Cross sections Raman scattering

We have seen that the strength of Raman scattered radiation is directly related to the Raman scattering cross-section (Oj ). The fact that this cross-section for Raman scattering is typically much weaker than that for absorption (oj limits conventional SR as a sensitive analytical tool compared to (Imear) absorption... 

The hyper-Raman scattering cross section is extremely small, typically of the order of 10 cm per molecule [24]. Therefore, an enhancement of signal intensity is essential in order to utilize this phenomenon as a practical spectroscopic tool in the field of molecular science. In a similar marmer to the enhancement of Raman scattering... 

The key requirements for ISRS excitation are the existence of Raman active phonons in the crystal, and the pulse duration shorter than the phonon period loq1 [19]. The resulting nuclear oscillation follows a sine function of time (i.e., minimum amplitude at t=0), as shown in Fig. 2.2e. ISRS occurs both under nonresonant and resonant excitations. As the Raman scattering cross section is enhanced under resonant excitation, so is the amplitude of the ISRS-generated coherent phonons. 

Raman scattering cross sections are small ( 10 cm ), thus fluorescence and luminescence can easily interfere with the data. This is particularly acute with surface carbon, which fluoresces strongly when the Raman is excited by visible wavelength lasers. In addition, catalyst... 

Fung, K. H., and I. N. Tang, Relative Raman Scattering Cross-Section Measurements with Suspended Particles, Appl. Spec-trosc., 45, 734-737 (1991). 

Raman spectra have been observed wherein the signals obtained are of the order of lO -lO greater than one would predict on the basis of the normal Raman scattering cross section of bulk molecules (11, J2). The latter effect has sometimes been termed "enhanced Raman scattering" and has been the subject of intensive theoretical and experimental investigation. We will first discuss the normal or general situation. 

Some surfaces geometries (rough surfaces) concentrate the electric fields of Raman scattering cross section so that it is surface-sensitive. This gives information on surface vibrational modes, and some information on geometry via selection rules. 

As mentioned previously, in addition to the (l/A4)-increase of the Raman scattering cross-section with UV excitation, further enhancement can also occur when the... 

There are several means of lowering the detection limit in vibrational spectroscopy. The intensity of a Raman band is for a given spectrometer and sample arrangement proportional to the flux of the exciting radiation the concentration c (in molecules per cm ), the effective molecular Raman scattering cross section and the effective thickness of the scattering sample d ... 

Sec. 2.4, Eq. 2.4-6 shows that the differential Raman scattering cross section of a vibration of degeneracy gt with the frequency i>A of molecules in a liquid, excited by radiation with the frequency Pq equals ... 

Redefining h as the reference frequency, and - dividing this equation by u,ef h), affords the absolute normalized Raman scattering cross section (Eq. 3.5-13). It has been tabulated for several compounds by Schrotter and Klockner, 1979. Typical values for liquids excited by radiation from an argon ion laser at 488 nm are listed in Table 3.5-1. Raman scattering cross sections of gases are discussed by Schrotter in Sec. 4.3.2.3. 

Table 3.5-1 Integrated absolute normalized Raman scattering cross sections of liquids, excited with radiation of A,e/ = 487.99 nm = hef = 20487 cm (Schrotter and Klockner, 1979)...

Eysel and Bertie (1988) considered the absolute normalized Raman scattering cross section of the ring breathing vibration of liquid benzene at 992 cm a standard for the measurement of relative Raman scattering cross sections of a number of compounds. When applying it as standard they made use of a divided spinning cell , developed by Kiefer and Topp (1974). 

The absolute Raman scattering cross section of this vibration is given by (Schrdtter and Klockner, 1979 Eysel and Bertie, 1988) ... 

The differential Raman scattering cross sections and depolarization ratios in the Fermi resonance region of carbon disulphide CS2 were measured and interpreted in terms of three bond polarizability parameters and the cubic force constant k 22 (Montero et al., 1984). 

The Raman spectrum of hydrogen sulfide H2S has been investigated at medium resolution in connection with the measurement of Raman scattering cross sections (Schrdtter and Kldckner, 1979). Later the g-branch of the v band was resolved using the cw CARS technique (Frunder et ah, 1985) and analysed by comparison with data from Fourier transform IR spectra (Lechuga-Fossat et ah, 1984). 

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