Lifetime intensity-averaged

In this definition, each decay time is weighted by the corresponding fractional intensity. This average is called the intensity-averaged decay time (or lifetime). 

The definition used depends on the phenomenon under study. For instance, the intensity-averaged lifetime must be used for the calculation of an average colli-sional quenching constant, whereas in resonance energy transfer experiments, the amplitude-averaged decay time or lifetime must be used for the calculation of energy transfer efficiency (see Section 9.2.1). 

The behavior of practically all luminescent materials is sensitive to various parameters of physical and chemical origin. The excited state lifetimes and average intensities of the fluorescence and/or phosphorescence of these materials are modulated, for example, by temperature, oxygen, pH, carbon dioxide, voltage, pressure, and ionic strength. Consequently, the luminescence of various materials could be used, in principle, to monitor parameters of interest in medicine, industry, research, and the environment. 

As shown in the next section, the intracellular pH can be evaluated from the fluorescence lifetime of BCECF inside cells without any ratio methods [10]. The relation between the intracellular pH and the lifetime of BCECF in Hb. salinarum indicates that the lifetime decreases with decreasing intracellular pH. Based on the correlation function between the intracellular pH and the fluorescence lifetime, the average value of the intracellular pH of Hb. salinarum was estimated to be 7.1, which is roughly the same as that obtained with the intensity ratio method [18]. 

Since the intensity average lifetime or the mean lifetime is the average amount of time a fluorophore spends in the excited state, it is normal that this time should be applied to determine the rotational correlation time of a fluorophore, the bimolecular diffusion constant of small molecules such as oxygen, iodide and cesium ions in macromolecules and in energy transfer studies. 

Fluorescence intensity of calcofluor, whether free in water or bound to ai-acid glycoprotein decays as a sum of four exponentials. When the fluorophore is free in solution, the intensity average lifetime is 0.85 ns. It increases to 4.8 and 3.9 ns when the fluorophore is bound to the sialylated and to the asialylated protein, respectively. 

On continued excitation in the phase fluorimeter, the fluorescence lifetime of polymer 1 films also decreased with time. The lifetime decrease was exponential with an average loss constant of 8.2 1.2 x 10 lf sec-1 (1.5 pm thick film) from measurements at different sites on the film. These findings constitute direct evidence for RET from the polymer to a photoproduct(s) in support of the fluorescence intensity measurements. 

Steady-state behavior and lifetime dynamics can be expected to be different because molecular rotors normally exhibit multiexponential decay dynamics, and the quantum yield that determines steady-state intensity reflects the average decay. Vogel and Rettig [73] found decay dynamics of triphenylamine molecular rotors that fitted a double-exponential model and explained the two different decay times by contributions from Stokes diffusion and free volume diffusion where the orientational relaxation rate kOI is determined by two Arrhenius-type terms ... 

A major advantage of fluorescence as a sensing property stems from the sensitivity to the precise local environment of the intensity, i.e., quantum yield (excited state lifetime (xf), and peak wavelength (Xmax). In particular, it is the local electric field strength and direction that determine whether the fluorescence will be red or blue shifted and whether an electron acceptor will or will not quench the fluorescence. An equivalent statement, but more practical, is that these quantities depend primarily on the change in average electrostatic potential (volts) experienced by the electrons during an electronic transition (See Appendix for a brief tutorial on electric fields and potentials as pertains to electrochromism). The reason this is more practical is that even at the molecular scale, the instantaneous electric... 

If now we consider a large number of molecules N0, the fraction still in the excited state after time t would be N/N0 — e kt where N is the number unchanged at time t. This exponential law is familiar to chemists and biological scientists as the first-order rate law and by analogy fluorescence decay is a first-order process—plots of fluorescence intensity after an excitation event are exponential and each type of molecule has its own characteristic average lifetime. 

When a semiconductor is illuminated, electrons may be excited into the conduction band and/or holes into the valence band, producing photoconductivity. This excited condition is not generally permanent, and when the illumination ceases, the excess current carriers will decay, or recombine. The average time which a photoelectron remains in the conduction band is termed the lifetime. As the lifetime increases, the photocurrent, for a given intensity of illumination, increases. 

The natural radiative lifetime is independent of temperature, but is susceptible to environmental perturbations. Under environmental perturbation, such as collisions with the solvent molecules or any other molecules present in the system, the system may lose its electronic excitation energy by nonradiative processes. Any process which tends to compete with spontaneous emission process reduces the life of an excited state. In an actual system the average lifetime t is less than the natural radiative lifetime as obtained from integrated absorption intensity. In many polyatomic molecules, nonradiative intramolecular dissipation of energy may occur even in the absence of any outside perturbation, lowering the inherent lifetime. 

Figure 7. Relative luminescence intensity and decay lifetime of Cu(I)Y during exposure to oxygen at 25°C and I atm. The decay times are average lifetimes except for the last point at which two distinct lifetimes, t = 28 /isec and r = 6 ysec (not shown here), were determined.

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