Raman Stokes resonance

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.)...

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. ... 

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. 

Fig. 1 Representation of the processes that occur (from left to right) in non-resonant Rayleigh scattering, Stokes and anti-Stokes Raman and resonance Raman spectroscopy.

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°... 

Fig. 3.10 Detection of Raman-Stokes scattering by photoionization of the excited level Ef either by one UV photon IP Ef) < h(o ]M < IP E )) or by resonant two-photon ionization...

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. ... 

The sensitivity of Raman spectroscopy in the gas phase can be greatly enhanced by combination with one of the detection techniques discussed in Chap. 6. For example, the vibrationally excited molecules produced by Raman-Stokes scattering can be selectively detected by resonant two-photon ionization with two visible lasers or by UV ionization with a laser frequency (o ], which can ionize molecules in level Ef but not in E (Fig. 8.8). 

Figure 12 Raman-REMPI (resonantly enhanced multiphoton ionization) spectrum of the °Qi(AJ = 0, AK = -2,K- 1) transitions of the Vie e2g) mode of benzene in a molecular beam. An energy-level diagram is shown for the double-resonance experiment. The ultraviolet source was tuned to 36,474 cm and the Raman wave-number calibration is adjusted to match the / = 6 line reported in Ref. 109. The expansion was 13% benzene in argon at 80 kPa and the sampling was done at XfD = 175 (D = 0.20 mm nozzle diameter) using pump and Stokes laser energies of 2 and 0.5 mJ. (From Ref. 117, with permission.)...

To see how interferometry can detect the difference between resonant and nonresonant processes, consider the experimental set-up shown in Figure 2 (a) and the shape of the anti-Stokes pulses produced by the long pump and short Stokes pulses previously mentioned. This combination of pulses is illustrated in Figure 2 (b). en the pump and Stokes pulses overlap, the molecule will be excited by SRS. This excitation will remain after the Stokes pulse passes. At the moment of the overlap, nonresonant four-wave-mixing processes can also be excited. However, because there is no persistent state associated with nonresonant processes, the nonresonant emission will end quickly after the Stokes pulse passes. With a Raman-active resonance, however, the pump can produce anti-Stokes radiation via SRS even after the Stokes has passed, because the resonance persists. Therefore, the nonresonant component can be discarded by rejecting any anti-Stokes radiation that occurs coincident with the Stokes pulse. 

Figure 5 Ladder graphs for four-wave mixing effects containing Raman processes. In all cases there is assumed an intermediate Raman-type resonance at the frequency Q. (A) The coherent anti-Stokes Raman (CARS) process. (B) The process responsible for stimulated Raman spectroscopy (SRS) as well as the Raman-induced Kerr effect (TRIKE). (C) The coherent Stokes Raman spectroscopy (CSRS). Adapted with permission from Levenson MD (1982), Introduction to Nonlinear Laser Spectroscopy. New York Academic Press.

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. 

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.

The nonlinear optical properties are determined using resonance Raman scattering, coherent antistokes Raman scattering and coherent stokes Raman scattering. The two-photon polarizability is found to be very large in these materials. General... 

In order to extend the range of 2laser excitation, both CARS (Coherent Anti-Stokes Raman Scattering) and CSRS (Coherent Stokes Raman Scattering) are used. In both cases <03 = 2003 -U2 In the CARS mode 0)3 > wj > (03 in the CSRS mode <02 > (1)3. One-photon resonance effects are the same in both cases as described later. Phase matching is also the same in both cases with 3 = 2 ... 

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