectroscopy for plasma diagnostic purposes. The aim of this article is therefore two-fold: (i) to summarize the challenges which arise in the application of quantum cascade lasers in such environments, and, (ii) to provide an overview of recent spectroscopic results (encompassing cavity enhanced methods) obtained in different kinds of plasma used in both research and industry.

TRIPLE Q: A three channel quantum cascade laser absorption spectrometer for fast multiple species concentration measurements

A compact and transportable three channel quantum cascade laser system (TRIPLE Q) based on mid-infrared absorption spectroscopy has been developed for time-resolved plasma diagnostics. The TRIPLE Q spectrometer encompasses three independently controlled quantum cascade lasers (QCLs), which can be used for chemical sensing, particularly for gas phase analysis of plasmas. All three QCLs are operated in the intra-pulse mode with typical pulse lengths of the order of 150 ns. Using a multiplexed detection, a time resolution shorter than 1 μs can be achieved. Hence, the spectrometer is well suited to study kinetic processes of multiple infrared active compounds in reactive plasmas. A special data processing and analysis technique has been established to account for time jitter effects of the infrared emission of the QCLs. The performance of the TRIPLE Q system has been validated in pulsed direct current plasmas containing N(2)O/air and NO(2)/air.

Evidence for surface oxidation on Pyrex of NO into NO2 by adsorbed O atoms

The surface of a Pyrex discharge tube was treated by a capacitively coupled RF plasma at low pressure. In cases where the plasma contained oxygen, O atoms deposition on the tube surface could be confirmed via the time-dependent conversion of NO to NO2 in a post-plasma experiment. Inside the discharge tube, the evolution of the concentrations of NO and of NO2 was measured using quantum cascade laser absorption spectroscopy in the mid-infrared spectral range. The surface density of atomic oxygen was estimated to be about 2 ? 1014?cm?2 based on NO oxidation in the closed reactor. The production rate of NO2 is in the range of 2 ? 1011?molecules?cm?3?s?1.

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.

Fluid modelling of CO2 dissociation in a dielectric barrier discharge

The dissociation of CO2 in a geometrically symmetric dielectric barrier discharge has been analysed by means of numerical modelling. A time- and space-dependent fluid model has been used, taking into account the spatial variation of the plasma between the plane-parallel dielectrics covering the electrodes. The main features of the model, including an extensive reaction kinetics for the vibrational states of CO2, are given. The modelling studies have been performed for different applied voltages, discharge frequencies, pressures, gas temperatures, and relative permittivities of the dielectrics. The model calculations show that the discharges in the positive and negative half-cycles are different for the considered standard condition, leading to a spatially asymmetric distribution of the stable neutrals like CO molecules and O atoms. The generation of CO mainly takes place during the discharge pulses, and it is dominated by electron impact dissociation. The specific energy input obtained for the broad range of parameters considered and determined for residence times reported in the literature agrees well with the corresponding experimental values. In accordance with these experiments, the calculated degree of CO2 conversion has been found to increase almost linearly with the specific energy input. Remaining discrepancies between the measured and calculated energy efficiencies are discussed.

NO kinetics in pulsed low-pressure plasmas studied by time-resolved quantum cascade laser absorption spectroscopy

Time-resolved quantum cascade laser absorption spectroscopy at 1897 cm−1 (5.27 µm) has been applied to study the NO(X) kinetics on the micro- and millisecond time scale in pulsed low-pressure N2/NO dc discharges. Experiments have been performed under flowing and static gas conditions to infer the gas temperature increase and the consequences for the NO line strength. A relatively small increase of ~20 K is observed during the early plasma phase of a few milliseconds. After some 10 ms gas temperatures up to 500 K can be deduced. The experimental data for the NO mixing ratio were compared with the results from a recently developed time-dependent model for pulsed N2–O2 plasmas which are well in accord. The early plasma pulse is determined by vibrational heating of N2 while the excitation of NO(X) by N2 metastables is almost completely balanced. Efficient NO depletion occurs after several milliseconds by N atom impact.

Molecule synthesis in an Ar-CH4-O2-N2 microwave plasma

The formation of new molecules in a microwave plasma, created from a mixture of Ar, CH4, N2 and O2, is investigated by means of an in-depth study of the molecular abundance in the plasma. The molecules are detected by means of tunable diode laser absorption spectroscopy and by absolute mass spectrometry. Three groups of molecules can be discerned in terms of molecular abundance: CO is predominantly formed, together with H2O, N2 and H2. The molecules CH4 and O2 are significantly depleted, but still abundant in a finite quantity. The third group is formed by several other species like NH3, NO, HCN etc. This tendency is expected to occur in every low temperature plasma containing C, O, H and N atoms. Furthermore, the combination of both techniques also allows us to make a clear distinction between the etching mode and deposition mode of the microwave reactor. Etching mainly occurs when the ratio of admixed gas flows Φ(O2)/Φ(CH4) > 0.5.