Stokes resonances

At resonance with an electric dipole allowed transition, the Stokes resonance Raman scattering, I(tt/2), associated with a single totally symmetric mode and its overtones is proportional to... 

Resonance Raman Spectroscopy. A review of the interpretation of resonance Raman spectra of biological molecules includes a consideration of carotenoids and retinal derivatives. Another review of resonance Raman studies of visual pigments deals extensively with retinals. Excitation profiles of the coherent anti-Stokes resonance Raman spectrum of j8-carotene have been presented. Resonance Raman spectroscopic methods have been used for the detection of very low concentrations of carotenoids in blood plasma and for the determination of carotenoid concentrations in marine phytoplankton, either in situ or in acetone extracts. ... 

Figure 5 presents the frequency ranges that, by optimistic estimates, can be covered by the optical techniques discussed above. We have intentionally left out of the discussion several other optical spectroscopic methods, such as coherent anti-Stokes resonant scattering (CARS), or induced transient grating (TG) spectroscopy, mainly because they are rarely used in the present context and/or are much less straightforward to interpret. 

Mizutani, Y. and Kitagawa, T. (1997) Direct observation of cooling of heme upon photodissociation of carbonmonoxy myoglobin. Science, 278, 443-446. Okamoto, H., Nakabayashi,T. andTasumi, M. (1997) Analysis of anti-Stokes resonance Raman excitation profiles as a method for studying vibrationally excited molecules. J. Phys. Chem. A, 101, 3488-3493. 

Resonance Raman and antisymmetric scattering are involved in a novel technique involving spin-flip Raman transitions in paramagnetic molecules that can function as Raman electron paramagnetic resonance. Figure 3.2a shows a conventional vibrational Stokes resonance Raman process, while 3.2b and 3.2c show the polarization characteristics of the two distinct spin-flip Raman processes for scattering at 90°... 

CARS can be resonantly-enhanced electronically when either the pump, Stokes or the CARS frequency Itself coincides with an electronic transition in the probed species. Stokes resonances are weighted by the excited vibrational state involved in the Raman resonance and this enhancement is generally weak even at flame temperatures. More typically one tries to achieve primary resonance with the pump laser. In so doing, Stokes resonances are automatically satisfied. The strength of the resonance scales as the product of the four dipole matrix elements Involved with each field in the wave mixing process. Thus only certain transitions tend to be enhanced leading in most cases to a simplification of the CARS spectrum. In the case of the combustion relevant OH molecule under study in our laboratory, a simple triplet spectrum is predicted since each Raman-resonant, downward Stokes transition must satisfy the appropriate dipole selection mles for strong electronic enhancement as shown in Fig. 10. ... 

CARS spectroscopy utilizes three incident fields including a pump field (coi), a Stokes field (CO2 C02nonlinear polarization at cOcars = 2c0i — CO2. When coi — CO2 coincides with one of the molecular-vibration frequencies of a given sample, the anti-Stokes Raman signal is resonantly generated [22, 23]. We induce the CARS polarization by the tip-enhanced field at the metallic tip end of the nanometric scale. 

The hyperpolarizability tensor is obtained in a way similar to the case of SHG. However, the selection rules for an SFG resonance at the IR frequency implies that the vibrational mode is both IR and Raman active, as the SF hyperpolarizability tensor elements involve both an IR absorption and a Raman-anti-Stokes cross-section. Conversely, the DFG hyperpolarizability tensor elements involve an IR absorption and a Raman-Stokes cross-section. The hyperpolarizability tensor elements can be written in a rather compact form involving several vibrational excitations as [117] ... 

Figure 3.24 Resonance Raman Stokes and anti-Stokes difference spectra of the photochemical ring opening of 1,3-cyclohexadiene. Anti-Stokes spectra were obtained with 284-nm pump and probe wavelengths, while the two-color Stokes spectra were generated with a 284-nm probe and a 275-nm pump. The line at 801 cm is due to the cyclohexane solvent. (Reprinted with permission from reference [122]. Copyright (1994) American Chemical Society.)...

Figure 2.52 Schematic representation of the transitions giving rise to the Raman effect. GS = ground electronic state, ES = excited electronic state, VS = virtual electronic stale, R = Rayleigh scattering, S = transitions giving rise to Stokes lines, AS = transitions giving rise to Anti-Stokes lines, RRS = transitions giving rise to resonance Raman.

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