Emission self-absorption

Atomic emission line at (a) low concentration of analyte, and (b) high concentration of analyte showing the effect of self-absorption. 

The expression is known as the transmission integral in the actual formulation, which is valid for ideal thin sources without self-absorption and homogeneous absorbers assuming equal widths F for source and absorber [9]. The transmission integral describes the experimental Mossbauer spectrum as a convolution of the source emission Une N(E,o) and the absorber response exp —cr( )/abs M - The substitution of N E,d) and cr( ) from (2.19) and (2.20) yields in detail ... 

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. 

Self-absorption is a phenomenon whereby emitted radiation is reabsorbed as it passes outwards from the central region of the flame (cf. arc/spark spectrometry). It occurs because of interaction with ground state atoms of the analyte in the cooler outer fringes of the flame and results in attenuation of the intensity of emission. It is particularly noticeable for lines originating from the lowest excited level and increases with the concentration of the analyte solution (Figure 8.24). 

Emission intensity of sodium in the acetylene-oxygen flame at 589 nm showing the effect of self-absorption on calibration curves. 

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

High intensity, microwave powered emission sources have recently been developed that are reported to provide substantially higher DUV output than classical electrode discharge mercury lamps 76). These sources suffer from self-absorption of the intense 254 nm emission but have a relatively high output in a band between 240 and 280 nm. They are extended sources of finite size rather than point sources, and they must also be an integral part of a tuned, resonant microwave cavity. Consequently, extensive condenser design work would be required in order to utilize the microwave powered sources in projection printers. 

Some shift of the Py maximum is observed in both cases of Fig. 13. This wavelength shift may be due to self-absorption and re-emission. In the case of Fig. 13a, the absorption depth increases with increasing crystal size despite of the constant ppy. In Fig. 13b, self-absorption and re-emission increases with increasing Py loading, which is easily comprehensible. This phenomenon has been discussed more quantitatively in Ref. 3. 

Thus, the intensity of atomic emission is critically dependent on the temperature. It also follows that when low concentrations of analyte atoms are used (i.e. when self-absorption is negligible), the plot of emission intensity against sample concentration is a straight line. 

For resonance lines, self-absorption broadening may be very important, because it is applied to the sum of all the factors described above. As the maximum absorption occurs at the centre of the line, proportionally more intensity is lost on self-absorption here than at the wings. Thus, as the concentration of atoms in the atom cell increases, not only the intensity of the line but also its profile changes (Fig. 4.2b) High levels of self-absorption can actually result in self-reversal, i.e. a minimum at the centre of the line. This can be very significant for emission lines in flames but is far less pronounced in sources such as the inductively coupled plasma, which is a major advantage of this source. 

In a flame, as the concentration of atoms increases, the possibility increases that photons emitted by excited atoms in the hot region in the centre will collide with atoms in the cooler outer region of the flame, and thus be absorbed. This self-absorption effect contributes to the characteristic curvature of atomic emission calibration curves towards the concentration axis, as illustrated in Fig. 4.4. The inductively coupled plasma (ICP) tends to be hotter in the outer regions compared with the centre—a property known as optical thinness—so very little self-absorption occurs, even at high atom concentrations. For this reason, curvature of the calibration curve does not occur until very high atom concentrations are reached, which results in a much greater linear dynamic range. 

Calibration for atomic emission showing self-absorption. 

Monochromatic UV radiation is emitted by excimer lamps, in which microwave discharge5 or a radio-frequency-driven silent discharge6 generates excimer-excited states of noble gas halide molecules, which decay by the emission of monochromatic UV radiation. In the ground state, the excimer molecules decay into atoms. Therefore, no self-absorption of the UV radiation can occur. All photons are coupled out of the discharge.4... 

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).

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... 

When the intensity of a hollow cathode lamp increases because of a reduction in the shunt resistance, the profile of the emission line changes. As the central part of the cathode becomes very hot, the line is broadened for several reasons. However, vaporised atoms emitted by the cathode will reabsorb in a colder part of the lamp in the form of a very fine line. The net result is that the emission curve dips in the middle because of self-absorption. This observation is the basis of the pulsed lamp technique for correction of background absorption (Fig. 14.15). 

For higher concentrations, we need all the terms in Equation 18-12. As concentration increases, a peak emission is reached. Then emission decreases because absorption increases more rapidly than the emission. We say the emission is quenched (decreased) by self-absorption, which is the absorption of excitation or emission energy by analyte molecules in the solution. At high concentration, even the shape of the emission spectrum can change, because absorption and emission both depend on wavelength. 

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. 

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