Self-reversal

Selbst-umkehr, /. self-reversal (as of a spectral line), -unterbrecher, m. (Elec.) automatic interrupter, -verbrenmmg, /. spontaneous combustion, -verdauung, /. autodigestion autolysis, -vergiftung, /. autointoxication. 

Also known as autopoisoning.. self poiz-on-iri self-reversal See self-absorption. jself ri v3r-s3l ... 

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 Monte Carlo computations, we do not calculate c but CJC °. Since, in the limit of large C °, CJCn° = c /a", where a is the number of choices for the equivalent unrestricted chain with no self-reversals, the following expression is given for C ... 

Direct evaluation of p(l, m) is not immediate. For example, in the restricted random walk on a square lattice in which squares are eliminated, the transition p 2, 2) is smaller than the transition p 3, 2), However, in an unrestricted random walk on a square lattice with no self reversals, p(2,2)=p(3,2) = i. 

Self-Reversal.—Disappearance of an expected source line at high pressures because of reabsorption in the lamp. 

The degree of intensity reduction and self-reversal increases with the centralization of the emitting atoms within the parent gas. 

When the line reverses, the maximum value of /, is independent of the degree of self-reversal in every cast. ... 

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. 

By using either a continuous or pulsed source of radiation and by measuring the amount of radiation absorbed by the reaction products, it is possible to determine product state distributions. The source of radiation can either be monochromatic (resonance lamp or laser) or broad-band (flash lamp or arc lamp) used in conjunction with a form of monochromator at the detector. The amount of absorption is monitored by an appropriate photosensitive or energy-sensitive detector. Particular care must be taken in the case of resonance lamps to avoid self-reversal of the output of the source, as this will complicate the quantitative analysis of product densities [17]. Similarly, laser sources must not be operated at such high output powers that the transitions involved become saturated, as this also complicates the analysis. Absorption measurements can be used for a wide range of reaction products, both ground and excited states of atoms, radicals and molecules [9,17, 22]. 

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 optical properties of the wall also contribute to the emitted spectra. The usual arc tube wall material, polycrystalline alumina (PCA), has rather low "in-line" transmission and scatters most of the incident radiation. That portion which is scattered back into the discharge medium and that is also in the spectral region inside or near the peaks of the self-reversed resonance lines, is likely to be absorbed. Thus we have found greater Na reversal for PCA arc tube lamps than for sapphire lamps with the same diameters, Na-Hg alloy composition and reservoir temperatures. Indeed, various authors have reported 3-8% higher efficacies for sapphire vs PCA lamps depending upon the quality of the PCA and its surface treatment 3). 

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. 

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... 

The lamps are usually sealed and may have auxiliary electrodes to excite atomic vapor. They are operated at DC but may be modulated and pulsed. If the primary current /p introduced is too high, the lines emitted are self-reversed. However, this may be avoided by keeping ip low and 4 (auxiliary current) high. 

Studies of profiles of Ba ion lines in plasmas formed by an Nd YAG laser at 1064 nm on YBa,Cu,0, in air at atmospheric pressure as a function of laser power showed the line at 455.4 nm to be strongly self-reversed at all laser powers, and a third peak to evolve in the centre of the self-reversed profile at power densities above 16 GW/cm This third peak was ascribed to fluorescence of Ba ions that absorbed emitted radiation, the absorption peaking 120 ns after the initial laser pulse. 

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... 

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