Porto Notation
With regard to Raman scattering or infrared absorption, one of the major tasks consists of assigning the experimentally observed lines to the different irreducible representations predicted by a previous group theory analysis based on the supposedly known space groups of the crystal. The species are labeled by the irreducible representations of the point group at the center of the first Brillouin zone because of the momentum conservation rule.
Second order effects can be present, as well as absorption due to impurities or imperfections, so that often the spectra exhibits more lines than predicted by group theory. A complete and correct band assignment is a matter of expertise and knowledge of complementary data concerning the substance.
In Raman scattering, one must achieve experimental geometrical arrangements in order to select the second-degree polynomial base function, related to polarizability. These functions correspond to the coupling between the incident light and scattered light polarizations.
By Ian R. Lewis, Howell Edwards
http://journals.aps.org/pr/pdf/10.1103/PhysRev.142.570