SPEAKER:
Dr Elena Zhukova, Moscow Institute of Physics and Technology
SYNOPSIS:
We have measured the terahertz-infrared (3 cm-1 to 7000 cm-1) spectra of the optical conductivity of a single crystals of beryl, Fe:Be3Al2Si6O18, that contain water molecules isolated within nanometer-sized cages formed by the ions of beryl crystal lattice. By comparing the spectra with those of dehydrated crystals we exclude phonon resonances and reconstruct the spectra determined exclusively by vibrations of the water molecules.
At liquid-helium temperatures well-known intramolecular H2O modes are observed above 1000 cm-1 and accompanied with satellite resonances that are combinations of intramolecular and external vibrations of H2O molecules. These external vibrations are seen in the form of a band between 100 cm-1 and 400 cm-1 that consists of narrow components whose origin can be connected with translational and librational vibrations of the H2O molecule within the crystalline nano-cage.
At terahertz frequencies, a broad bump centered at 20 cm-1 (at 5 K) is observed with three rather narrow resonances at its high-frequency shoulder (38 cm-1, 42 cm-1 and 46 cm-1). The origin of these low-energy excitations is discussed.