Stokes Raman

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 room temperature the thermal population of vibrational excited states is low, although not zero. Therefore, the initial state is the ground state, and the scattered photon will have lower energy than the exciting photon. This Stokes shifted scatter is what is usually observed in Raman spectroscopy. Figure la depicts Raman Stokes scattering. 

The accuracy of the temperature pdf data obtained with the Raman Stokes/anti-Stokes technique has been assessed by tests made on a known and well-calibrated laminar premixed flame source, viz., a porous plug burner (20). These data, which were checked by analytical calculations based upon the optical and electronic properties of our detection system, showed a roughly 5-7% standard deviation, which has been considered acceptable for present measurement purposes (2 7). However, additional problem not considered in this type of test, can exist. For example ... 

Figure 3.16. A few interactions contributing to the Raman-Stokes process (with creation of one phonon). The propagators are labeled with the notation of the text. The process a is of the first order b could be included by renormalization of the created exciton c would require the renormalization of the interaction (or the vertex).

If the intensity of the Raman Stokes beam at I si is high enough, this beam itself can generate a new beam at the second Stokes frequency v 2 = si = in a man-... 

Superlicially, the Raman Stokes shifts to lower energies (longer wavelengths) are analogous to the Stokes shifts found in molecular lluorescenee (sce Section 6C-6). We shall see. however, that Raman and... 

The NIR laser source line is at 1064 nin, so the Raman-Stokes lines occur at longer wavelengths. This is beyond the detection range of the materials used in array detectors. The detector for an NIR FT-Raman system is a liquid nitrogen-cooled photoconductive detector such as Ge or InGaAs. 

The sensitivity of Raman spectroscopy in the gas phase can be greatly enhanced by combination with one of the detection techniques discussed in Chap. 1. For example, the vibrationally excited molecules produced by Raman-Stokes scattering can... 

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...

FIGURE 20 (a) Generation of a Stokes wave in singie-pass stimulated Raman scattering, (b) Generation of a Raman-Stokes wave in a Raman laser oscillator. 

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). 

Raman Stokes-scattered intensity for the mode Q reads ... 

Given the character table work out the symmetry types of the vibrations of CCU. Which vibrations will be allowed in infrared absorption and which in Raman spectroscopy Compare the number and type of your vibrations to the Raman Stokes lines in Figure 12.23. 

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