Infrared absorption in LiF polymers and microcrystals

Publisher Website
Google Scholar
Abstract
Infrared absorption spectra are reported for LiF in various stages of aggregation. Starting with monomers and dimers the degree of aggregation is increased to include more complex polymers until the transition is made to microcrystals and finally to bulk. The samples were prepared at helium temperature by the simultaneous condensation of argon and LiF vapor onto a transparent substrate. The argon matrix served the purpose of separating the LiF aggregates without strongly interacting with them. Although LiF vapor contains only monomers and dimers, larger polymers could be formed in the matrix by increasing the concentration of LiF. For concentrations of 2%, more than 20 distinct polymer lines could be seen in the infrared absorption spectrum. At concentrations of 7%, the LiF polymer lines abruptly disappear and a single broad surfacephonon absorption, characteristic of microcrystals, emerges. The surface-phonon frequency slowly shifts with increasing LiF concentration until it coincides with the bulk absorption peak at the long-wavelength transverse-optical phonon frequency. This shift is due to the increased dipolar interaction between microcrystals as they are brought closer together. The infrared absorption spectra of small regular polymers of LiF were calculated and compared with experiment. It appears that the infrared spectrum is much more influenced by the shape of the polymer than by its size.