Abstract

An application of the optogalvanic effect in a premixed H₂-O₂-Ar flame, called laser-enhanced ionization (LEI), employs a dye laser, which is tuned to resonance with the 3<i>S</i>_1/2-3<i>P</i>_3/2 transition of sodium analyte. The ionization mechanism is dominated by the collisional process following resonant excitation. With addition of a second N₂ laser, the resulting dual-laser ionization (DLI) technique displays enhanced sensitivity; furthermore, the multiphoton ionization process dominates the ionization mechanism. A theoretical model based on the density matrix formalism (or master equation approach) has been employed to interpret ac Stark broadening and intensity dependence observed in Na DLI line shapes, and to determine the ionization rates. The DLI photoionization rate obtained from the model agrees satisfactorily with that produced by other authors. It is well known that the density matrix formalism can properly take into account the collisional effect and the saturation effect observed in the power dependence measurement of DLI.

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