Anti-stokes scattering, intensity ratio

For any vibrational mode, the relative intensities of Stokes and anti-Stokes scattering depend only on the temperature. Measurement of this ratio can be used for temperature measurement, although this application is not commonly encountered in pharmaceutical or biomedical applications. Raman scattering based on rotational transitions in the gas phase and low energy (near-infrared) electronic transitions in condensed phases can also be observed. These forms of Raman scattering are sometimes used by physical chemists. However, as a practical matter, to most scientists, Raman spectroscopy means and will continue to mean vibrational Raman spectroscopy. 

Before melting and for some time after only the band at 625 cm of the AA [C4CiIm]+ cation was observed in the 600-630 cm i region. Gradually 603 cm i band due to the GA conformer became stronger. After about 10 min the AA/GA intensity ratio became constant. The interpretation [50] is that the rotational isomers do not interconvert momentarily at the molecular level. Most probably it involves a conversion of a larger local structure as a whole. The existence of such local structures of different rotamers has been found by optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES) [82], Coherent anti-Stokes Raman scattering (CARS) [83],... 

Figure 2. Intensity ratio of anti-Stokes to Stokes vibrational Raman scattering for a trapezoidal slit function. Center position of Stokes bandpass at 6072 A.

Block the laser beam or spectrometer entrance slit and adjust the spectrometer to an anti-Stokes shift of 1000 cm Caution Exposure of the sensitive phototube to the intense Rayleigh scattering line can seriously damage the detector. Scan the anti-Stokes spectrum from 1000 to 150 cm in the parallel polarization configuration and, using appropriate sensitivity expansion, j measure the ratio of anti-Stokes to Stokes peak heights for each band. 

The sample temperature may be controlled by measuring the Stokes I anti-Stokes intensity ratio of Raman scattered radiation (Schrader et al., 1990), Eq. 2.4-10. This is of importance when temperature-sensitive samples or phase transitions are investigated. 

Another intriguing quality of Raman spectroscopy is its capability to measure local temperature quantitatively and precisely. This is possible in two distinct ways, arising due to two different characteristics of the Raman spectra in crystalline solids. The first characteristic is the presence of the phonon occupation number in the Raman scattering cross section in accordance with (17.3). While the relation to temperature of the strict intensity of a particular phonon peak is obfuscated by the numerous other components of the Raman scattering cross section, taking the ratio of integrated intensities of the Stokes (1 ) and anti-Stokes (Ias) peaks provides the following relationship by which to measure temperature ... 

The temperature dependences of many optical properties of the resin, e.g. fluorescence and Raman scattering (the ratio of Stokes to anti-Stokes intensities), provide an opportunity to use this as a way of monitoring temperature by comparison with a known standard material. Other systems are based on the properties of the fibre itself or a deliberately added dopant rather than the resin being probed. Table 6.4 shows the commercially available temperature probes that are based on optical phenomena and the use of fibre-optics (Fernando and Degamber, 2006). 

Stokes (o)q+(0 ) scattering processes. The partial derivative factor, (3aij/9Qk)e evaluated at the equilibrium position of the normal coordinate comprises a necessary condition for Raman activity of the normal mode Q. Raman effects occur only for those normal modes that cause the molecule to undergo a net change in polarizability during the course of the vibration. While equation (7) implies that both Stokes and anti-Stokes components should appear with equal intensity, a quantum mechanical derivation shows that the Stokes/ anti-Stokes intensity ratio is proporti onal to the Boltzmann factor (7), and can be used to determine the molecular temperature of a collection of molecules. The statistical derivation is based upon the thermal population of ground and excited molecular vibrational states according to a Boltzmann distribution. 

Raman spectroscopy has recently gained popularity for advanced chemical analysis of surfaces. In nanoscience, Raman spectroscopy is used to characterize surface properties of materials, measure temperature, and determine crystallinity. Raman spectroscopy is a spectroscopic technique used in material science to study vibrational and rotational frequencies in a system. The technique measures shifts in inelastic scattering, or Raman scattering, of light from a visible, near infrared or near ultraviolet light source and the shift in energy provides information about the material s surface characteristics. The Raman signal unit is a measurement of the ratio between the Stokes (down-shifted) intensity and anti-Stokes (up-shifted) intensity peaks. 

Although each Stokes line and its anti-Stokes counterpart are equally separated from the Rayleigh line, they are not of equal intensity. This is because the intensity of each transition is proportional to the population of the energy level from which the transition originates under equilibrium conditions the ratio of populations is given by the Boltzmann distribution. With the fourth-power dependence on the scattering frequency, the ratio of intensities of the Stokes line and its anti-Stokes partner in a Raman spectrum is given by... 

---