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

• Published in: arXiv.org 2014-02
• Main Authors: Zhukova, Elena S, Torgashev, Victor I, Gorshunov, Boris P, Lebedev, Vladimir V, Shakurov, Gilman S, Kremer, Reinhard K, Pestrjakov, Efim V, Thomas, Victor G, Fursenko, Dimitry A, Prokhorov, Anatoly S, Dressel, Martin
• Format: Article
• Language: English
• Subjects: Absorption spectra; Banded structure; Beryllium aluminum silicates; Bonding strength; Cages; Coupled walls; Crystal lattices; Crystal structure; Dehydration; Dipole moments; Energy levels; Fine structure; Hydrogen bonds; Ionic crystals; Lattice vibration; Manganese; Permittivity; Quantum tunnelling; Single crystals; Vibrational states; Water chemistry
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1401.5760

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.