Raman active transitions

Not all vibrational transitions can be accessed by Raman scattering. Raman-active transitions are those associated with a change in polarizability of the molecule. In classical terms, this can be viewed as a perturbation of the electron cloud of the molecule. 

Eq. (35-7)]. For molecules of symmetry, these elements belong to the symmetry species, Fly, and so that the condition for a Raman-active transition is that the product F(i/fj) X include one of these species. Thus, from the Xg ground state of acetylene, Raman transitions to the (10000) Xg, (01000) 2, and (00010) Il levels are allowed and can be used to determine the nj, V2, and V4 fundamental frequencies, respectively. As can be seen in Table 1, these three modes do not produce a dipole change as vibration occurs, and thus these transitions are absent from the infrared spectrum. This is an example of the rule of mutual exclusion, which applies for IR/Raman transitions of molecules with a center of symmetry... 

Photoacoustic Raman spectroscopy (PARS) Photoacoustic Raman spectroscopy (PARS) is again a nonlinear spectroscopic technique. In this technique, selective population of a given energy state of a system (transitions must involve change in polarizability) is amplified using coherent Raman amplification (also known as stimulated Raman scattering). In this process, it is also important that the frequency difference of the two incident laser beams must be adjusted to equal the frequency of Raman-active transition. 

In contrast, the dipole moment of carbon dioxide fluctuates in phase with the asymmetric vibrational mode. Thus, an IR absorption band arises from this mode. On the other hand, as the polarizability of one of the bonds increases as it lengthens, the polarizability of the other decreases, resulting in no net change in the molecular polarizability. Thus, the asymmetric stretching vibration is Raman inactive. For molecules with a center of symmetry, such as CO2, no IR active transitions are in common with Raman active transitions. This is often called the mutual exclusion principle. 

Raman-active transition in the spectra of polyenes. Comparison of ab initio calculated Raman intensities for n-alkanes and rrnns-polyenes gives parallel information [91,92]. Note that when the number of carbon atoms in the chains increases, the Raman intensities of saturated and conjugated chains differ remarkably in their behav-... 

Several other forms of nonlinear spectroscopy have been developed that are not strictly based on Raman-active vibrations or rotations. Degenerate four-wave mixing (DFWM) is a technique where all input and output frequencies are identical. Because it does not involve the generation of light at new frequencies, it can rely on non-local mechanisms other than the local electronic polarizability (e.g. electrostric-tion). The selection rules for DFWM are closely related to those of one-photon techniques (e.g. absorption). DFWM using infrared beams is therefore used to probe infrared absorbing transitions instead of Raman-active transitions. 

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