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Applied Optics

Possibility of lifetime measurement in a flowing medium with phase fluorometry

Jianbang Liu, Changsheng Liu

Published May 1, 1991• 0 citations
Publisher

Abstract

The possibility of lifetime measurement in a flowing medium with phase fluorometry is investigated theoretically. A 3-D time dependent partial differential equation of the number density of atoms (or molecules) in the upper level of the fluorescence transition is solved analytically, taking flow, diffusion, optical excitation, decay, Doppler shift, and thickness of the excitation light sheet into account. An analytical expression of the intensity of the fluorescence signal in the flowing medium is deduced. Conditions are given, in which the principle of lifetime measurement with phase fluorometry in the static sample cell can be used in a flowing medium.

DOI
10.1364/ao.30.001688

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The possibility of lifetime measurement in a flowing medium with phase fluorometry is investigated theoretically. A 3-D time dependent partial differential equation of the number density of atoms (or molecules) in the upper level of the fluorescence transition is solved analytically, taking flow, diffusion, optical excitation, decay, Doppler shift, and thickness of the excitation light sheet into account. An analytical expression of the intensity of the fluorescence signal in the flowing medium is deduced. Conditions are given, in which the principle of lifetime measurement with phase fluorometry in the static sample cell can be used in a flowing medium.
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Lifetimes of 13 upper states of optical-resonance transitions in selected metal atoms have been measured using the phase-shift method. Data were taken over a range of at least 12 of fluorescence intensity, to take into account the effects of radiation entrapment and exciting light scattering. The following results were obtained: Na 3p2P0 16.2 nsec, Tl 7s ²S12 7.65 nsec, Tl 6d²D32 6.9 nsec, In 6s ²S12 7.5 nsec, In 5d ²D32 7.9 nsec, In 5d ²D52 7.9 nsec, Ga 5s ²S12 7.6 nsec, Ga 4d2D 7.7 nsec, Cu 4p2P0 7.2 nsec, Ag 5p ²P120 6.7 nsec, Ag 5p ²P120 7.5 nsec, Pb 7s3P10 6.05 nsec, and Bi 7s ⁴P12 5.9 nsec. Estimated absolute accuracies vary between 3 and 7%.
In this work we expose a recently developed theoretical model for the analysis of the measurements obtained with the so-called pinhole method, for the determination of the upper level lifetime in materials where the superimposition between fluorescence and absorption spectra determines significant radiation trapping effect. Under fairly general conditions the fluorescence decay curve after a pulsed excitation of the sample obtained with this experimental set-up can be conveniently described by a double exponential functional form. The faster of the two decay times is close to the intrinsic fluorescence lifetime, with a difference that can be calculated with geometrical considerations. The theoretical results were tested with a suitably designed experiment, where the upper level lifetime of the laser transition of some Yb doped samples were investigated, obtaining results that are consistent with literature data and in good agreement with the theoretical results as for the temporal dependence of the fluorescence decay.
The study solves the problem of the luminescent photons propagation in a resonantly absorbing medium consisting of atoms of the studied substance and a buffer inert gas. The Monte Carlo method was used in numerical experiments carried out to simulate real processes that occur in a chamber designed to determine the lifetime of an individual atom in an excited state by the method of measuring the luminescence intensity decay time. Findings of the research show that when luminescent photons are repeatedly scattered (scattering means the process of absorption and re-emission of a photon by an atom) in a medium, the luminescence decay time noticeably increases, reaching a value greater than the average lifetime of an individual atom in an excited state. The reflection of photons from the walls that make up the chamber can lead to errors in measuring the lifetime. The process of luminescence decay is studied theoretically for various detunings of the laser frequency from the resonant transition frequency. A three-level model of the atom and a model of complete frequency redistribution were applied. The study describes an algorithm based on the Monte Carlo method, which is used to model the three-level population kinetics, laser radiation transfer, radiation trapping, and frequency redistribution