Use of a rotating single probe in studies of ionization of metal additives to premixed flames. Part 1.—Measurement of total positive ion concentrations, and ionization of gallium, indium and thallium

Abstract

A procedure, based on the observed kinetics of ionization of alkali metals, is described for the quantitative calibration of a rotating single probe for total positive ion concentrations (in the range 4 × 10⁷–4 × 10¹² ions cm^–3) in premixed H₂+ O₂+ N₂ flames. Points of agreement with, and departure from, predictions based on the probe theory of Su and Lam are noted.

The rates of ionization of Ga, In and Tl have been measured in H₂+ O₂+ N₂ flames, in the temperature-range 2200–2800°K. The results are consistent with operation of the overall process M + X → M⁺+ e^–+ X, where M represents free Ga, In or Tl atoms and X represents any flame gas molecules. When appropriate allowances for (any) compound formation are quantitively made, the activation energy for this process is determined, within experimental error, to be equal to the ionization potential of M for all three metals. Simple kinetic theory cross-sections of ∼10,000 Ų are implied by these observations. The rates of recombination of M⁺ with e^–, i.e., the reverse of the above process, were also determined in the same flames. (Above equilibrium concentrations of M⁺ were induced through addition of traces of C₂H₂.) The results confirm that no significant deviations from the rate quotient law are obtained with the above ionization and recombination processes.