Quantum efficiency fluorescence

The fluorescent emission for quinine at 450 nm can be induced using an excitation frequency of either 250 nm or 350 nm. The fluorescent quantum efficiency is known to be the same for either excitation wavelength, and the UV absorption spectrum shows that 250 is greater than 350- Nevertheless, fluorescent emission intensity is greater when using 350 nm as the excitation wavelength. Speculate on why this is the case. 

The requited characteristics of dyes used as passive mode-locking agents and as active laser media differ in essential ways. For passive mode-locking dyes, short excited-state relaxation times ate needed dyes of this kind ate characterized by low fluorescence quantum efficiencies caused by the highly probable nonradiant processes. On the other hand, the polymethines to be appHed as active laser media ate supposed to have much higher quantum efficiencies, approximating a value of one (91). 

Table II. A Comparisoa of Fluorescence Quantum Efficiencies (4>p) of OPA, Danysl, and CBI Derivatives in Aqueous-Organic Mixed Solvents...

The ratio of the fluorescence quantum efficiency in the absence of the heavy atom to that in the presence of the heavy atom is given by... 

The intensity of the fluorescence emission detected at the photodetector stage was plotted as a function of temperature over the same range, and this is shown in Figure 11.22. It falls off rapidly with temperature increase over the whole temperature region. This does not contradict the experimental evidence of Burns and Nathan(56) who showed that the fluorescence quantum efficiency of the ruby fluorescence integrated over the entire band from 620 to 770 nm is independent of temperature (to 5%) in the region from-196 to 240°C, for the emission detected here is only the A-line part of the total fluorescence emission. 

To test the above ideas, Weitz etal.(i2) performed experiments on the fluorescence decay from a thin layer of europium(III) thenoyltrifluoracetonate (ETA) deposited on a glass slide covered with Ag particles approximately 200 A in diameter. The fluorescence decay rate was found to increase by three orders of magnitude in comparison with that of ETA in solid form. In addition to the large increase in decay rate, there was also evidence for an increase in overall fluorescence quantum efficiency. It is not possible from Eq. (8.11) to say anything about the manner in which is partitioned between radiative and nonradiative processes, y should be written in terms of a radiative part yr and a nonradiative part ynr y = yr + y r. Since the radiative rate for dipole emission is given by... 

Compounds and materials, including mixtures, which can undergo radioluminescence and are used in the measurement of radioactivity. Characteristics of these substances include a high fluorescence quantum efficiency and a short fluorescence lifetime. 

A fluorescence decay of the ensemble of many CV molecules on a PMMA film is shown in Fig. 12. The decay was not fitted to a single exponential function and a stretched exponential function, but was well fitted to a biexponential function I(t) = Af exp(-t/xf) + Asexp(—t/ts), where tf and ts are time constants and Af and As are pre-exponential factors. We obtained tf = 0.43 ns and ts = 1.76 ns, and the ratio As/Af = 1.14. Compared with the excited state lifetime of CV (2-3 ps) in methanol and ethanol [5-8,58-68], the fluorescence lifetime of CV on a PMMA film increased more than two orders of magnitude [9-12] thus, so did the fluorescence quantum efficiency. The enhancement of the fluorescence efficiency of CV on a PMMA film made it possible to observe single CV molecules. Figure 13 shows fluorescent spots on a PMMA film on which a drop of 1-nM CV in methanol was spin-coated. The number of fluorescent spots in an image linearly increased with increasing concentration of a CV methanol... 

The fluorescence quantum efficiency will be the ratio of the radiative transition rate to the total rate for all three processes. It is therefore important to know the factors that control the two nonradi-ative relaxation processes. 

Variables 3 and 4 are found in the familiar relationship, i.e., that fluorescence in dilute solutions is directly proportional to the absorbance of the solute AX at the wavelength selected for excitation, times the quanta of light IoX available at the wavelength selected for excitation, times the fluorescence quantum efficiency X for the solute. Oftentimes the wavelengths available for maximum illumination in an excitation source do not... 

In TPA induced fluorescence spectroscopy the excited state population for fluorescence is created by the simultaneous absorption of two photons instead of the one-photon absorption in standard fluorescence experiments. The principles of this technique are described in Ref. [23]. It allows an experimental determination of two-photon absorption cross-sections provided the material is fluorescent and that its two-photon fluorescence quantum efficiency is known (which is usually assumed to be equal to the one-photon fluorescence quantum efficiency if the same excited state is reached). In this case the method generally provides high sensitivity. 

Taking as a reference level, the fluorescence quantum efficiency of the crystal in the absence of triplet exciton interactions, relative quantum efficiency can be defined by the ratio [Pg.100]

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