Ozone production in oxygen by means of F2-laser irradiation at λ=157.6 nm

Ozone is generated in pure oxygen (p5 kPa), synthetic air (p7 kPa) and oxygen-argon mixtures (p3 kPa) by irradiation of these gases with the VUV light of a repetitively pulsed (f _L15 Hz) F₂-laser at =157.6 nm with maximum about 4 mJ/pulse. An absorption photometer measurement operating at 253.7 nm (Hg line) determines the ozone concentration as a function of oxygen and/or additive gas pressure, the repetition frequency of the laser and the wall temperature of the reaction chamber. The temporal development of the ozone concentration as a function of these parameters is calculated by means of rate equations for the species O(³ P), O₂(X ³ _g ^– ), O₃(¹ A ₁), O(¹ D), O₂(a ¹_g), O₂(b ¹ _g ⁺ ) and vibrationally excited O ₃ ^* (¹ A ₁) and the photon distribution. The maximum concentration of O₃ in the sealed-off chamber reaches 1.6% in pure O₂, 4.1% in air and 1.2% in a 1:5 O₂-Ar mixture at 3 kPa. The annihilation of O₃ by the wall and temperature dependent volume processes (300 KT395 K) is studied and the experimental and theoretical results are compared.     

Temperature measurements in CO2-laser-induced pyrolysis flames for SiC and ternary SiC/C/B powder synthesis by means of CARS

The hot luminescent reaction zones of CO₂-laser-induced pyrolysis flames using SiH₄/C₂H₂ gas mixtures with different silane to acetylene ratios and with and without diborane additives were investigated by means of H₂ Q-branch CARS spectroscopy, leading to spatial temperature profiles in gas flow direction. In the case of B₂H₆ additive to the stoichiometric SiH₄/C₂H₂ mixture a high temperature plateau ( 800–1000 K) of the reactant gas volume develops already several millimetres before reaching the CO₂-laser focus line. This precursor preheating zone could be explained by the catalytic effect of boron atoms or boron-containing intermediate species in the flame. A similar behaviour for acetylene-rich flames operating at half laser power was not observed.     

Experimental and Theoretical Studies of a Pulsed Microwave Excited Ar/CF4 Plasma

The present work deals with a pulsed microwave discharge in an Ar/CF ₄ gas mixture under a low pressure (1–10 mbar). The discharge chamber developed has a cylindrical geometry with a coupling window alternatively made of quartz or alumina. The setup allows one to investigate the plasma–wall interactions (here etching of the quartz window) and the ignition process of the pulsed microwave plasma. Microwave pulses with a duration of 50–200 s and repetition rate between 1 and 10 kHz are typical for the experiments. The space-time behavior of the fluorine number density in the discharge has been investigated experimentally by optical actinometry. The discharge kinetics is modeled using electron-transport parameters and rate coefficients derived from solutions of the Boltzmann equation. Together with the solution of the continuity and electron balance equations and the rate equations describing the production of CF _x (x=2, 3, 4) radicals and F atoms, a good agreement between experimental and theoretical data can be achieved.     

Eigenschaften und Parameterabhängigkeit der Temperaturverteilung und der Charakteristik eines zylindersymmetrischen Stickstoffbogens

The purpose of this work is to examine the properties and dependence upon parameters of the temperature distribution, and to ascertain the characteristic of a stationary, cylindersymmetrical nitrogen arc with negligible convection. Proceeding from the local balance of energy (the Elenbaas-Heller differential equation), the article makes known a process designed to determine the axial field strength in the arc discharge tube explicity with the aid of the boundary conditions of the temperature and the material functions. To this end, the differential equation is converted to a nonlinear integral equation. This equation can then, provided that the material functions are known and the parameters — radius of discharge tube, axis temperature, wall temperature — are established, be solved either numerically or graphically by a method analogous with the Picard's method of successive approximations. The numerical results, parameter limitations, and assumptions concerning material functions, enable the graphic and analytical relationship between temperature distribution and characteristic to be ascertained within a predetermined range of parameters. Thus we are enabled to interpret and theoretically record a few empirical beginnings and laws.     

Infrared absorption of silane,ammonia, acetylene and diborane in the range of the CO2 laser emission lines: Measurements and modelling

Absorption spectra of the gases SiH₄, NH₃, C₂H₂ and of SiH₄/Ar and SiH₄/B₂H₆ mixtures have been measured in the spectral range of the CO₂ laser from 9.2 to 10.8 µm. In agreement with literature, silane shows the highest absorption (absorption coefficient = 3.3 × 10^–2 Pa^–1 m^–1). The deviation of the measured absorption behaviour of silane from literature, as far as the pressure dependence is concerned, can be explained by the enhanced spectral energy density in our experiment. This is confirmed by a rate-equation model involving the basic mechanisms of V-V and V-T energy transfer between vibrationally excited silane molecules. In contrast to silane, the absorption coefficient of NH₃ at the 10P(20) laser line is 4.5 × 10^–4 Pa^–1 m^–1 atp = 20 kPa and has its maximum of 4.5 × 10^–3 Pa^–1 m^–1 at the 10R(6) laser line. For C₂H₂ and B₂H₆, is even less ( 2.1 Ò 10^–5 Pa^–1 m^–1 for C₂H₂).     

Coherent Anti-Stokes Raman Scattering Applied to Hydrocarbons in a Microwave Excited Process Plasma

Coherent anti-Stokes Raman scattering (CARS) carried out using the BOXCARS technique is applied to quantify the density and temperature of hydrogen, methane, and methyl in a process plasma generated by a surfatron in the pressure range from 30 to 3000 Pa. Below approximately 500 Pa all species and their temperatures have a flat distribution dominated by diffusive processes. At higher pressure the plasma localizes near the inner surface of the surfatron tube. A strong depletion of the methane ground state density is observed, where the gas temperature remains at 400 K on a moderate level. On the other side the methyl density is high at the position of high methane depletion. The measured methyl density is a factor of 200 lower than the methane density, in contrast to the factor 10–20 often reported in the literature. The measured data can be interpreted by proper modeling by means of the Boltzmann equation of electrons, balancing of chemical reactions and diffusive processes. The calculated methane and methyl densities agree with the measured ones only if the electron temperature is assumed to be near 1 eV and thus much lower than predicted in previous papers.     

Studies on Gas Purification by a Pulsed Microwave Discharge at 2.46 GHz in Mixtures of N2/NO/O2 at Atmospheric Pressure

Pulsed microwave discharges operated at atmospheric pressure in gas mixtures containing N₂, O₂, and NO are investigated experimentally and theoretically for various gas mixture constituents and operating conditions with respect to the ability of exhaust gas purification. The rotational gas temperature and the vibrational temperature of N₂ are derived from CARS measurements. The composition of the exhaust gas after treatment is monitored using FTIR spectroscopy. The processes of the chemical, electronic, and vibrational kinetics are described by a model that has been developed to calculate the species densities. The results obtained show that in N₂/NO gas mixtures an overall reduction of NO_x takes place. In the case of N₂/O₂/NO gas mixtures, no net reduction of NO_x is achieved for a pulsed microwave power below 3600 W, a pulse length of 50 s, and a typical repetition frequency of 2 kHz.     

Absolute Calibration of TALIF of Atomic Nitrogen by NO Titration—Experimental and Theoretical Analysis

The method of absolute calibration of the TALIF signal of atomic nitrogen by NO titration in the afterglow of a flow tube reactor has been analyzed using TALIF and emission spectroscopy. An increase in production of atomic nitrogen at titration beginning and an asymmetric parabolic curve for emission of NObands have been observed. Of three possible explanations of these effects the model of wall recombination was favored: it led to fitted wall recombination coefficients referring to wall loss factors, which where in the same dimension as literature values and was able to simulate a hysteresis effect, which has been observed experimentally. According to the observations the titration end point cannot be used to determine the atomic nitrogen density in the afterglow before any NO has been added into the system as the titer produces part of this density itself. But for absolute calibration purposes referring to the wall recombination model the linear decrease of the TALIF signal with increasing titer concentration can still be used as straight line of calibration. Only a correction of the calibration factor by a factor of 0.59 for 5 mbar titrations has to be taken into account as the titer shows wall interaction not only in a catalytic way, but is partly consumed.     

Resonance enhanced coherent anti-stokes raman scattering and laser induced fluorescence applied to CH radicals: a comparative study

Resonance enhanced CARS and LIF have been applied to the CH radicals in a microwave excited Ar/H/CH plasma ( Pa, kW). Both techniques yield similar nonthermal rotational population distributions of CH(X) in its vibrational ground state (), which can be described by two rotational temperatures, K being in the order of the gas temperature for rotational states with , and a considerably higher for the higher rotational states. This result is in good agreement with previous resonance CARS and LIF measurements in similar plasmas. With resonance CARS additional measurements on CH in the state could be performed yielding a vibrational temperature of 2440 K, the total CH density was about m. The detection limits of both techniques are determined, in our case about CH radicals per quantum state in the detection volume, and their advantages and disadvantages are discussed. Received: 29 April 1996 / Accepted: 19 June 1996