Raman scattering critical points

Marsault-Herail F., Marsault J. P., Michond G., Levi G. Raman scattering Orientational motion and collision frequency in liquid CF4 from the triple to critical point, Chem. Phys. Lett. 31, 335-9 (1975). 

This behavior matches quite well to the scenario that has recently been developed for 2D spin systems in the proximity to a quantum critical point [16] and is a direct fingerprint of the tendency to form local correlations. It has to be mentioned, however, that the lowest energy excitation still has triplet character. For compounds with closer proximity to quantum criticality, such as the 1/6 depleted triangular lattice (Kagome) [6] and the 1/5 depleted square lattice, also exists a dense singlet spectrum within the singlet triplet gap [17-20], Unfortunately no Raman scattering data for the related compounds are available up to now. 

The spectral function leading to a quasi-first-order Raman spectrum close to the critical point of a displacive system contains basically three contributions from 1) the soft mode, 2) the central peak and 3) higher order Raman scattering arising from processes involving hard and soft phonons... 

From the above discussion when dpjdq, or more rigorously, VqPy is zero or has a slope discontinuity, there are likely to be slope discontinuities in the combined density of states, as revealed by infrared and Raman spectra of two-phonon processes. Points in the Brillouin zone where each of the components of VqP = 0 are known as critical points. The intensity of infrared absorption or Raman scattering depends upon quantum mechanical matrix elements which are, in general, not simple to evaluate. However, by using symmetry considerations and group theoretical methods, the various modes can be assigned as infrared or Raman active. 

The OHS spectra of water, as measured by Raman scattering and Infrared absorption from the LDA phase to nearly the first critical point of water, have been described by the following Gaussian component peak positions (wave numbers) (I) 3120 cm , 01) 3220 cm , (III) 3400 cm , (IV) 3540 cm , and (V) 3620 cm . All have been unambiguously classified as HB, NHB, or OHS oscillators. The situation may be summarized as follows (see, e.g.. Fig. 11c) ... 

RM Martin. Resonance Raman scattering near critical points. Phys Rev B 10 2620-2631, 1974. 

Y Garrabos, R Tufeu, B Le Neindre, G Zalczer, D Beysens. Rayleigh and Raman scattering near the critical point of carbon dioxide. J Chem Phys 72 4637, 1980. 

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