Raman spectroscopy anti-Stokes lines

Section 2.3). The scattered light consists of two types one, called Rayleigh scattering, is strong and has the same frequency as the incident beam (vo), and the other, called Raman scattering, is very weak ( 10 5 of the incident beam) and has frequencies vo vm, where vm is a vibrational frequency of a molecule. The vo — vm and vo + vm lines are called the Stokes and anti-Stokes lines, respectively. Thus, in Raman spectroscopy, we measure the vibrational frequency (vm) as a shift from the incident beam frequency (vo). In contrast to IR spectra, Raman spectra are measured in the UV-visible region where the excitation as well as Raman lines appear. 

The vibrational selection rules are the same for Raman spectroscopy as for infrared spectroscopy. In the Stokes process, the intense, monochromatic radiation t es a molecule from the v = 0 state to a virtual state, VO, from which it falls back to the v = 1 state. Similarly, in the anti-Stokes process, the virtual state VI is involved in the overall transfer of the molecule from the v = 1 to the v = 0 state. The Stokes and anti-Stokes transitions lie on the low and high wavenumber sides, respectively, of the exciting radiation. The intensity of the anti-Stokes line, relative to the Stokes transition is very low because of the lower population of the v = 1 state, compared to that of the v = 0 state. Consequently, Raman spectroscopy uses only the Stokes transitions. 

Equation (A8.6) shows that the Stokes/anti-Stokes lines depend on the change of molecular polarizability Aa for the vibrational modes of the molecule. This allows us to develop the selection rules for Raman spectroscopy in Section 6.2.4. 

A recent approach is the use of anti-Stokes Raman spectroscopy [10]. The advantage of this anti-Stokes measurement is that it does not restrict the Raman excitation wavelength as does FT-Raman. The disadvantage of the anti-Stokes method is the extremely weak signal obtained. In order to surmount this problem, a new instrumental system was devised which increases the density of the anti-Stokes lines by using a chromatic aberration focal lens with a highly sensitive spherical integration cell and finally with a laser power reference system. 

The best resolution of Q-branch rotational structure in a N2-Ar mixture was achieved by means of coherent anti-Stokes/Stokes Raman spectroscopy (CARS/CSRS) at very low pressures and temperatures (Fig. 0.4). A few components of such spectra obtained in [227] are shown in Fig. 5.9. A composition of well-resolved Lorentzian lines was compared in [227] with theoretical description of the spectrum based on the secular simplification. The line widths (5.55) are presented as... 

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