Fluorescence spectra efficiency

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. 

Polarization effects The transmission efficiency of a monochromator depends on the polarization of light. This can easily be demonstrated by placing a polarizer between the sample and the emission monochromator it is observed that the position and shape of the fluorescence spectrum may significantly depend on the orientation of the polarizer. Consequently, the observed fluorescence intensity depends on the polarization of the emitted fluorescence, i.e. on the relative contribution of the vertically and horizontally polarized components. This problem can be circumvented in the following way. 

Currently used nonlinear optical crystals are potassium dihydrogen phosphate (KDP) and barium borate (BBO). Compared to KDP, the advantages of BBO are its transparency in the UV and its larger quantum efficiency of up-conversion by a factor of 4—6. For a given position of the crystal, only a narrow band of the fluorescence spectrum is up-converted. Therefore, if the full fluorescence spectrum is of interest, the crystal must be rotated at a series of angles. An example of experimental set-up is presented in Figure 11.2. The fwhm of the response is 210 fs. 

To calculate the ratio of phosphorescence efficiency to fluorescence efficiency (see Sec. III-G) the area under the corrected phosphorescence spectrum was first divided by the appropriate value of PD/P and the resulting value was then divided by the area under the corrected fluorescence spectrum. For phosphorescence lifetimes greater than 1 msec, the value of Pu/P differed by less than 5% from the maximum value of 1/3. Most of the delayed emissions to be described later had lifetimes greater than 1 msec, and the value of PD/P was therefore generally taken to be 1/3. For a lifetime of 0.25 msec., the observed emission in the out-of-phase position was reduced to about one-half of the value corresponding to long lifetimes. As the lifetime decreased below 0.25 msec, the phosphorescence observed in the out-of-phase position decreased very rapidly. 

Fig. 2. A portion of the experimentally observed fluorescence spectrum from a 2 pm radius PMMA microparticle (top) and the calculation of scattering efficiency (bottom) for a microsphere of radius 1.77 pm (ref. index - 1.49). First order MDR are dominant in the experiment.

A fluorescence spectrum is characteristic of a given compound. It is observed as a result of radiative emission of the energy absorbed by the molecule. The observed spectrum does not depend on the wavelength of the exciting light, except that the spectrum will be more intense if irradiation occurs at the absorption maximum. The spectral intensity is called the quantum efficiency and is usually abbreviated as . The quantum yield or quantum efficiency, d>, which is solvent dependent, is the ratio Approximate values of quantum efficiencies are as follows naphthalene, 0.1 anthracene, 0.3 indole, 0.5 and fluorescein, 0.9. 

Thermolysis of peroxide [29c] in benzene solution generates a chemiluminescent emission whose spectrum is identical to the fluorescence spectrum of photoexcited p-dimethylaminobenzoic acid under similar conditions. Thus the direct chemiluminescence is attributed to the formation of the singlet excited acid. The yield of directly generated excited acid is reported to be 0.24% (Dixon and Schuster, 1981). Since none of the other peroxybenzoates generate detectable direct chemiluminescence it was not possible to compare this yield to the other peroxides. However, by extrapolation it was concluded that the dimethylamino-substituted peroxide generates excited singlet products at least one thousand times more efficiently than does the peroxyacetate or any of the other peroxybenzoates examined. 

Tridentate N-donor ligands are efficient in separating actinides from lanthanides selectively by solvent extraction, an area of potential great importance in treatment of used nuclear fuel rods. The tridentate ligand 2,2 6 ,2 -terpyridyl (terpy) forms a range of complexes. The perchlorate complexes [Ln(terpy)3] ( 104)3 contain nine-coordinate cations with near- )3 symmetry, a structure initially deduced from the fluorescence spectrum of the europium compound (Section 5.4) and subsequently confirmed by X-ray diffraction smdies (Figure 4.7)... 

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