Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

330 DETECTION AND ESTIMATION OF INTERMEDIATESthe angular microwave frequency, w, should be equal to the cyclotron frequency,w,, given by w, = eH/mc. The collision frequency, v, of electrons (with atoms ormolecules) is required for an evaluation of the electron density from microwaveattenuation measurements. However, the integral absorption in a cyclotronresonance experiment is a function of the electron concentration alone, so that thecyclotron method is, in this respect, more satisfactory than the microwave attenuationmethod. The relative half-width, AH, of the cyclotron resonance curve does,in fact, give a measure of the collision frequency, and it may be shown that AH/H=2vlw. Cyclotron resonance in flames was first observed by Schneider and HofmannZs4with low pressure acetylene flames. The resonance curve becomes broaderand less intense as the pressure (and, therefore, v ) increases, and with an acetyleneairflame, operating in the pressure range 30-200 torr, resonance could be detectedonly when potassium chloride was added to the flame. Stable flames couldbe established in the pressure range 6-25 torr with acetylene-oxygen, and atthe lowest pressures resonance could be observed without added alkali metal.Bulewicz and Padley have used the cyclotron resonance technique to investigateflames of acetylenez85*z86, cyanogenzS5 and a variety of other fuelsz8'. Usefulinformation has emerged about the collision cross-sections of electrons withother species; in the study of acetylene flameszs6 the cross-sections were obtainedfor collision with nitrogen, helium, argon, neon, water and carbon dioxide aswell as with the reactant gases.Radio frequencies (say in the range 10-100 Mc.sec-') are of the same order as thecollision frequencies of the more massive ions in flames, and experiments usingthese frequencies can yield information about both electrons and heavier ions.Smith and Sugden"' first applied the technique to flames, and gave an analysisof the effects expected on the resonant circuit characteristics when electrons orior,-, or both, are present in the flame. The resonant frequency and the selectivityof the tuned circuit may be affected by the presence of charged species in the flame.At a frequency of about 100 Mc.sec-' the resonant frequency shift and selectivitychange induced by electrons in the flame show a parallel effect, whereas if massiveions are present in sufficient concentration they will markedly affect the resonantfrequency but have little effect on the selectivity. From observations made on ahydrogen-air flame containing alkali metal salts it was possible to infer the presenceof ions. The results indicated that the ions were twenty times as numerousas the electrons, and by dividing the ions equally into alkali metal and hydroxylions (to retain a charge balance) it was possible to estimate collision cross-sectionsfor these species. More recently, ~ ~ r k ehave r ~ tended ~ ~ to ~ use * the ~ rf ~ ~resonance method for its relatively high sensitivity to electrons rather than toexploit its potential use in the study of heavy ions. One reason for this bias is thatchanges in the ohmic resistance of the flame (which results mainly from theelectrons) will swamp the effect of the heavier ions on the resonant frequency.SugdenZg1 has, however, suggested that an rf experiment be performed with a