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Low-energy excitations of a single water molecule are studied when confinedwithin a nano-size cage formed by the ionic crystal lattice. Optical spectraare measured of manganese doped beryl single crystal Mn:Be3Al2Si6O18, thatcontains water molecules individually isolated in 0.51 nm diameter voids withinthe crystal lattice. Two types of orientation are distinguished: water-Imolecules have their dipole moments aligned perpendicular to the c axis, anddipole moments of water-II molecules are parallel to the c-axis. The opticalconductivity and permittivity spectra are recorded in terahertz and infraredranges, at frequencies from several wavenumbers up to 7000cm-1, at temperaturesfrom 5K to 300K and for two polarizations, when the electric vector E of theradiation is parallel and perpendicular to the c-axis. Comparative experimentson as-grown and on dehydrated samples allow to identify the conductivity andpermittivity spectra caused exclusively by water molecules. In the infraredrange well-known internal modes nu1, nu2 and nu3 of the H2O molecule areobserved for both polarizations, indicating the presence of water-I andwater-II molecules in the crystal. Spectra recorded below 1000cm-1 reveal arich set of highly anisotropic features in the low-energy response of H2Omolecule in a crystalline nanocage. While for E parallel to c only twoabsorption peaks are detected, at 90cm-1 and 160cm-1, several absorption bandsare discovered for E perpendicular to c, each consisting of narrowerresonances. The bands are assigned to librational and translational vibrationsof water-I molecule that is weakly, via hydrogen bonds, coupled to the nanocagewalls. A model is presented that explains the fine structure of the bands bysplitting of the energy levels due to quantum tunneling between the minima in asix-well potential relief felt by a molecule within the cage.

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