Collisional ionization of Na atoms excited by one- and two-photon absorption in H2---O2---Ar flames

Abstract

An ionization signal was detected when a flashlamp-pumped dye laser was tuned on resonance with various one- or two-photon transitions of Na atoms in an H2---O2---Ar flame of 1800 K at atmospheric pressure. Ionization signals were obtained by collecting the electric charge with two thin iridium probes which were immersed into the flame. Relative ionization signals were measured as a function of dc probe voltage, laser tuning, Na-solution concentration, and free-O2 concentration. Fluorescence signals served to monitor the relative populations of the higher Na levels. A preliminary qualitative interpretation is given on the basis of collisional ionization from the saturated as well as the higher levels, which are mutually connected by population redistribution. With the assumption of a quasi-steady-state ionization, an effective Saha equation is derived, which connects the concentration of the atoms in the saturated level with the ionic concentration by an effective ionization constant. Values of this constant were estimated from the experiments and compared for the different excitation modi investigated. Additional one-photon wing excitation of the 3P–5S transition might play a role as an intermediate step in the ion production when the 3P level is saturated. The presence of excess O2 molecules enhances the degree of ionization, whereas the addition of comparable quantities of N2 molecules does not have this effect. This is explained by the positive electron affinity of O2, which shifts the effective ionization equilibrium. The role of multiphoton processes which directly transfer Na atoms from the 3S or 3P state to the continuum state is shown to be negligible. Also the role played by the associative ionization reaction: Na(3P) + Na(3P) → Na2+ + e− is shown to be negligible under our experimental conditions.