Self-reversal of the line

Resonance absorption is most sensitive and its application is most straightforward when the emission line is not self-reversed and when it has a Doppler profile corresponding to a low translational temperature. " For molecules, the strength of an electronic transition is dissipated over many individual lines with the result that the emission and absorption intensity of any single line is generally less than for atoms. Although this lowers the sensitivity of the method, it also tends to reduce the problems which can arise from self-reversal of the lines emitted by the source. The OH radical is well suited to a study by resonance absorption because the intensity from a simple lamp emitting the A l,+ — X Il resonance system is concentrated in a few lines of the (0,0) band with low values of K, which are quite widely separated. present address Thornton Research Centre, Shell Research Ltd., P.O. Box 1, Chester. 

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. 

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). 

When the emitted radiation leaves the laser focal volume the possibility exists that it will be absorbed, or trapped, by gas molecules along the path to the collection optics. This effect will manifest itself as self-reversal of individual fluorescence lines or band absorption of the fluorescence spectrum. 

It is of crucial importance to the success of the experiments that the resonance lines used to excite the fluorescence be of high and constant intensity and of small breadth and that they not be self-reversed. When the gaseous mixture in the fluorescence vessel is kept at pressures of at most a few... 

The effect of Ar, He, N, and air on the self-reversal of Cu emission lines is illustrated in Fig. 9.13A, where spectra (recorded with a delay time 300 s) are normalized to the... 

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 detectable, because it is at the very center of the emitted resonance line and very difficult to resolve. It is, of course, exactly the radiation that is most easily absorbed by the atoms of the sample. In practice, if the HCL is operated at too high a current, the self-reversal decreases the sensitivity of the analysis by removing absorbable light. It also shortens the life of the lamp significantly. 

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... 

Figure 6.28 Distortion of spectral line shape in an HCL due to self-reversal, (a) Shape of the spectral line emitted by the HCL. (b) Shape of the spectral energy band absorbed by cool atoms inside the HCL. (c) Shape of the net signal emerging from an HCL. showing self-absorption of the center of the emission band.

This technique of background correction makes use of the fact that resonant atomic spectral lines emitted by a hollow cathode lamp may display self-reversal when the lamp is operated at high discharge current. A hollow cathode lamp operates during the first part of the measurement... 

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... 

---