Fluorescence self-absorption

In X-ray fluorescence, self-absorption due to optical quenching of the sample is always observed. For solid samples, radiation can be observed only at a depth of a few micrometres. The intensity P of radiation, after travelling a distance dx in a material which a lineic absorption coefficient p (cm ), will decrease by dP for a penetration angle of 90°. The integrated form of the expression dP = —pP dx is comparable to that described in colorimetry ... 

Figure 4.13 Fluorescence self-absorption effects for sodium molybdate NajMoO,.

The lower pair of traces shows aqueous solutions at concentrations of 1 mM Mo (no self-absorption, solid line) in a 10 mm path-length XAS cuvette (Figure 4.12) and a computed spectrum including fluorescence self-absorption for 50 mM Mo. The upper pair of traces shows an undistorted transmittance spectrum (solid line) and a computed self-absorption distorted fluorescence spectrum (broken line). All calculations were done starting from undistorted data using the program fluorgen of the EXAFSPAK software suite. 

Ema data can be quantitated to provide elemental concentrations, but several corrections are necessary to account for matrix effects adequately. One weU-known method for matrix correction is the 2af method (7,31). This approach is based on calculated corrections for major matrix-dependent effects which alter the intensity of x-rays observed at a particular energy after being emitted from the corresponding atoms. The 2af method corrects for differences between elements in electron stopping power and backscattering (the correction), self-absorption of x-rays by the matrix (the a correction), and the excitation of x-rays from one element by x-rays emitted from a different element, or in other words, secondary fluorescence (the f correction). 

Figure 6. Tempcraiure dependence of the fluorescence lifetime of BMPC in 1 1 ethanol-mcihanol. Measurements were carried out at the LENS laboratory of Florence by a picosecond apparatus using as an excitation source (at 380 nm) a dye laser pumped by a frequency-doubled cw Nd-YAG laser and recording the fluorescence time jirofiles by a streak camera. Since the overall insuumental response time was 75-80 ps, decays with t>200 ps, observed at T<130 K, were analyzed without deconvolution. At 177, 178 and 193 K, the lifetimes were roughly estimated as i=(FWHM -77 ), where FWHM was the width at half maximum of the decay. Because of the rather high sample absorbances (An,x=2), self absorption may have reduced the lifetimes to some extent.

Concentration At high concentrations fluorescence emission l ecomes non-linear due to self-absorption by the sample itself or complete absorption of the excitation energy before it reaches the cell center. High fluorescence Intensity may overload the photomultiplier tube which returns slowly to its normal operating conditions and misrepresents the actual fluorescence signal until restabilized. 

Emission inner filter effect (self-absorption) The fluorescence photons emitted in the region overlapping the absorption spectrum can be absorbed (radiative energy trans-... 

Self-absorption occurs when the fluorescence due to some excited molecular entity(ies) is absorbed by other molecular entities of the same species which are in the ground state. 

The relation between fluorescence intensity and structure was studied by Goldzieher et al. [113]. For this work, 0.2 mL of an ethanolic steroid solution was added to 1 mL of 90% (v/v) sulfuric acid, the mixture heated at 80°C for 10 minutes, and finally diluted with 4.0 mL of 65% (v/v) sulfuric acid. The resultant solution contained 25-250 pg/ml, and was measured at an excitation wavelength of 436 nm. No correction was made for self-absorption of the solutions. 

Fig. 17. Delayed fluorescence spectrum of 5 X 10-63/ anthracene in ethanol.84 Half-bandwidth of analyzing monochromator was 0.05 ju-1 at 2.5 n K Intensity of exciting light was approximately 1.4 X 10 einstein cm. a sec.-1 at 2.73m-1 (366 mju). (1) Normal fluorescence spectrum (distorted by self-absorption). (2) Delayed emission spectrum at sensitivity 260 times greater than for curve 1. (3) Spectral sensitivity of instrument (units of quanta and frequency).

Fig. 18. Normal fluorescence of pyrene in ethanol.42 (1) 3 X 10 W, (2) 10-5M, (3) 3 X 10 4JI/, (4) 2 X 10 Af, The instrumental sensitivity settings for curveB 1 and 4 were approximately 0.6 and 3.7 times that for curves 2 and 3. The short wavelength ends of the spectra in the more concentrated solutions are distorted by self-absorption.

The relationship between the weight concentration of an element and the intensity of one of its characteristic lines is complex. Several models have been developed to correlate fluorescence to weak, atomic concentrations. Many corrections have to be made due to inter-element interactions, preferential excitation, self-absorption, and fluorescence yield (heavy elements relax more quickly by internal conversion without emission of photons). All of these factors require the reference sample to be practically the same structure and atomic composition for all elements present as the... 

Fig. 2. Time resolved fluorescence spectra of all-trans PRSB in methanol (black) and octanol (grey) for a) t<50 fs and b) t>50 fs. The intensity of the octanol spectra is adjusted the methanol spectra. The spectra are not corrected for self-absorption (for >19.500 cm 1), or for the detector response function. A residual signal appearing at energies <14.000 cm"1 is due to incomplete background subtraction (see above).

Maximum response of the fluorescent product was observed with an excitation wavelength of 378 nm and an emission wavelength of 518 nm in Figure 18-23. Emission is proportional to concentration only up to —0.1 xg Se/mL. Beyond 0.1 pig Se/mL, the response becomes curved, eventually reaches a maximum, and finally decreases with increasing concentration as self-absorption dominates. This behavior is predicted by Equation 18-12. 

Figure 18-23 Fluorescence calibration curve for the selenium-containing product in Reaction 18-15. The curvature and maximum are due to self-absorption. [From m.-c. Sheffield and T M. Nahir, "Analysis of Selenium in Bran Nuts by Microwave Digestion and Fluorescence Defection J. Chem. Ed. 2002, 79. 1345.]...

Figure 2 shows in the full curves the relative fluorescence spectrum of PST with the maximum at 32,000 cm"1 and of 10"4 mole % TPB in PST with the maximum at 23,000 cm"1. The spectra were drawn with maxima of the same height. The dashed curves show the spectral transmittance of the same samples. As seen from the overlapping of transmittance and the fluorescence quantum distribution, part of self-absorption is large, whereas it is much smaller in the case of TPB fluorescence. 

The spatial distribution of emitting species produced in the electron-hole recombination process is one of important reasons for a difference between the PL and EL spectra, and a characteristic determining the EL quantum efficiency. The self-absorption of the short-wavelength part of the fluorescence can be utilized for determining the spatial distribution of EL. The principle of the method, as discussed in Sec. 3.1 and used for photoexcited states in Sec. 3.2, has been adapted to the recombination radiation as follows [41] the unknown spatial distribution of the EL light intensity, if/ x) from a plate-shaped emitting sample, is related to the experimentally observed EL signal, El(Zo), by the expression... 

Self-absorption Absorption of part of the fluorescence from excited molecular entities by molecular entities of the same species in the ground state. The mechanism operating is a radiative energy transfer... 

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