Origin of Raman Spectra

if v = 4,160cm-1 (H2 molecule), P(v = 1 )/P(v = 0) = 2.19 x 10-9. Therefore, almost all of the molecules are at 0 = 0. On the other hand, if v = 213 cm-1 (J2 molecule), this ratio becomes 0.36. Thus, about 27% of the h molecules are at 0 = 1 state. In this case, the transition n = 1 — 2 should be observed on the low-frequency side of the fundamental with much less intensity. Such a transition is called a hot band since it tends to appear at higher temperatures. 

As stated in Section 1.1, vibrational transitions can be observed in either IR or Raman spectra. In the former, we measure the absorption of infrared light by the sample as a function of frequency. The molecule absorbs A E = hv from 

/o and I denote the intensities of the incident and transmitted beams, respectively, e is the molecular absorption coefficient, and c and d are the concentration of the sample and the cell length, respectively (Fig. 1-7). In IR spectroscopy, it is customary to plot the percentage transmission (T) versus wave number (v)  

It should be noted that T (%) is not proportional to c. For quantitative analysis, the absorbance (A) defined here should be used  

The origin of Raman spectra is markedly different from that of IR spectra. In Raman spectroscopy, the sample is irradiated by intense laser beams in the UV-visible region (v0), and the scattered light is usually observed in the direction perpendicular to the incident beam (Fig. 1-7 see also Chapter 2, 

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The origin of Raman spectra can be explained by an elementary classical theory. Consider a light wave of frequency v with an electric field strength E. Since E fluctuates at frequency v, we can write... 

This research by Dumont et al. (2002) describes the development of Raman spectra from a parent pitch to a resultant coke. The RMS spectrum of the coke originates from within one (embryonic) graphene layer which most probably forms part of a cross-linked carbon network. The proposed existence of a graphitic crystallite" composed of several grapherte layers stacked parallel to each other is not required. Hence, the frequent use of the term "graphitic microcrystallite within the RMS literature is simply an uncritical carry-over of words from WAXD studies and which unfortunately impart seriously misleading ideas. 

Fig. 24 Raman spectra of solid S12 (bottom) and of Si2-CS2 (top) at -80 °C. The weak line at 651 cm (with isotopic satellite) originates from CS2 [79]...

Fig. 28 Raman spectra of polymeric sulfur (S ) prepared by various methods [109,173], of large disordered rings (S ) [182], and of photo-induced amorphous sulfur (a-S) [119], respectively. The spectrum of a-S has been smoothed for clarity. The position of the stretching vibration of a-S is pressure-shifted to higher wavenumbers. The very weak signals in the spectra of Sj, at ca. 150 and 220 cm are probably caused by the presence of Sg. In addition, the weak shoulder at ca. 470 cm observed in spectra of Sj, may originate from Sg, too...

Fig. 30 Raman spectra of high-pressure low-temperature sulfur at two different pressures [184]. The peaks marked by asterisks were reported as originating from another high-pressure allotrope (presumably p-S). The intensities of the spectra at lower wavenumbers (below ca. 300-350 cm ) have been magnified (factor has not been reported)...

To check the identity and purity of the products obtained in the above reactions it is not sufficient to analyze for the sulfur content since a mixture may incidentally have the same S content. Either X-ray diffraction on single crystals or Raman spectra of powder-like or crystalline samples will help to identify the anion(s) present in the product. However, the most convincing information comes from laser desorption Fourier transform ion cyclotron resonance (FTICR) mass spectra in the negative ion mode (LD mass spectra). It has been demonstrated that pure samples of K2S3 and K2S5 show peaks originating from S radical anions which are of the same size as the dianions in the particular sample no fragment ions of this type were observed [28]. 

For Raman spectra of hexa- [30, 40] and heptasulfides [46] with complex univalent cations, see the original literature. 

Taking into consideration that antenna xanthophylls not only possess original absorption but also resonance Raman spectra, and the fact that the Raman signal is virtually free from vibrational spectroscopy artifacts (water, sample condition, etc.), it seemed of obvious advantage to apply the described combination of spectroscopies for the identification of these pigments. 

Fig. 10.4 Fossilized cellular filamentous microorganisms (two examples of Primaevifilum amoenum). They are 3.456 billion years old and come from the Apex chert region in northwestern Australia. As well as the original images, drawings and the Raman spectra and Raman images, which indicate that the fossils have a carbonaceous (organic) composition, are shown. With kind permission of J. W. Schopf...

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