Raman scattering preresonance

There have been extensive theoretical treatments of resonant and preresonant Raman scattering, which reveal the important factors affecting the magnitude of p. However, it is difficult to theoretically predict p from molecular structure, and the theory is normally used to predict laser frequency dependence and deduce the relationship between resonance Raman spectra and molecular structure. Albrecht and Hutley (22) derived an expression for the laser frequency dependence of the cross section ... 

If the exciting line is close to but not inside an electronic absorption band, the process is called preresonance Raman scattering. 

Fig. 2. Four types of light scattering processes used in molecular spectroscopy. Raman scattering takes place if the incident light corresponds to a transparent region of the molecular absorption spectrum the intermediate state is then virtual in that it does not closely resemble any particular molecular state. Preresonance Raman scattering takes over when the incident beam approaches an electronic absorption band, so that the corresponding electronic state dominates the intermediate state. It turns into resonance Raman scattering when the intermediate state is dominated by a few (ro-)vibronic levels in the vicinity of the incident light frequency. Ultimately the resonance fluorescence limit is reached when the incident beam coincides with a single sharp level of the electronic manifold.

Fig. 17. Same plot as Fig. 3 showing interference between resonance and preresonance Raman scattering for a totally symmetric mode (B = 2). The preresonance scattering is due to a strong (M , = 10Af , parallel moments) remote (Eg = 10) transition. The remote state is coupled to the resonant state (K = 1) but not displaced relative to the ground state (B = 0).

The terms proportional to A will lend to reduce the maximum of p, and to extend the range of incident light frequencies for which p, exceeds further into the preresonance region. Thus for large y, corresponding to preresonance Raman scattering, Eq. (189) reduces to... 

For realistic molecular polarizabilities a close proximity to the surface is needed. For a normal Raman scatterer, generally only first-layer enhancement is expected (as seen below, roughness can increase this range to 1-2 nm) for a preresonance scatterer, the effect can be at longer distances, however, the enhancement will be smaller than for a NR scatterer. 

Doom and Hupp have used preresonance Raman spectra in an analysis of the vibronic components which contribute to the intervalence absorption maximum of [(CN)5Ru -CN-Ru (NH3)5] and to the MLCT absorption maximum of [(bpy)Ru(NH3)4] ". These authors employ the time-dependent scattering approach of Heller to obtain the nuclear displacements of several vibrational modes coupled to the electronic transitions. They find in each case that several vibrational modes, spanning a wide range of frequencies, do contribute significantly to the photoinduced electron transfer processes. Hopkins and co-workers have used a two-color, ps Raman technique to investigate interligand electron transfer in Ru(II)-tn5-polypyridyl complexes, and they find vibrational relaxation of the electronically excited mole ule occurs within about 30 ps of excitation, after which interligand equilibration occurs more slowly than 5 x 10 s. 

Figure 1 The different arrangements for SERRS the curves represent (A) molecular absorbance and (B) plasmon resonance. In (A) the molecular absorbance maximum and the plasmon absorbance maximum do not coincide position 1 represents excitation at the absorbance maximum, 2 that at the plasmon maximum. In (B) the molecular absorbance and the plasmon maximum coincide position 1 represents excitation away from the absorbance and plasmon maximum where the spectrum has a preresonant component, 2 that at the absorbance maximum and plasmon maximum. (Rodger C, Smith WE, Dent G, and Edmondson M (1996) Surface-enhanced resonance-Raman scattering An informative probe of surfaces. Journal of Chemical Society, Dalton Transaction 791-799 reproduced by permission of The Royal Society of Chemistry.)...

In the following we focus on preresonance and resonance Raman scattering, i.e., we limit the number of intermediate electronic states considered explicitly to one, two, or (in Section VII) three. 

Since the H-T terms are developed from the first-order terms active in the nonresonance Raman effect, they are usually the most important source of preresonance enhancement whereas the FC terms are active in proportion to the extent to which closure fails for the vibrational levels of the intermediate state. Some physical insight into these different scattering mechanisms is furnished by the following interpretation of the energy denominators of the scattering tensor. 

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