Spectroscopy line spectra

Hi) Line Spectra Line spectra are usually encountered when the light emitting substance i.e., the radiating species are separate atomic entities (particles) which are distinctly separated from one another, as in gas. Therefore, it is invariably known as atomic spectrum . As the line spectrum depends solely upon the type of an atom, hence it enjoys the status of a predominant type of emission spectroscopy. 

The absorption spectrum of the violet soln. is not very different from that of iodine vapour, although one is a band and the other a line spectrum. In his Handbuch der Spectroscopie (Leipzig, 1905), H. Kayser thus summarizes the observations on the absorption spectra of red and brown soln. of iodine ... 

Modern atomic theory received a shot in the arm when it was recognized that the individual atom has light absorption and emission spectra occurring at narrow lines of the spectrum at specific wavelengths, as opposed to the broad bands typical of the polyatomic molecules and compounds. Since the line spectrum of each element is characteristic of that element, atomic spectroscopy can be used for precise elementary analysis of many types of chemically simple and complex materials. These studies make use of the wave character of light, as well as light s particle character. 

The example of B5H9 serves to show how the chemical shift may be used as an aid to determining the structure of a molecule and, in particular, in deciding between alternative structures. There are many examples in the literature of this kind of application which is reminiscent of the way in which the chemical shift in NMR spectroscopy may be employed. However there is one important difference in using the two kinds of chemical shift. In XPS there are no interactions affecting closely spaced lines in the spectrum, however close they may be. Figure 8.15 illustrates this for the C lx lines of thiophene. In NMR spectroscopy the spectrum becomes more complex, due to spin spin interactions, when chemical shifts are similar. 

Spectroscopy is a very useful technique for chemists. Sometimes only a very small amount of an element in a sample can be tested. And that amount may be too small to see or weigh easily. But the element can still be detected by heating the sample. The element will give off its characteristic line spectrum. The line spectrum shows that the element is present. 

The discussion of emission spectroscopy will be concluded by a description of a rather unusual application. Bay and Steiner have measured atomic hydrogen concentrations in the presence of molecular hydrogen by microwave excitation of the atomic hydrogen line spectrum. With low power fed into the gas (ca. 5 watts), there is not enough energy available for the dissociation of molecular hydrogen and subsequent excitation. Thus the measured intensities of the atomic hydrogen lines correspond to the concentrations of atoms already present in the reaction mixture. The method is curiously similar to that adopted to detect atoms and free radicals by mass spectrometry (see Section 3). 

Historically important in the development of modern atomic theory was the recognition that although polyatomic molecules show more or less broad bands of absorption and emission in the visible and ultraviolet regions of the spectrum, the characteristic light absorption or emission by individual atoms occurs at fairly narrow lines of the spectrum, which correspond to sharply defined wavelengths. The line spectrum of each element is so uniquely characteristic of that element that atomic spectroscopy can be used for precise elementary analysis of many types of chemically complex materials. 

Fig. 8i-4ii The 4-helix bundle as shown for the a-heUx (i) and the 3io-heUx (ii). iii Nuclear Overhauser Enhancement Spectroscopy (NOESY) spectrum of 4-heUx bundle (aN Region) at pH 4 in 25% MeCN/phosphate buffer corresponding to Fig. 7i (line 2) showing aN(i- i+2 green) and aN(i- r+3 red) NOE correlations typical of 3io-helicity...

Emission and absorption spectra may show a continuous spectrum, a line spectrum, ora band spectmm. A continuous spectrum contains an unbroken sequence of frequencies over a relatively wide range it is produced by incandescent solids, liquids, and compressed gases. Line spectra are discontinuous lines produced by excited atoms and ions as they fall back to a lower energy level. Band spectra (closely grouped bands of lines) are chmacteristic of molecular gases or chemical compounds. See aiso action spectrum SPECTROSCOPY. 

Emission spectroscopy provides an ideal method for qualitative analysis, since each atomic species has its own unique line spectrum. Spectral lines have two characteristics useful for qualitative analysis (1) their wavelengths and (2) their intensities. It is the pattern of wavelength distribution that is primarily used for qualitative analysis, although the relative intensity distribution also can be helpful to verify spectral lines to identify an element. About 70 elements are easily identified by spectral methods. Those that are more difficult to identify include the gases and a few nonmetals, primarily because sensitive lines lie in the short ultraviolet portion of the spectrum that is difficult to observe. 

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