Measurement of collisional ionization cross-sections for metal atoms in flames

Abstract
The rates of collisional ionization of gaseous metal atoms, M, in the temperature range 2000 to 2800 K have been measured by optical and electrostatic probe studies in over 100 fuel-rich, atmospheric pressure hydrogen + oxygen flames diluted with each of the gases argon, nitrogen, carbon monoxide and carbon dioxide. Individual cross-sections for the process M + Xj → M+ + e- + Xj, with Xj = Ar, H2, N2, CO, CO2 and H2O, and M = Li, Na, K, Rb, Cs and Tl, have been determined from a detailed analysis of the dependence of the collisional ionization rate constants on burned gas composition. They lie between 10 and 145 nm2 and are given in table 4. They show no correlation with the corresponding optical quenching cross-sections for the first excited electronic states of M. The implications of these cross-sections are discussed with regard to both their large overall magnitude and their dependence on the nature of M and Xj. Elastic collision, translational de-excitation of Xj contributions appear to be at least as important as processes involving rotational or vibrational de-excitation of Xj.