Raman scattering intensity

In the stimulated Raman effect it is only the vibration that gives the most intense Raman scattering that is involved this is the case for Vj in benzene. 

The small HOMO-LUMO band gap and presence of other close-in-energy MOs results in fullerenes being easily polarized. They all give very intense Raman scattering lines and have relatively large x values useful for NLO applications (11). Indeed, C60 is one of the best materials known to date for optical limiting. 

The excitation in the deep-UV results in more intense Raman scattering. Deep-UV laser excitation has the ability to avoid fluorescence background in the Raman spectra. Tunability of the UV source could allow exploitation of resonance Raman effect. Resonance Raman Effect enhances the intensity of Raman lines. Deep-UV LIF is... 

Computer programs such as Gaussian 03 can simultaneously calculate several other parameters including normal modes (shown in Table 1.15), IR intensities, Raman scattering intensities, and depolarization ratios. 

Other indicators of a species concentration include fluorescent intensity, infrared intensity, Raman scattering intensity, and electron spin resonance. These will not be discussed in detail. 

Raman spectroscopy can differentiate between internal and external bonds as well as cis and trans isomerism and conjugation in compounds with ethylenic linkages. The type of unsaturation in butadiene and isoprene rubbers can be determined from the intense Raman scattering of the C=C stretching modes [55]. The trans- and cw-l,4-polybutadiene structures scatter at 1664 and 1650 cm , respectively. The 1,2-vinyl structure of polybutadiene scatters at 1639 cm and this scattering is well-resolved from that of the 1,4-polybutadiene structures. For poly-isoprene, a slightly different situation prevails. The cis- and ran -l,4-polyisoprene structures are riot resolved, and they scatter at 1662 cm , but the 3,4-polyisoprene structure scatters at 1641 cm , and the 1,2-vinyl stracture scatters at 1639 cm [56,57]. 

Due to the rather stringent requirements placed on the monochromator, a double or triple monocln-omator is typically employed. Because the vibrational frequencies are only several hundred to several thousand cm and the linewidths are only tens of cm it is necessary to use a monochromator with reasonably high resolution. In addition to linewidth issues, it is necessary to suppress the very intense Rayleigh scattering. If a high resolution spectrum is not needed, however, then it is possible to use narrow-band interference filters to block the excitation line, and a low resolution monocln-omator to collect the spectrum. In fact, this is the approach taken with Fourier transfonn Raman spectrometers. 

Shreve A P and Mathies R A 1995 Thermal effects in resonance Raman-scattering—analysis of the Raman intensities of rhodopsin and of the time-resolved Raman-scattering of bacteriorhodopsin J. Phys. Chem. 99 7285-99... 

Measurement of the total Raman cross-section is an experimental challenge. More connnon are reports of the differential Raman cross-section, doj /dQ, which is proportional to the intensity of the scattered radiation that falls within the element of solid angle dQ when viewing along a direction that is to be specified [H]. Its value depends on the design of the Raman scattering experiment. 

RRS has also introduced the concept of a Raman excitation profile (REPy for thefth mode) [46, 4lZ, 48, 49, 50 and M]. An REP. is obtained by measuring the resonance Raman scattering strength of thefth mode as a fiinction of the excitation frequency [, 53]. Flow does the scattering intensity for a given (thefth) Raman active vibration vary with excitation frequency within an electronic absorption band In turn, this has led to transfomi theories that try to predict... 

Another related issue is the computation of the intensities of the peaks in the spectrum. Peak intensities depend on the probability that a particular wavelength photon will be absorbed or Raman-scattered. These probabilities can be computed from the wave function by computing the transition dipole moments. This gives relative peak intensities since the calculation does not include the density of the substance. Some types of transitions turn out to have a zero probability due to the molecules symmetry or the spin of the electrons. This is where spectroscopic selection rules come from. Ah initio methods are the preferred way of computing intensities. Although intensities can be computed using semiempirical methods, they tend to give rather poor accuracy results for many chemical systems. 

The incident radiation should be highly monochromatic for the Raman effect to be observed clearly and, because Raman scattering is so weak, it should be very intense. This is particularly important when, as in rotational Raman spectroscopy, the sample is in the gas phase. 

In FT-Raman spectroscopy the radiation emerging from the sample contains not only the Raman scattering but also the extremely intense laser radiation used to produce it. If this were allowed to contribute to the interferogram, before Fourier transformation, the corresponding cosine wave would overwhelm those due to the Raman scattering. To avoid this, a sharp cut-off (interference) filter is inserted after the sample cell to remove 1064 nm (and lower wavelength) radiation. 

Raman scattering is normally of such very low intensity that gas phase Raman spectroscopy is one of the more difficult techniques. This is particularly the case for vibration-rotation Raman spectroscopy since scattering involving vibrational transitions is much weaker than that involving rotational transitions, which were described in Sections 5.3.3 and 5.3.5. For this reason we shall consider here only the more easily studied infrared vibration-rotation spectroscopy which must also be investigated in the gas phase (or in a supersonic jet, see Section 9.3.8). 

Laser radiation is very much more intense, and the line width much smaller, than that from, for example, a mercury arc, which was commonly used as a Raman source before 1960. As a result, weaker Raman scattering can now be observed and higher resolution is obtainable. 

The selection mles for CARS are precisely the same as for spontaneous Raman scattering but CARS has the advantage of vastly increased intensity. 

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