Flow atom concentration measurement

If electronically excited species can be made to give up their excess energy to some active material, then their concentration may be determined by measurement of the rate of heat liberation. The method is, of course, well established for the measurement of oxygen and hydrogen atom concentrations, and the most accurate experimental technique is to use the isothermal hot-wire calorimeter developed by Tollefson and LeRoy.42 The amount of power needed to maintain a catalytic probe at a constant temperature is reduced if heat is liberated at the probe, and no correction is needed for heat losses. The flow of energy-rich species, [Pg.325]

Absolute H-atom measurements also were made using the Na/Li method (1(3) in sulfur free flames. An aerosol of an equimolar solution of NaCl and LiCl was added to the central core flow through the nebulizer. Relative intensity measurements were made of the Na 589.0 nm and Li 670.8 nm emission from which the H-atom concentrations were calculated. The H-atom measurements could only be made in the sulfur free flames. Reaction of Na or Li with sulfur species would render the technique inoperative. 

Mahan and Solo studied the reaction in a stirred continuous flow reactor, in which O atoms produced by a microwave discharge through pure O2 or 02-inert gas mixtures reacted with CO. O atom concentrations were measured by titration with NO2. They found that radiation accompanied reaction 0.29 % of the time for the process at 298 °K, and concluded that the reaction was second-order. They proposed the mechanism... 

Rodebush and Klingelhoeffer measured the rate of the reaction Cl+H2 HCl-f-H by producing atoms in an electrodeless discharge and flowing the Cl into an hydrogen stream. The concentration of Cl atoms was measured with a diffusion gauge of the Wrede type. The HCl produced was collected and subsequently determined by titration. The calculated rate coefficients at 289 °K and 273 °K are... 

Some reactions of ground-state fluorine (2P) atoms generated by 2.45 GHz discharge of dilute F2 + He mixtures have been studied mass spectrometrically with a beam inlet system from a fast flow reactor.3 Fluorine atom concentrations were measured accurately from the consumption of Cl2 in the simple and extremely rapid bimolecular F + Cl2 — FC1 + Cl reaction, for which AL/298 = —9 kJ mol-1, rate constant = 1.1 x 10-10cm3 molecule-1 s-1 at 300 K. The recombination rate of fluorine atoms has been determined at 295 K and at pressures of 10—81Torr, with Ar as carrier gas.4 The data are consistent with a third-order homogeneous reaction whose rate constant is significantly lower than that predicted theoretically and also lower than that for most other atom-recombination reactions under similar conditions. 

Fig. 48. Atomic oxygen concentration, measured by optical emission actinometry, as a function of radius in a 13.56 MHz oxygen discharge sustained in a diode reactor. The electrode is covered with a reactive film up to a radius of 3.75 cm. This film acts as a sink for atomic oxygen (loading) resulting in significant radial concentration gradients. Such gradients are responsible for etch non-uniformity. Solid lines show the result of mathematical model predictions. After [231]. Pressure 2 torr, gas flow 100 seem.

In order to demonstrate the performance of the Doppler-burst envelope integral value method for the estimation of the instantaneous particle velocity vector and the particle mass flux or concentration, measurements were performed in a liquid spray issuing from a hollow cone pressure atomizer (cone angle 60°) and a swirling flow which exhibits complex particle trajectories (Sommerfeld and Qiu 1993). All the measurements were conducted using the one-component phase-Doppler anemometer. The integration of the mass flux profiles provided the dispersed phase mass flow rate which agreed to 10 % with independent measurements of the mass flow rate (Sommerfeld and Qiu 1995). 

Braun and co-workers [32] at the National Bureau of Standards have recently developed a powerful new method of studying H atom reactions. The H atoms are generated from the flash photolysis of an olefin with <1% of photodecomposition and the H atom concentration monitored from the resonance fluorescence by atomic H excited by absorption of Lyman-a 1216 A radiation. So far only one transfer reaction has been studied over a range of temperatures by this method but others are likely to follow. The technique has also been extended to measurements of H atom concentrations in fast flow systems [42]. 

In addition to the use of spatially-resolved concentration measurements for the determination of rate constants for reactions of ground state atoms, the discharge-flow method has been extensively applied to kinetic and spectroscopic studies of chemiluminescent phenomena. In these cases, the flow parameters in the flow tube are of no great importance, as time resolution is not obtained from axial displacements consequently, the total pressures and flow rates, and tube diameters may be varied over wide limits, since it is unnecessary to ensure adherence to the conditions for plug flow. 

In the first section of this chapter the methods of production of atoms and the determination of their concentrations in discharge-flow systems are discussed, with particular reference to two important problems. Firstly, the identification of secondary active species which may accompany the primary active species (a ground state atom) in the products of an electric discharge and secondly, a critical discussion of various methods for the measurement of atom concentrations. 

Rate constants measured in discharge-flow systems depend upon measurements of (a) the velocity of flow u, and (b) the partial pressures of reactants. For a reaction first-order in atom concentration [A], the rate constant is given by = iJ d In [A]/dx = — (RTZFlAp)d In [A]/ dx, where SF is the total flow rate, A is the cross-section area of the flow tube, p is the total pressure, and x is the displacement along the tube axis. For simple reactions of higher overall orders, the dependences of rate constants upon the parameters SF, A, p and reagent flow rate F< are summarized in Table 4.2. The importance of accurate measurements of flow rates and of total pressure, particularly for reactions of overall second and third orders, is clear. For example, realistic random errors of 1 % in/ , and of 3% in SFand F<, lead to an error of 12% in Atj. 

The methods listed above all enable relative concentrations of atoms or radicals to be measured. It is a much more difficult problem to measure absolute magnitudes of atoms and radicals in discharge-flow systems, or indeed in any other systems such as flash photolysis experiments. Two principal methods are used for the derivation of absolute concentrations (a) the combination of spectrometric measurements with calculated transition probabilities or (b) the use of the stoichiometry of rapid titration reactions. Of these methods, (b) is probably the most frequently used at the present time. Emphasis will be given to the possibilities of absolute concentration measurements in the discussion of the methods which follows. 

Consider an elementary transfer reaction of an atom with a stable molecule, of simple stoichiometry, and sufficiently rapid for at least 99% extent of reaction to occur within the time resolution of a discharge-flow system (1 to 100 ms). Such a reaction constitutes a possible titration reaction for the measurement of atom concentrations if some means of detecting atoms in the system is also available. An atom indicator is sometimes provided conveniently by a chemiluminescent emission associated with the titration reaction. 

Whilst the rate constant k may thus be deduced from measurement of [A] at different distance (times) along the length of the flow tube, a more satisfactory technique for the measurement of pseudo-first-order rate constants k is the fixed observation point method. In this method, the atom concentrations [A] remaining at a fixed observation point are measured when the same (excess) concentration of reagent [R] is added in turn at each of several inlets along the flow tube. 

The abstraction of H from HOF by atomic F in a discharge-flow apparatus at 296 to 298 K was measured mass spectrometrically by determining F concentrations. In about 1 Torr of He carrier gas with an initial F atom concentration of -1.5 xIO cm , the reaction... 

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