Stokes-shifted Raman bands

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. 

Figure 3.17 presents ps-TR spectra of the olehnic C=C Raman band region (a) and the low wavenumber anti-Stokes and Stokes region (b) of Si-rra i-stilbene in chloroform solution obtained at selected time delays upto 100 ps. Inspection of Figure 3.17 (a) shows that the Raman bandwidths narrow and the band positions up-shift for the olehnic C=C stretch Raman band over the hrst 20-30 ps. Similarly, the ratios of the Raman intensity in the anti-Stokes and Stokes Raman bands in the low frequency region also vary noticeably in the hrst 20-30 ps. In order to better understand the time-dependent changes in the Raman band positions and anti-Stokes/Stokes intensity ratios, a least squares htting of Lorentzian band shapes to the spectral bands of interest was performed to determine the Raman band positions for the olehnic... 

Physically, the Brillouin spectrum arises from the inelastic interaction between a photon and the hydrodynamics modes of the fluid. The doublets can be regarded as the Stokes and anti-Stokes translational Raman spectrum of the liquid. These lines arise due to the inelastic collision between the photon and the fluid, in which the photon gains or loses energy to the phonons (the propagating sound modes in the fluid) and thus suffer a frequency shift. The width of the band gives the lifetime ( 2r)-1 of a classical phonon of wavenumber q. The Rayleigh band, on the other hand, represents the... 

As the time scale of the Raman scattering event ( 3.3 x 10 14 s for a vibration with a Stokes wave number shift of 1000 cm 1 excited in the visible) is much shorter than that of the fastest conformational fluctuations, an ROA spectrum is a superposition of snapshot spectra from all the distinct conformations present in a sample at equilibrium. Since ROA observables depend on absolute chirality, there is a cancellation of contributions from enantiomeric structures arising as a mobile structure explores the range of accessible conformations. Therefore, ROA exhibits an enhanced sensitivity to the dynamic aspects of biomolecular structure. In contrast, conventional Raman band intensities are blind to chirality and so are generally additive and therefore less sensitive to conformational mobility. Ultraviolet circular dichroism (UVCD) also demonstrates an enhanced sensitivity to the dynamics of chiral structures ... 

Figure 3. Calculated band profiles of Stokes vibrational Raman scattering from Nt at 2000 K assuming a triangular slit function with FWHM = 5.0 cm 1. The bottom curve includes the isotropic part of the Q-branch only. The top curve is a more exact calculation including O- and S-branch scattering, the anisotropic part of the Q-branch and line-strength corrections owing to centrifugal distortion. The base lines have been shifted vertically for clarity.

The reason for this rivalry is that the spontaneous Raman scattering is a weak effect and thus it is essential to optimise the experimental set-up. The crucial factor is that the efficiency of the Raman scattering process has one of the highest power dependencies on frequency of any optical effect. This efficiency is proportional to frequency to the fourth power and the intensity of a Stokes Raman band of a shift frequency is governed by... 

Vibrational Raman band intensities and frequencies are also dependent on temperature, applied pressure, and the intrinsic microstructure of the material. These second-order parameters may be extracted from measured spectra. Both X-ray diffraction lines and Raman bands from polycrystalline materials show increased broadening as the microcrystallite grain sizes decrease. In fact, for the hexagonal phase of BN, bandwidths vary linearly with the reciprocal grain size (13). Inherent stress in thin films is manifested in vibrational line shifts. Based on pressure-dependent measurements of vibrational frequencies in bulk solids, inherent stress and stress inhomogeneity can be determined in thin films. Since localized stress can influence the optical and electronic properties of a thin film, it appears to be an important parameter in film characterization studies. Vibrational features also exhibit temperature-dependent frequency shifts. Therefore, an independent measurement of temperature is sometimes necessary to deconvolute these effects. Reference to Figure 1 shows that the molecular temperature of a material may be determined from the Stokes/anti-Stokes... 

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