Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 195

In the appropriate frequency ranges electromagnetic radiations can interact with the vibrational modes of a condensed phase whose energy depends on interatomic forces and on the geometry of the structural units involved. Specific structural units such as Q ⁿ species thus have a characteristic vibrational signature whose changes as a function of chemical composition or temperature and pressure yields valuable structural information. With infrared (IR) and Raman spectroscopy, one probes in this way the interaction between atomic entities undergoing vibrational motion and an incident electromagnetic radiation whose energy is in the IR or visible regions of the spectrum. The difference between these techniques is that the interaction of the atoms and entities undergoing vibrational motion involves absorption and scattering of photons in IR spectroscopy and Raman spectroscopies, respectively, the energy of the incident photons varying by an amount (or a multiple of it) E = hν = hc/λ, where c is the speed of light, h Planck’s constant, ν and λ the frequency and wavelength of the vibrational mode probed. When recorded against the frequency of the light source, a spectrum thus shows a series of bands or peaks that are the fingerprints of specific vibrational modes. In contrast, Brillouin spectroscopy does not bring direct structural information because it relies on the interaction of photons with acoustic waves, which probes the bulk and not some specific units of the substance. But it yields elastic coefficients, which themselves strongly depend on structure.