Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

7 ELECTRICAL METHODS FOR CHARGED SPECIES 329insulation properties, has been reported’76. Langmuir probe studies have beenused also in indirect determinations of negative ion concentration^^^^. The electronconcentration, measured with a probe, in low-pressure acetylene-oxygenflames, is only a few per cent of the positive ion concentration, and the negativeion concentration may be obtained by difference. The very high apparent concentrationsof negative ions calculated in this manner do, however, appear to be atvariance with mass spectrometric values. Travers and Williams276 believe thatsingle probe methods are not particularly suitable for the measurement of electronconcentrations, and describe a double probe technique which they have developed.Unfortunately, it seems that although the double probe technique can be used tomeasure positive ion concentrations (with less spatial resolution than the singleprobe technique) yet it is unsuitable for the measurement of electronconcentrations.In reply to the criticisms of the use of single probes, Calcote has said’” “Therefore,for determining local electronconcentrations in a flame, one is left with twochoices:the use of single probes which is fraught with experimental difficulties, or to makeno measurements at all”. It seems, then, that at the present time single probeexperiments are at least as useful as double probe studies. The use of Langmuirprobes has been extended to the study of ionization in shock tubesz7’.Another way in which the conductivity of a flame, and hence the electron concentration,may be determined is by the use of microwaves. Studies of this kindhave the great advantage that there is no interference with the plasma, but at thesame time the spatial resolution is rather poor (since it depends on the size of thewave guides used). In a series of papers Sugden et al. have examined microwavemethods for the determination of electron concentrations in flames containingtraces of alkali metals. The earliest crude results’79 were extended to studies ofcoal-gas flames’”. In the latter experiments the flame was placed in the path ofthe microwave radiation and the attenuation determined. Sugden and ThndZa1improved the sensitivity of the microwave technique by introducing the conductingcolumn of flame gases into a resonant cavity. The “Q” of the cavity is reduced, andthe conductivity of the gases may be calculated from the “Q” values. Sufficientsensitivity was available in this apparatus to make possible determinations of theelectron concentration in “pure” acetylene-air flames (Le. free from alkali metalsalts). Refinements in microwave technology have subsequently allowed muchgreater sensitivity in the instruments. Thus Shuler and Weber’” used a bridgecircuit, slightly off balance, with the flame between two electromagnetic horns inone arm of the bridge. Concentrations as low as 5 x electrons cm-j could bedetected with this apparatus, and hence the electron density could be measured in“pure” hydrogen-oxygen and. acetylene-oxygen flames. Sugden” has given areview of this work, together with the simplified-but sufficiently accuratetheoryrequired for the interpretation of the results.The microwave absorption of an electron cloud may have a resonance characterif a magnetic field be applied. The condition for this “cyclotron” resonance is thatReferences pp. 336-342