Lifetime Measurements of Metastable Levels of Thallium and Lead in the Air-Acetylene Flame by Laser-Enhanced Ionization Spectrometry

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Abstract
Laser-enhanced ionization spectrometry with two-step excitation has been used to evaluate the collisional lifetime of the metastable P levels of thallium and lead in an air-acetylene flame burning at atmospheric pressure and supported by a three-slot burner head fitted on a conventional nebulizing chamber. A water-cooled molybdenum electrode immersed in the flame was maintained at high negative potential with respect to the burner body. The ionization current resulting after the two-step excitation was amplified and measured with a digital storage oscilloscope and a boxcar averager. The two excimer lasers were triggered externally with two trigger pulses, one being delayed in time with respect to the other one. In this way the second laser photon, tuned at a transition starting from the metatable level under study, could be correspondingly delayed from the first photon tuned at a transition starting from the ground state. The lifetimes measured were found to be 81 ns and 360 ns for T1 and Pb, respectively.