Atomic spectra fine spectrum

A fine spectrum is an absorption or emission spectrum that displays a series of vertical fines indicating that only certain narrow wavelength bands (fines) are absorbed or emitted. A fine spectrum results when atoms are measured. This is the case because there are no vibrational levels in atoms, and therefore only very few transitions are allowed. 

Thus, with allowance for both the first-order and second-order processes, the secondary electron spectrum fine structure is formed by oscillations of two types, which are determined by the same local atomic structure but different wave numbers this is the main difference of SEFS spectra from EXAFS and EELFS spectra. It is just this qualitative difference that must determine the characteristic features of SEFS spectra, and it must be taken into account in obtaining parameters of the local atomic structure from experimental data. However, it should be pointed out that a signal from two final states can be observed also in EXAFS and EELFS spectra in the case of the excitation of two closely spaced levels. And though the mechanism of appearance of these signals differs from that in the case of SEFS, nevertheless conceivably the analogous problem must be solved also for these traditional methods. 

The oscillating part of the secondary electron spectrum fine structure in the expression obtained is determined by two interference terms resulting from scattering of secondary electrorrs of final and intermediate states (the latter are due to the second-order process only). Here intensities of oscillating terms are determined by the amplitudes and intensities of electron transitions in the atom ionized. In this section we make estimations of these values within the framework of the simple hydrogen like model using the atomic unit system as in the preceding section. This section s content is based on papers [20,22,29-31,33,35,37,45-47]. 

A C-NMR spectrum of a random copolymer (Figure 7.19) reveals several multiple lines for various carbon atoms suggesting their sensitivity to the structural arrangements of the comonomer units along the polymer chain. Of the different carbon atoms, the fine structures exhibited by the backbone quaternary carbon atom due to methacrylonitrile units and the methine carbon atom of the styrene units are particularly significant and have been utilised for comonomer sequence analyses. 

Figure B2.5.12 shows the energy-level scheme of the fine structure and hyperfme structure levels of iodine. The corresponding absorption spectrum shows six sharp hyperfme structure transitions. The experimental resolution is sufficient to detennine the Doppler line shape associated with the velocity distribution of the I atoms produced in the reaction. In this way, one can detennine either the temperature in an oven—as shown in Figure B2.5.12 —or the primary translational energy distribution of I atoms produced in photolysis, equation B2.5.35.

The hydrogen atom and one-electron ions are the simplest systems in the sense that, having only one electron, there are no inter-electron repulsions. However, this unique property leads to degeneracies, or near-degeneracies, which are absent in all other atoms and ions. The result is that the spectrum of the hydrogen atom, although very simple in its coarse structure (Figure 1.1) is more unusual in its fine structure than those of polyelectronic atoms. For this reason we shall defer a discussion of its spectrum to the next section. 

In Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), a gaseous, solid (as fine particles), or liquid (as an aerosol) sample is directed into the center of a gaseous plasma. The sample is vaporized, atomized, and partially ionized in the plasma. Atoms and ions are excited and emit light at characteristic wavelengths in the ultraviolet or visible region of the spectrum. The emission line intensities are proportional to the concentration of each element in the sample. A grating spectrometer is used for either simultaneous or sequential multielement analysis. The concentration of each element is determined from measured intensities via calibration with standards. 

Extended X-ray absorption fine structure (EXAFS) measurements based on the photoeffect caused by collision of an inner shell electron with an X-ray photon of sufficient energy may also be used. The spectrum, starting from the absorption edge, exhibits a sinusoidal fine structure caused by interferences between the outgoing and the backscattered waves of the photoelectron which is the product of the collision. Since the intensity of the backscattering decreases rapidly over the distances to the next neighbor atoms, information about the chemical surroundings of the excited atom can be deduced. 

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