Kinetics of thermal ionization of alkali metals in flames

Abstract

The rate of ionization of alkali metal atoms M in flames of H₂+ O₂+ N₂ has been measured using a quadrupole mass spectrometer to determine concentrations of each ion in a flame. The results confirm that the atoms ionize by collisions with flame gas molecules X in M + X → M⁺+ e^–+ X. The measured activation energy is, within experimental error, the ionization potential of the alkali metal and the associated cross-sections πσ² are anomalously large, being similar for each metal and in the range 2.5 ± 1.1 × 10^–16 m². These cross-sections are in excellent agreement with Kelly and Padley's values and are discussed in terms of both a “ladder-climbing” excitation mechanism and a process with an ionic complex as intermediate.