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Vibrational states of a water molecule in a nano-cavity of beryl crystal lattice

Low-energy excitations of a single water molecule are studied when confined within a nano-size cage formed by the ionic crystal lattice. Optical spectra are measured of manganese doped beryl single crystal Mn:Be3Al2Si6O18, that contains water molecules individually isolated in 0.51 nm diameter voids within the crystal lattice. Two types of orientation are distinguished: water-I molecules have their dipole moments aligned perpendicular to the c axis, and dipole moments of water-II molecules are parallel to the c-axis. The optical conductivity and permittivity spectra are recorded in terahertz and infrared ranges, at frequencies from several wavenumbers up to 7000cm-1, at temperatures from 5K to 300K and for two polarizations, when the electric vector E of the radiation is parallel and perpendicular to the c-axis. Comparative experiments on as-grown and on dehydrated samples allow to identify the conductivity and permittivity spectra caused exclusively by water molecules. In the infrared range well-known internal modes nu1, nu2 and nu3 of the H2O molecule are observed for both polarizations, indicating the presence of water-I and water-II molecules in the crystal. Spectra recorded below 1000cm-1 reveal a rich set of highly anisotropic features in the low-energy response of H2O molecule in a crystalline nanocage. While for E parallel to c only two absorption peaks are detected, at 90cm-1 and 160cm-1, several absorption bands are discovered for E perpendicular to c, each consisting of narrower resonances. The bands are assigned to librational and translational vibrations of water-I molecule that is weakly, via hydrogen bonds, coupled to the nanocage walls. A model is presented that explains the fine structure of the bands by splitting of the energy levels due to quantum tunneling between the minima in a six-well potential relief felt by a molecule within the cage.