Normal Raman Scattering

Raman scattering is normally of such very low intensity that gas phase Raman spectroscopy is one of the more difficult techniques. This is particularly the case for vibration-rotation Raman spectroscopy since scattering involving vibrational transitions is much weaker than that involving rotational transitions, which were described in Sections 5.3.3 and 5.3.5. For this reason we shall consider here only the more easily studied infrared vibration-rotation spectroscopy which must also be investigated in the gas phase (or in a supersonic jet, see Section 9.3.8). 

The probability of Raman scattering is quite small. This normally requires the use of intense laser sources and concentrated samples. A high-resolution double or triple monochromator is used to separate the Raman lines from the intense Raleigh line. 

The intensities of Raman scattering depend on the square of the infinitesimal change of the polarizability a with respect to the normal coordinates, q. Since the polarizability itself is already the second derivative of the energy with respect to the electric field - see equa-... 

Narayanan V., Begun G., Stokes D., Sutherland W., Vo-Dinh T., Normal Raman and surface-enhanced Raman scattering (SERS) spectra of some fungicides and related chemical compounds,/. Raman Spectrosc., 1992 23 281-286. 

Raman spectroscopy is primarily useful as a diagnostic, inasmuch as the vibrational Raman spectrum is directly related to molecular structure and bonding. The major development since 1965 in spontaneous, c.w. Raman spectroscopy has been the observation and exploitation by chemists of the resonance Raman effect. This advance, pioneered in chemical applications by Long and Loehr (15a) and by Spiro and Strekas (15b), overcomes the inherently feeble nature of normal (nonresonant) Raman scattering and allows observation of Raman spectra of dilute chemical systems. Because the observation of the resonance effect requires selection of a laser wavelength at or near an electronic transition of the sample, developments in resonance Raman spectroscopy have closely paralleled the increasing availability of widely tunable and line-selectable lasers. 

Vibrational spectroscopies such as Raman and infrared are useful methods for the identification of chemical species. Raman scattering [4] is a second-order process, and the intensities are comparatively low. A quick estimate shows that normal Raman signals generated by species at a surface or an interface are too low to be observable. Furthermore, in the electrochemical situation Raman signals from the interface may be obscured by signals from the bulk of the electrolyte, a problem that also occurs in electrochemical infrared spectroscopy (see Section 15.3)... 

Unfortunately, the different selection rules that apply to resonant and normal Raman scattering were not taken into account in this spectral interpretation. In the following, it is shown that the conclusions and assignments mentioned above have to be modified on the basis of symmetry considerations as discussed by Ricchiardi et al. (41). 

The electrochemical cell used in our laboratory has been fully described elsewhere (5). The cell body is made of chemically inert Kel-F and the electrode is mounted on a piston so that its surface can be pushed to the optical window, to a spacing of the order of 1-3 microns, in order to minimize the signal from the bulk electrolyte. For Raman scattering spectroscopy the window is of flat fused quartz, and the exciting laser beam is incident at about 60°. The scattered light is collected off-normal, but the geometry is not critical for SERS due to the high sensitivity. Details on the SERS measurements in our laboratory have been reported previously (6,7). 

In the frequency region where the i/(0H) vibrations of interfacial H20 are observed, the normal Raman scattering from the bulk solution can obscure the SERS of interfacial H20 if appropriate precautions are not taken. In the studies reported here, the SERS of interfacial H20 was acquired with the electrode surface positioned as close to the electrochemical cell window as possible to minimize contributions from the bulk solution. When altering the electrode potential to deposit Pb onto the Ag electrode surface, the electrode was pulled away from the window several mm, the surface allowed to equilibrate at the new conditions, and the electrode repositioned near the cell window for spectral acquisition. 

Figure 7.1 Energy level diagram Illustrating changes that occur in IR, normal Raman, resonance Raman, and fluorescence. Notation on the figure stands for Rayleigh scattering (R), Stokes Raman scattering (S), and anti-Stokes Raman scattering (A). Reprinted from Ferraro et al. (2003) [4] with permission from Elsevier.

The DFT calculations led to a structure of C3 , symmetry for the model compound S(NMe)3 (Figure 9 and Table 7) with 42 normal vibrations, whose irreducible representation is given by 8A (Ra) + ISff (Ra/IR) + 6A" (IR) + WE" (Ra). The brackets indicate activities in Raman scattering and/or infrared absorption. 

Figure 26 compares the spectrum of free MYKO 63 with that of MYKO bound to calf thymus DNA. Both spectra have been normalized to the same Raman scattering intensity and drug concentration. The spectrum of free MYKO 63 was obtained by subtracting the solvent spectrum from the MYKO solution spectrum. The spectrum of the bound drug was obtained by a two-step process (i) the solvent spectrum was subtracted both from the spectrum of the MYKO-DNA complex and from the DNA solution spectrum (ii) subtraction of these two new spectra was... 

Fig. 26. Raman spectra of unbound (A) and bound (B) MYKO 63 after subtraction of calf thymus DNA and NaC104 backgrounds and normalization to the same Raman scattering intensity and drug concentration...

Raman scattering activities (A 4 /AMU), Raman depolarization ratios, reduced masses (AMU), force constants (mDyne/A) and normal coordinates ... 

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