Anomalous Raman scattering

Figure 4.8-3 Anomalous Raman scattering of ds-poly(acetylene). The upper spectrum is excited with a blue laser, the lower spectrum with a red laser, as described in Kuzmany et al., 1981.

Chew and Wang(39) have pointed out the possibility of double resonance, that is, that the frequencies of both the excitation and inelastically scattered radiation are resonant. They presented the results of calculations which indicate that double resonance can have a significant effect on the angular intensity distribution of inelastically scattered radiation. This case is of some practical interest, particularly in Raman studies, where coincidence may lead to anomalous Raman band intensities, if both the excitation and the shifted frequency are resonant. 

In resonance Raman scattering (ga 0), it is possible to have pp >. For example, if ctxy — —ctyx and the remaining off-diagonal elements are zero, g° = gs = 0 and ga 0. Then, (1-49) gives pp —> oo. This is called anomalous (or inverse) polarization (abbreviated as ap or ip). As will be shown in Section 1.15, resonance Raman spectra of metallopophyrins exhibit polarized (Aig) and depolarized (B g and B ) vibrations as well as those of anomalous (or inverse) polarization () ... 

Williams GM, Becker PC, Edelstein N, Boatner LA, Abraham MM (1989b) Excitation profiles of resonance electronic Raman scattering in ErP04 crystals. Phys Rev B 40 1288-1296 Williams GM, Edelstein N, Boatner LA, Abraham MM (1989c) Anomalously small Af-5d oscillator strengths and 4/-4/electronic Raman scattering cross sections for Ce in crystals of LUPO4. Phys Rev 6 40 4143-4152... 

The first evidence for a pseudogap in the Raman scattering response of the cuprates was actually inferred from the anomalous temperature dependences of Raman- and infrared-active phonons (Litvinchuk et al. 1992). More recently, evidence for a pseudogap has... 

While the calculations of Devereaux et al. and others appear to show that many of the anomalous features observed in the superconducting-state electronic Raman scattering response of high-Fc cuprates can be explained assuming a d 2 pairing state, there are several noteworthy exceptions to this interpretation. 

Okamoto, S., Ishihara, S., and Maekawa, S. Theory of Raman scattering from orbital excitations in manganese oxides. Phys. Rev. B 2002, 66,014435-1-9. Templeton, D.H. and Templeton, L.K. X-ray dichroism and polarized anomalous scattering of the uranyl ion. Acta Crystallogr. A 1982, 38, 62—67. 

Unless y 2 = q (nonresonant Raman), measurement with circularly polarized light is needed to determine the three invariants. The results are usually expressed as the depolarization ratio / (O/, vi)/4"(0/] Vi). For nonresonant Raman scattering this equals 3/4 for a = is lower otherwise, and reaches zero if a = a ( ) (totally symmetric vibration). In resonant Raman, can be different from zero, and the depolarization ratio can exceed 3/4 ( anomalous polarization ). 

Figure 10 Parallel ( ) and perpendicular ( ) scattered resonance Raman spectra from NiP in CS2 showing the anomalously polarized bands (labeled) associated with the ring A2g vibrations...

These NCS experiments [Chatzidimitriou-Dreismann 1997 (a) Chatzidimi-triou-Dreismann 1999 Karlsson 1999], which were motivated by the theoretical work of C. A. Chatzidimitriou-Dreismann [Chatzidimitriou-Dreismann 1991 Chatzidimitriou-Dreismann 1997 (b)] on protonic quantum entanglement in condensed systems and by the results of a an earlier Raman experiment on liquid H O / D O mixtures [Chatzidimitriou-Dreismann 1995], were followed by a series of other experiments on liquid and solid organic materials [Chatzidimitriou-Dreismann 2000 (b) Chatzidimitriou-Dreismann 2001 Chatzidimitriou-Dreismann 2002 (a)], various metallic hydrides [Abdul-Redah 2000 Karlsson 2002 (b) Karlsson 2003 (b)], liquid hydrogen [Chatzidimitriou-Dreismann 2004 (b)] and among others an ionic solid [Abdul-Redah 2004] using the same experimental technique, i.e., neutron Compton scattering. All these experiments confirmed the anomalous results found earlier and also revealed certain new aspects of the considered effect. 

Thus the value of p depends on the degree of asymmetry in the tensor. For pure symmetric scattering, as in the normal Raman effect for instance, Opp = pp and Pi = 3/4 (depolarised scattering). For pure antisymmetric scatteringOpp = - pp and therefore G = 0, giving Pi = (inverse polarisation). For an asymmetric tensor lappi Iflppl and < Pi < ° , which is called anomalous polarisation. 

Fig. 21. Resonance Raman spectra of oxyhaemoglobin (bottom pair of curves) and ferrocyto-chrome c (top pair). The scattering geometry is shown schematically in the diagram at the top. Both the direction and the polarisation vector of the incident laser radiation are perpendicular to the scattering direction. The scattered radiation is analysed into components perpendicular (Ij ) and parallel (I ) to the incident polarisation vector. The exciting wavelength was 568.2 nm for oxyhaemoglobin and 514.5 nm for cytochrome c. The slit width was about 10 cm". The concentrations were about 0.5 mM for each. The anomalously polarised, polarised and depolarised bands, are indicated by ip, p, and dp respectively. [From Spiro and Strekas, Ref. (42) ...

The variations in the refractive index can cause anomalous features in infrared spectra. In Raman spectroscopy, refractive index variations are not a problem, since the excitation frequency is far removed from any absorption bands. Therefore, it is easier to record Raman rather than infrared spectra from samples such as cellulose which scatter light strongly. 

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