Self-reversal broadening

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. 

It has been found, however, in practice that a perfectly straight analytical working curve (— log T plotted against concentration) is seldom obtained in atomic absorption spectroscopy. The reasons for this are usually a combination of instrumental problems broadening of the emission line of the light source due to self-reversal, Doppler and pressure broadening of the absorption lines of the atoms in the flame, failure to exclude flame emission entirely, use of a focused instead of a parallel... 

Pulsed lamp background correction A very simple method of background correction has been proposed by Smith and Hieftje [25] and is therefore known as the Smith—Hieftje method. It is based on the self-reversal behaviour of the radiation emitted by hollow cathode lamps when they are operated at high currents. This ef feet is seen when a large number of non-excited atoms are brought into the vapor phase. These atoms absorb the characteristic radiation emitted by the excited species. At the same time, a significant broadening of the emission line is observed. 

It is assumed that only the background absorption is measured at high lamp current (which is an only partially valid assumption), however, the broadening of the line profile is limited and self-reversal is not complete. Thus, radiation is still emitted at the centre of the emission line and is absorbed by the analyte, and will subsequently be subtracted from the gross absorption of the analyte. This significantly reduces the sensitivity of the determination, with an average loss of sensitivity of ca. 45 % being observed for the elements most commonly determined by... 

If the HCL is run at a high current, an abundance of free atoms form. These free atoms absorb at precisely the resonance lines the hollow cathode is intended to emit an example is shown in Figure 6.28, with the free atoms absorbing exactly at K over an extremely narrow bandwidth (Figure 6.28b). The result is that the line emitted from the HCL is as depicted in Figure 6.28c instead of the desired emission line depicted in Figure 6.28a. The emitted lines are broadened by the mechanisms discussed in Section 6.1.1. The phenomenon of absorption of the central portion of the emission line by free atoms in the lamp is called self-reversal. Such absorption is not easily... 

In most laboratory sources the excitation temperature decreases towards the boundary of the discharge region. Consequently the absorption in this part of the source is increased and the profile of the emitted radiation is not only appreciably broadened but often shov/s a pronounced dip at the centre of the line. This effect is known as self reversal and it has been studied in detail by Cowan and Diecke (1948). It is particularly important that self reversal is avoided in the lamps used in the resonance fluorescence experiments described in Chapters 15-17, for the strengths of the signals are proportional to the intensity at the line centre frequency... 

Reversed Radiation.—A pressure broadened resonance emission line with radiation in the middle of the line virtually absent because of self-absorption near the walls, (e.g., the 2537 A. line obtained from medium pressure mercury lamps). 

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