Search

Rotationally resolved spectra of the fundamental band of the CN free radical in four isotopic forms have been measured using tunable diode laser absorption spectroscopy. The source of the radical was a microwave discharge in a mixture of isotopically selected methane and nitrogen diluted with argon. The lines were measured to an accuracy of 5 x 10(-4) cm(-1) and fitted to the formula for the vibration rotation spectrum of a diatomic molecule, including quartic distortion constants. The band origins of each of the isotopomers from the five parameter fits were found to be 12C14N: 2042.42115(38) cm(-1), 13C14N: 2000.08479(23) cm(-1), 12C15N: 2011.25594(25) cm(-1), 13C15N: 1968.22093(33) cm(-1) with one standard deviation from the fit given in parenthesis. Some of the lines showed a resolved splitting due to the spin rotation interaction. This was averaged for fitting purposes. The average equilibrium internuclear distance derived from the upsilon = 0 and 1 rotational constants of the four isotopomers is 1.171800(6) A which is in good agreement with the value determined from microwave spectroscopy.

The line strengths of nine Q-branch lines in the ν2 fundamental band of the methyl radical in its ground electronic state have been measured by diode laser absorption spectroscopy. The vibration–rotation spectrum of methyl was recorded in a microwave discharge in ditertiary butyl peroxide heavily diluted in argon. The absolute concentration of the radical was determined by measuring its kinetic decay when the discharge was extinguished. The translational, rotational, and vibrational temperatures, also required to relate the line strengths to the transition dipole moment, were determined from relative integrated line intensities and from the Doppler widths of the lines after allowing for instrumental factors. The line strengths of the nine Q-branch lines were used to derive a more accurate value of the transition dipole moment of this band, μ2=0.215(25) D. Improved accuracy over earlier measurements of μ2 (derived from line strengths of single lines) was obtained by integrating over the complete line profile instead of measuring the peak absorption and assuming a Doppler linewidth to deduce the concentration. In addition, a more precise value for the rate constant for methyl radical recombination than available earlier was employed. The new value of μ2 is in very good agreement with high-quality ab initio calculations. Furthermore, the ratio of the transition dipole moments of the ν2 and ν3 fundamental bands in the gas phase is now in highly satisfactory agreement with the ratio determined for the condensed phase. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this paper Ar/H2/CH4 microwave discharges used for nanocrystalline diamond chemical vapor deposition in a bell-jar cavity reactor were characterized by both experimental and modeling investigations. Discharges containing 1% CH4 and H2 percentages ranging between 2% and 7% were analyzed as a function of the input microwave power under a pressure of 200 mbar. Emission spectroscopy and broadband absorption spectroscopy were carried out in the UV-visible spectral range in order to estimate the gas temperature and the C2 density within the plasma. Infrared tunable diode laser absorption spectroscopy was achieved in order to measure the mole fractions of carbon-containing species such as CH4, C2H2, and C2H6. A thermochemical model was developed and used in order to estimate the discharge composition, the gas temperature, and the average electron energy in the frame of a quasihomogeneous plasma assumption. Experiments and calculations yielded consistent results with respect to plasma temperature and composition. A relatively high gas temperature ranging between 3000 and 4000 K is found for the investigated discharge conditions. The C2 density estimated from both experiments and modeling are quite high compared with what is generally reported in the literature for the same kind of plasma system. It ranges between 1013 and 1014 cm-3 in the investigated power range. Infrared absorption measurements and model predictions indicate quite low densities of methane and acetylene, while the atomic carbon density calculated by the model ranges between 1013 and 1015 cm-3. The methane and hydrogen introduced in the feed gas are subject to a strong dissociation, which results in a surprisingly high H-atom population with mole fraction ranging between 0.04 and 0.16. Result analysis shows that the power coupling efficiency would range between 70% and 90%, which may at least explain the relatively high values obtained, as compared with those reported in the literature for similar discharges, for gas temperature and C2 population. The high H-atom densities obtained in this work would indicate that growing nanocrystalline diamond films would experience a very high etching. Simulation results also confirm that sp species would play a key role in the surface chemistry that governs the diamond growth. [ABSTRACT FROM AUTHOR]

Plasmas in interaction with surfaces are of key importance in a wide range of applications, which include materials technology, microelectronics, environmental issues, and biomedicine. The intense use of plasma technological processes demands proper plasma diagnostic techniques for monitoring, controlling and optimization purposes in industrial environments. In particular, for the improvement of the efficiency of a production process, in situ diagnostic techniques with online capabilities are favorable. From the middle of the last decade a variety of phenomena in molecular non-equilibrium plasmas in which many short-lived and stable species are produced have been successfully studied with quantum cascade laser absorption spectroscopy (QCLAS) in the mid-infrared spectral range. It has been possible to determine absolute concentrations of species, temperatures, degrees of dissociation, dynamics of reaction processes and phenomena involving plasma-surface interactions using spectroscopy, thereby providing a link with chemical and kinetic modelling of the plasma. Since quantum cascade lasers (QCLs) emit near room temperature, i.e., without the need of cryogenic cooling, very compact and robust spectroscopic instruments can be designed. This has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. Recent applications of infrared absorption spectroscopy using QCLs for in situ monitoring of plasma processes in industrial environments are reviewed. Examples which emphasize the capabilities of QCLAS as plasma diagnostic technique in industrial plasma processes will be given. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Low-pressure pulsed DC N2-H2 plasmas with admixtures of CH4 were investigated in a comparative study in two different types of reactors, (i) a cylindrical industrial scale active screen plasma nitriding (ASPN) reactor and (ii) a laboratory scale plasma nitriding monitoring reactor (PLANIMOR) with a linear electrode configuration. Applying infrared laser absorption spectroscopy (IRLAS) the evolution of the molecular concentrations of four stable molecules, CH4, HCN, NH3, and C2H2 has been monitored. The degree of dissociation of the carbon containing precursor, CH4, varied between 80 and 95%. The concentrations of the molecular reaction products have been found in a range of 1011 to 1016 molecules cm-3. By analyzing the development of the molecular concentrations at changes of the screen plasma power, the gas mixture and the admixture of CH4, a similar behavior of the monitored species depending on the varied parameters has been found in both reactors with NH3 and HCN as the main products of plasma conversion. The fragmentation efficiency of methane has been determined, decreasing from 2.4 to 0.8 × 1015 molecules J-1 with the increasing power of the screen plasmas in both reactors. With the help of optical emission spectroscopy (OES) the rotational temperature of the N+2 ions in the plasma could be determined, being in a range of 650...920 K where the rotational temperature of the screen plasma in the ASPN reactor is approximately 100 K higher than that in PLANIMOR. Also with power the ionic component of nitrogen molecules, i.e. the intensity of the N+2 -(0-0) band of the first negative system, which has been used as the reference parameter for comparing both reactors, increases strongly in relation to the intensity of the neutral component, represented by the N2-(0-0) band of the second positive system. In addition, steel samples have been treated in both the ASPN reactor and PLANIMOR leading to comparable nitriding results. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Microwave (MW) H2 plasmas with small admixtures of CH4 and/or B2H6 used for doped diamond deposition have been studied by infrared absorption and optical emission spectroscopy (OES) techniques. The experiments were carried out in order to analyze the dependence of the plasma chemistry on several parameters in the MW discharges, such as power, pressure and gas mixture, at relatively high pressures, i.e. up to 300 mbar, and power values, i.e. up to 4 kW. The evolution of the concentrations of five stable molecules, CH4, C2H2, C2H4, C2H6 and B2H6, was monitored in the plasma processes by in situ infrared tunable diode laser absorption spectroscopy. OES was used simultaneously to gather complementary information about the concentration of the boron atom. One target was the determination of the precursors' fragmentation rates and the conversion rates to the molecular reaction products. The analysis of correlations between the power and pressure of the plasma and molecular concentrations, and the differences found with CH4 and B2H6 admixtures were another focus of interest. The degree of dissociation of the carbon containing precursor molecule CH4 varied between 35% and 75%, while the decomposition of diborane, the boron precursor, varied between 85% and 97%. By analyzing the development of the molecular concentrations with changes of gas mixtures, pressure and plasma power values, it was found that (i) C2H2 is the most abundant reaction product in the plasma in the case of methane admixture and (ii) the boron atom concentration depends strongly on the presence of hydrocarbons in the plasma. The fragmentation efficiencies of methane and of diborane (RF(CH4) ≈ (1–2) × 1015 molecules J−1, RF(B2H6) ≈ (0.4–1.4) × 1013 molecules J−1) and the conversion efficiencies to the molecular products (RC(product) ≈ 1013–1015 molecules J−1) have been determined for different gas mixtures and plasma power, and pressure values. The boron atom balance, in relation to the dissociated B2H6 molecules was found to vary between 10% and 40% depending on the discharge conditions. [ABSTRACT FROM AUTHOR]

Trace gas sensing in the mid-infrared using quantum cascade lasers (QCLs) promises high specificity and sensitivity. We report on the performance of a simple cavity enhanced absorption spectroscopy (CEAS) sensor using a continuous wave external-cavity QCL at 7.4 μm. A noise-equivalent absorption coefficient αmin of 2.6 × 10-8 cm-1 in 625 s was achieved, which corresponds to a detection limit of 6 ± 1 ppb of CH4 in 15 millibars air for the R(3) transition at 1327.074 cm-1. This is the highest value of noise-equivalent absorption and among the longest effective path length (1780 m) reported to date with QCL-based CEAS. [ABSTRACT FROM AUTHOR]

Abstract: A hot filament plant for chemical vapor deposition of crystalline diamond featuring new operating stages has been built. It allows (i) a separate methane feed locally at substrate position and (ii) supplying a forced gas flow towards the substrate. To understand the effect of these two features on diamond growth, the results of systematic diamond growth experiments are discussed. To reveal the effects of these features on the gas phase, infrared tunable diode laser absorption spectroscopy (IR-TDLAS) was employed. Using a forced gas flow showed a remarkable increase in the diamond growth rate of a factor >6 compared to standard coating setups. By lowering the methane content in the forced flow diamond quality factors >95% were achieved. IR-TDLAS showed an increase of all measured carbon-containing species CH4, C2H2, CH3 and CO when applying the forced flow. The mass transport dominated by diffusion in the standard setup shifts to a convective gas transport in the forced flow setup. The induced laminar flow causes a more effective transport of the growth species to the substrate and leads to higher growth rates. Application of feeding methane locally at substrate position leads to exceptionally high growth rates (0.68μm/h) at correspondingly high diamond quality (91%). For this, the methane content has to be lowered, though, which at the same time leads to a more homogenous deposition lateral on the surface. From the IR-TDLAS gas phase measurements, a more effective precursor dissociation, a higher CH3 density and a rise in the CH3⧸C2H2 ratio above the substrate surface can be derived. [Copyright &y& Elsevier]

Abstract: In hot-filament CVD the gas-phase composition is a vital parameter for diamond coating results. Concentrations of carbon-containing species have significant influence on growth rate, quality and morphology of deposited diamond. To learn more about the correlations between process parameters and gas species concentrations we applied the highly sensitive infrared tunable diode laser absorption spectroscopy (IR-TDLAS) technique. With a sophisticated compact IR-TDLAS unit, relative and absolute concentrations of CH4, C2H2 and CO were simultaneously measured. Also, the absolute concentration of the methyl radical was determined in dependence on process parameters. Concentrations of CO2, C2H6 and HCN were investigated but found to be lower than the detection limit. The influence of the typical diamond CVD-process parameters on various species concentrations is discussed. The applicability of IR-TDLAS for hot-filament process monitoring is evaluated. In context with diamond growth results, information for CVD process refinement was deducted from the IR-TDLAS measurements. [Copyright &y& Elsevier]

Rotationally resolved spectra of the fundamental band of the CN free radical in four isotopic forms have been measured using tunable diode laser absorption spectroscopy. The source of the radical was a microwave discharge in a mixture of isotopically selected methane and nitrogen diluted with argon. The lines were measured to an accuracy of 5×10-4 cm-1 and fitted to the formula for the vibration rotation spectrum of a diatomic molecule, including quartic distortion constants. The band origins of each of the isotopomers from the five parameter fits were found to be 12C14N: 2042.42115(38) cm-1, 13C14N: 2000.08479(23) cm-1, 12C15N: 2011.25594(25) cm-1, 13C15N: 1968.22093(33) cm-1 with one standard deviation from the fit given in parenthesis. Some of the lines showed a resolved splitting due to the spin rotation interaction. This was averaged for fitting purposes. The average equilibrium internuclear distance derived from the v=0 and 1 rotational constants of the four isotopomers is 1.171800(6) Å which is in good agreement with the value determined from microwave spectroscopy. [Copyright &y& Elsevier]