Influence of the shape of the exciting laser pulse on fluorescence saturation in the quantitative analysis of dissolved trace organic substances

Excitation nonlinearities in emission reabsorption laser-induced fluorescence techniques

“…This assumption is somewhat unrealistic in that most lasers used in fluorescence produce temporal and spatial pulses with Lorenzian, Gaussian, or similar shapes. However, it was shown by Georgiev et al 12 that the influence of laser pulse shape on the fluorescence saturation condition is small for pulses with high peak intensity ͓greater than 10 25 photons͞ s͞cm 2 ͔ and short duration ͑less than 6 ns͒. Both of these conditions are met by the Nd:YAG laser system used for ERLIF purposes, whose average pulse intensity is of the order of 10 25 photons͞s͞cm 2 ͑the peak intensity is therefore higher͒ and duration is 3 ns.…”

“…͑12͒ along with its decomposition into the terms within the braces ͑denoted Terms 1-5͒ as a function of excitation photon flux. The following dye and laser parameters ͑representative of the pyrromethene group and frequency doubled Nd:YAG͒ were used [12][13][14] A ϭ A 21 ͞⌽ ϭ 5.3763 ϫ 10 8 s Ϫ1 , p ϭ 5 ϫ 10 Ϫ9 s, and EM ϭ c͞ EM ϭ 4.8354 ϫ 10 14 s Ϫ1 . In terms of the total number of dye molecules present, which is given by n ϭ CV ϭ CA f t, a concentration of 2.4 ϫ 10 Ϫ2 mol͞L, a fluorescence area of 4.9087 cm 2 , and a film thickness of 50 m, which correspond to conditions employed below in our experimental validation, give n ϭ 3.5473 ϫ 10 17 .…”