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InfraRed Tunable Diode Laser Absorption Spectroscopy technique has been implemented in a H2/CH4 Micro-Wave (MW frequency f=2.45 GHz) plasma reactor dedicated to diamond deposition under high pressure and high power conditions. Parametric studies such as a function of MW power, pressure, and admixtures of methane have been carried out on a wide range of experimental conditions: the pressure up to 270 mbar and the MW power up to 4 kW. These conditions allow high purity Chemical Vapor Deposition diamond deposition at high growth rates. Line integrated absorption measurements have been performed in order to monitor hydrocarbon species, i.e., CH3, CH4, C2H2, C2H4, and C2H6. The densities of the stable detected species were found to vary in the range of 1012-1017 molecules cm-3, while the methyl radical CH3 (precursor of diamond growth under these conditions) measured into the plasma bulk was found up to 1014 molecules cm-3. The experimental densities have been compared to those provided by 1D-radial thermochemical model for low power and low pressure conditions (up to 100 mbar/2 kW). These densities have been axially integrated. Experimental measurements under high pressure and power conditions confirm a strong increase of the degree of dissociation of the precursor, CH4, associated to an increase of the C2H2 density, the most abundant reaction product in the plasma. [ABSTRACT FROM AUTHOR]

In Ar and Ar/N2 radio frequency (RF) discharges with admixtures of aluminium tri-isopropoxide (ATI) the fragmentation of this metal-organic precursor was studied by means of Fourier Transform Infrared (FTIR) spectroscopy using an optical long path cell providing an optical length of l = 17.2 m. The experiments were performed in an asymmetric capacitively coupled process plasma at a frequency of f = 13.56 MHz and at pres-sure values in the range of p = 1-10.5 Pa. The discharge power was chosen between P = 10-100 W. Using FTIR spectroscopy the evolution of the concentrations of ATI and of six stable molecules, CH4, C2H2, C2H4, C2H6, CO and HCN, was monitored under flowing conditions at gas flows of Φ total = 0.5-14.5 sccm in the discharge. The concentrations of the reaction products were measured tobe between 2 x 1012 molecules cm-3 as e.g. found for C2H4and C2H6, and 5 x 1013 molecules cm-3, as e.g. in the case of CO. In the plasma a complete dissociation of the precursor ATI was found at a power value of about P = 80 W independent on the admixture of Ar or N2. The fragmentation efficiency (F E) of the reaction products which originate from the ATI molecules ranges between 0.2 and 4 x 1016 molecules J-1 while the fragmentation rate (F R) reached up to 2.5 x 1018 molecules s-1. The multi component detection ability of the spectrometer served to analyse the carbon balance of the by-product formation. For all experiments, the carbon balance never exceeded 25%. Therefore, in the plasmas the majority of the provided carbon is most likely deposited at the reactor walls or forms dust particles or higher molecular C xH y. The conversion efficiencies (C E) of the produced molecular species ranges between 0.1 x 1015 molecules J-1 for C2H4 and 5 x 1015 molecules J-1 for C2H6 depending on the discharge conditions of the RF plasma. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Low-pressure pulsed dc H2–N2 plasmas with admixtures of CH4 or CO2 used for conventional (CPN) and active screen plasma nitriding (ASPN) were studied by infrared absorption and optical emission spectroscopy (OES) techniques. The experiments were performed in two modes: (i) the plasma at an internal model probe, driven by a bias voltage operated only, representing a CPN approach, and (ii) the screen plasma operated only, which corresponds to an ASPN experiment. Combining in situ tunable diode laser absorption spectroscopy with ex situ Fourier transform infrared spectroscopy the evolution of the concentrations of the methyl radical and of eight stable molecules, C2H2, CH4, C2H4, CO, CO2, NH3, HCN and H2O was monitored. The degree of dissociation of the carbon-containing precursor molecules varied between 40% and 98%. The methyl radical concentration was found to be in the range 1011–1012 molecules cm−3. By analysing the development of molecular concentrations with changes in gas mixtures and plasma power values, it was found that (i) C2H2, HCN and NH3 were the main products of plasma conversion in the case of methane admixture and (ii) CO, HCN and NH3 in the carbon dioxide case. The fragmentation efficiencies of methane and carbon dioxide (RF (CH4) ≈ (0.5–4) × 1016 molecules J−1, RF (CO2) ≈ (0.5–3.4) × 1016 molecules J−1) and the respective conversion efficiencies to the product molecules (RC (product) ≈ 1013–1015 molecules J−1) were determined for different gas mixtures and plasma power values. With the help of OES the rotational temperature of the screen plasma could be determined, which increased with power from 600 to 850 K. Also with power the ionic component of nitrogen molecules, i.e. the intensity of the -(0–0)-band, increased strongly in relation to the intensity of the neutral component, represented by the N2-(0–2)-band. [ABSTRACT FROM AUTHOR]

Abstract: The kinetics of a low pressure pulsed DC discharge in dry air is experimentally studied using simultaneous measurements of the concentrations of NO and NO2 downstream the plasma region. The measurements have been performed via infrared tunable diode laser absorption spectroscopy. The gas pressure was 3.95mbar and the pulse duration and frequency range from 20μs to 3ms and from 50 to 1000Hz, respectively. The peak current was varied from 10 to 80mA. It is shown that the NO and NO2 densities depend only on the averaged current. In order to explain this feature a simplified analytical calculation has been developed which considers about 30 kinetic reactions involved in the NO x formation. The calculations lead to a fair agreement with the experimental results. [Copyright &y& Elsevier]

The introduction of room temperature continuous wave external-cavity quantum cascade lasers (EC-QCLs) with narrow linewidths has greatly facilitated high resolution spectroscopy over wide spectral ranges in the mid-infrared (MIR) region. Using the wide tuning range of an EC-QCL we have measured the absolute line strengths of many P-branch transitions of the stretching dyad of the ν 3 fundamental band of 28 SiH 4 between 2096 and 2178 cm −1 . Furthermore, the high spectral resolution available has enabled us to resolve and measure representative examples of the tetrahedral splittings associated with each component of the P-branch. The positions of these components are in excellent agreement with spherical top data system (STDS) predictions and theoretical transitions from the TDS spectroscopic database for spherical top molecules. These are the first measurements of these line strengths of 28 SiH 4 and are an example of the applicability of high-powered, widely tunable EC-QCLs to high resolution spectroscopy. [ABSTRACT FROM AUTHOR]