Fundamentals
Selection rules
Selection Rules
Both Raman spectroscopy and infrared spectroscopy provide a unique spectral fingerprint of a material. The patterns of the spectra are caused by molecular or lattice vibrations. Although the spectral features of Raman and infrared spectra can be interpreted in a similar way the spectra look slightly different. The whole vibrational picture of a material is given by the complementary information of both Raman and infrared spectra.
Using selection rules, it can be predicted whether a molecular vibration is Raman or infrared active. During the interaction between a molecule and a photon the total angular momentum in the electronic ground state has to be conserved. As a consequence of this requirement only specific vibrational transitions are possible.
Rule of mutual exclusion
In general, molecular vibrations symmetric with regard to the centre of symmetry are forbidden in the infrared spectrum, whereas molecular vibrations which are antisymmetric to the centre of symmetry are forbidden in the Raman spectrum. This is known as the rule of mutual exclusion.
Selection rules for an infrared absorption and for Raman scattering
Infrared absorption can be detected if the dipole momentum µ in a molecule is changed during the normal vibration. The intensity of an infrared absorption band IIR depends on the change of the dipole moment µ during this vibration:
where q is the normal coordinate.
A Raman active vibration can be detected if the polarizability a in a molecule is changed during the normal vibration. The intensity of a Raman active band IRaman depends on the change of polarizability a during this vibration:
As a consequence of the selection rules, infrared spectroscopy provides detailed information about functional groups and Raman spectroscopy especially contributes to the characterization of the carbon backbone of organic substances or polymers.
