Molecular inertial effects in liquids: Poley absorption, collision-induced absorption, low-frequency Raman spectrum and Boson peaks

The far-infrared absorption spectra of low-viscosity liquids contains a broad peak whose frequency decreases with increase in temperature and height increases. This is known as the Poley absorption. In the theory of itinerant oscillations of molecules in a liquid, the frequency of the Poley absorption peak is inversely proportional to the square root of a molecule's moment of inertia. Dipolar molecules confined to symmetrical cages in the structure of an ice clathrate crystal also show a peak in the far-infrared region. This has also been interpreted in terms of rotational oscillations of the dipolar molecule with its frequency being inversely proportional to the square root of the molecule's moment of inertia. The low-frequency Raman and the far-infrared absorption spectra (or the dielectric loss spectra in the THz frequency range) of a glass and a supercooled liquid also show a peak and several other features depending on the manner of presenting the Raman scattering and infrared absorption data. Data on these features are collected from the literature and compared here. It is shown that despite the differences arising from molecule-specific effects, these three features are remarkably similar, thus indicating that the vibrational effects attributed to the Boson peak and to other features of the low-frequency Raman scattering are likely to admit to the same underlying mechanisms as the Poley absorption. There is also a collision-induced absorption in liquids in the far-infrared region. Its equivalence in the Raman-scattering studies is yet to be recognized.