Strontium atoms in the air-acetylene flame are directly photoionized in two steps provided by one dye laser tuned at the resonance ground-state transition (460.733 nm) and by the excimer pump beam at 308 nm, partially split from the amplifier section of the dye laser. The ions produced are then monitored by a third laser beam, colinear and counterpropagating in the flame, tuned to an ionic fluorescence transition and delayed in time with respect to the ionizing beams. In this way a fast decay, which is not affected by variations in the electron number density in the flame and therefore attributed to ion chemistry, and a slow decay, due to recombination, could clearly be observed. The fast decay is affected by variations in the flame stoichiometry and the slow decay by the number density of electrons in the flame, as shown by the addition of varying concentrations of an easily ionized element like caesium. The advantages of this optical probing of the laser-induced ionization in flames are discussed.
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