Frequency shifts Raman

Fig. 11. Low frequency shift Raman Specfra in the (zy) scattering geometry for various temperatures in FeFj ss)...

RS Raman spectroscopy [210, 211] Scattered monochromatic visible light shows frequency shifts corresponding to vibrational states of surface material Can observe IR-forbidden absorptions low sensitivity... 

First, it is possible to excite a chromophore corresponding to the active site, and detennine which modes interact with it. Second, by using UV excitation, the amino acids with phenyl rings (tryptophan and tyrosine, and a small contribution from phenylalanine) can be selectively excited [4], The frequency shifts in the resonance Raman spectrum associated with them provide infomiation on their enviromnent. 

The metal substrate evidently affords a huge ( 10 and even as high as 10 [84, 85]) increase in the cross-section for Raman scattering of the adsorbate. There are two broad classes of mechanisms which are said to contribute to this enhancenient [, and Ml- The first is based on electromagnetic effects and the second on cheniicaT effects. Of these two classes the fomier is better understood and, for the most part, the specific mechanisms are agreed upon the latter is more complicated and is less well understood. SERS enhancenient can take place in either physisorbed or chemisorbed situations, with the chemisorbed case typically characterized by larger Raman frequency shifts from the bulk phase. 

For most purposes only the Stokes-shifted Raman spectmm, which results from molecules in the ground electronic and vibrational states being excited, is measured and reported. Anti-Stokes spectra arise from molecules in vibrational excited states returning to the ground state. The relative intensities of the Stokes and anti-Stokes bands are proportional to the relative populations of the ground and excited vibrational states. These proportions are temperature-dependent and foUow a Boltzmann distribution. At room temperature, the anti-Stokes Stokes intensity ratio decreases by a factor of 10 with each 480 cm from the exciting frequency. Because of the weakness of the anti-Stokes spectmm (except at low frequency shift), the most important use of this spectmm is for optical temperature measurement (qv) using the Boltzmann distribution function. 

Raman spectroscopy, long used for quaHtative analysis, has been revitalized by the availabiHty of laser sources. Raman spectroscopy is based on scattering of light with an accompanying shift in frequency. The amount by which the frequency is shifted is characteristic of the molecules that cause the scattering. Hence, measurement of the frequency shift can lead to identification of the material. 

Raman spectrometry is another variant which has become important. To quote one expert (Purcell 1993), In 1928, the Indian physicist C.V. Raman (later the first Indian Nobel prizewinner) reported the discovery of frequency-shifted lines in the scattered light of transparent substances. The shifted lines, Raman announced, were independent of the exciting radiation and characteristic of the sample itself. It appears that Raman was motivated by a passion to understand the deep blue colour of the Mediterranean. The many uses of this technique include examination of polymers and of silicon for microcircuits (using an exciting wavelength to which silicon is transparent). 

Raman spectra have also been reported on ropes of SWCNTs doped with the alkali metals K and Rb and with the halogen Br2 [30]. It is found that the doping of CNTs with alkali metals and halogens yield Raman spectra that show spectral shifts of the modes near 1580 cm" associated with charge transfer. Upshifts in the mode frequencies are observed and are associated with the donation of electrons from the CNTs to the halogens in the case of acceptors, and downshifts are observed for electron charge transfer to the CNT from the alkali metal donors. These frequency shifts of the CNT Raman-active modes can in principle be u.sed to characterise the CNT-based intercalation compound for the amount of intercalate uptake that has occurred on the CNT wall. 

Toth, Quist and Boyd [354] investigated the Raman spectra of LiF - NaF -ZrF4 melts and showed that increasing the ZrF4 concentration from 14% to 40% (mol), leads to a shift in the intensive polarized band from 555 to 593 cm 1. A similar effect was observed also for LiF - NaF - ThF4 melts [355], whereby a frequency shift of the symmetric vibration occurred when concentration of polyvalent metal was increased. This phenomenon was explained by the reduction of the polyvalent metal s coordination number. 

Wang C. H., Wright R. B. Effect of density on the Raman scattering of molecular fluids. I. A detailed study of the scattering polarization, intensity, frequency shift, and spectral shape in gaseous N2, J. Chem. 

Frequency doubled/Raman shifted fiber laser to generate 589 nm. This is a diode pumped, CW format currently under development. 

Rhombohedral Se was found as a high-pressure allotrope of sulfur above 9-10 GPa by several groups [58, 137, 150, 184, 186, 188, 191]. The pressure dependence of frequencies [137, 150, 184] as well as the kinetics of the transition from p-S to Ss [186] have been investigated systematically by Raman spectroscopy. The pressure dependent frequency shifts of chemically prepared Ss and of high-pressure Ss have been found to be identical [137, 150]. 

Figure 3. Frequency shift of the Raman band at 612 cm for Fe-TsPc adsorbed on a sliver electrode as a function of the applied potential vs. SCE In 0.05 M H2S0. Laser excitation line 514.5 nm potential sweep rate 10 mV s electrode area 0.27 cm. See caption Fig. 2.

The driving force for the temperature-dependent spin crossover (SCO) is the entropy difference between the HS and the LS isomers which arises mainly from a shift of the vibrational frequencies when passing from the HS to the LS state [97-99]. This frequency shift has been studied by IR- and Raman-spectroscopy and recently also by NIS [23, 39, 87]. The NIS method is isotope ( Fe) selective and, therefore, its focus is on iron-ligand bond-stretching vibrations which exhibit the most prominent contribution to the frequency shift upon SCO [87]. 

A small fraction of the molecules are in vibrationally excited states. Raman scattering from vibrationally excited molecules leaves the molecule in the ground state. The scattered photon appears at higher energy, as shown in Figure lb. This anti-Stokes-shifted Raman spectrum is always weaker than the Stokes-shifted spectrum, but at room temperature it is strong enough to be useful for vibrational frequencies less than about 1500 cm 1. The Stokes and anti-Stokes spectra contain the same frequency information. 

While the BC configuration for the B—H complex is now accepted, several aspects of the vibrational spectra of the acceptor-H complexes are not understood. The temperature dependence of the B—H complex has been examined by Raman spectroscopy (Stutzmann and Herrero, 1987) and IR absorption (Stavola et al., 1988a). The H-stretching vibration shifts from 1875 to 1903 cm 1 between room temperature and liquid He temperature. Frequency shifts of just a few cm 1 are more typical for local vibrational modes. The vibrational bands are also surprisingly broad. 

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