The electrical conductivity is calculated for some typical seeded gases of magnetohydrodynamic interest. The electron density is obtained using Saha's equation. A method is developed for calculating electron mobility in gas mixtures, taking account of the variation of the scattering cross section with energy. Scattering by ions is also considered. Results are presented for several gases seeded with small amounts of Cs or K in the temperature range from 2000° to 4000°K.

Conductivity of Seeded Atmospheric Pressure Plasmas

Vaporization of individual solute particles in flame spectrometry

We describe a novel approach for measuring fluid bulk properties such as refractive index (RI) and temperature changes in tubes of capillary dimensions based on a simple optical configuration which uses an unfocused He-Ne laser, a cylindrical tube, and a photodetector. Side illumination of a fused silica capillary produces a fan radiation in a 360° plane normal to the central axis of the capillary that is spatially well defined in the lateral direction. It is shown that, upon viewing a high-contrast interference pattern contained in the beam profile on a flat imaging plane in a direct backscattering configuration, the physical characteristics of the capillary and the bulk properties of the fluid contained within the tube can be determined. Position changes in the maxima and minima in the intensity-modulated profile (interference fringes) are directly related to the refractive index of the fluid in the tube. Measurement of the fringe shift by a slit-photodetector assembly facilitates the determination of dn/n at the level of 1.8 x 10 -7 , while temperature changes can be measured using the fringe centroid position at a level of 5.8 x 10 -5 °C (DL = 3σ). Each determination can be made within a probe volume of 2.6 nL. Also, awide range of tube diameters can be employed (75-775 μm i.d.) with no modification to the optical train or measurable degradation in system performance. We report results for an optical ray trace model for predicting fringe pattern energy distributions, optimization of the optical configuration, and general system performance predictions. Comparison of our four-beam interference model with experimental results illustrates the utility of the model.

Capillary-Scale Refractive Index Detection by Interferometric Backscatter

It is shown how Langmuir probes can be used in flames to obtain not only the positive ion concentration but electron concentrations and electron temperatures. An internal method is presented for checking the results by comparing the wall potential calculated from the above three quantities with the observed wall potential. Satisfactory agreement between these two values in hydrocarbon air/or oxygen flames from 1.5 to 760 mm Hg increases the confidence in the use of Langmuir probes to obtain plasma profile properties in flames. The accuracy of positive ion and electron collision cross sections appears to be the major factor limiting the accuracy of the probe.

Detailed results are presented for a number of flames. The positive ion concentration always exceeds the electron concentration indicating the formation of negative ions. Electron temperatures exceed the gas temperature and do not decay as rapidly as might be expected.

A mass spectrometric technique for obtaining ion profiles of good spatial resolution is outlined and detailed profiles are presented for an acetylene–oxygen flame at 2.5 mm Hg. The ion CHO+; peaks ahead of C3H3+; which precedes H3O+;. Many other ions are observed to peak at about the same position in the flame as C3H3+;. There are still problems, however, with respect to interpreting the results in terms of the first ion produced from neutral species and the sequence of ion molecule reactions which follows.

Ion and electron profiles in flames

The broadening and shift of the sodium D2 line emitted from an acetylene-oxygen flame with N2, Ar, CO2 and He, respectively, as the perturbing gases have been investigated using a Fabry-Perot interferometer. In the case of He, the results show a significant deviation from values calculated on the basis of Lindholm's impact broadening theory when a van der Waals potential is assumed for the interaction between emitter and perturber. An attempt is made to describe the experimental results by the named theory with a Lennard-Jones potential. The constants of the van der Waals and repulsive forces between the sodium emitter and the helium atom are evaluated.

Broadening and shift of the sodium D2 line by various perturbing gases under flame conditions

A method for determining optical cross sections, based on finding the point of intersection of the low- and high-density asymptotes of an intensity-density curve, is extended to elements of low ionization potentials. These elements suffer partial thermal ionization in the flame gases at the relatively high temperatures of 2500° and 2760°K which are used as spectroscopic light sources. Since the degree of ionization of the carrier atoms depends on their partial pressure, the intensity-density curves may be distorted. This difficulty is overcome by increasing the concentration of electrons in the flame gases and thereby shifting the ionization equilibrium to the side of neutral atoms. For the alkaline earth elements the existence of equilibrium ionization is tested for by direct, independent measurements of the partial pressures Avhich enter into the Saha equilibrium quotient. Optical cross sections are measured for the resonance lines of Li, Na, K, Rb, Cs, Ca, Sr, and Ba. The values for the alkalis exhibit a moderate increase with atomic weight which is in accordance with the Weisskopf-Lindholm theory of impact broadening.

Optical Cross Section of Resonance Lines Emitted by Flames under Conditions of Partial Thermal Ionization

Absorption and Dispersion of Microwaves in Flames

Direct determinations of the concentration of electrons in flame gases were needed to justify the method, described in the preceding paper, to eliminate the distortion of intensity-density curves due to ionization of the metallic flame additives. Measurements of the attenuation of a microwave beam upon crossing a column of flame gases were performed, and electron concentrations in an acetylene-air flame (T = 2500°K) containing one or mixtures of two alkali elements were obtained. The results indicate that the ionization of the metallic flame additives is thermally equilibrated in the flame zone chosen for the optical line-intensity measurements.

Fritz W. Hofmann

Papers

Optical Cross Section of Resonance Lines Emitted by Flames under Conditions of Partial Thermal Ionization

Absorption and Dispersion of Microwaves in Flames

Microwave Determination of Electron Concentrations in Flame Gases Used as a Spectroscopic Light Source

Cyclotron Resonance in Flames