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

作者：Zhukova Elena S^*; Torgashev Victor I; Gorshunov Boris P; Lebedev Vladimir V; Shakurov Gil' man S; Kremer Reinhard K; Pestrjakov Efim V; Thomas Victor G; Fursenko Dimitry A; Prokhorov Anatoly S; Dressel Martin

来源：Journal of Chemical Physics, 2014, 140(22): 224317.

DOI：10.1063/1.4882062

Low-energy excitations of a single water molecule are studied when confined within a nano-size cavity 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 sigma(nu) and permittivity epsilon%26apos;(nu) spectra are recorded in terahertz and infrared ranges, at frequencies from several wavenumbers up to nu = 7000 cm(-1), at temperatures 5-300 K 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 spectra of sigma(nu) and epsilon%26apos;(nu) caused exclusively by water molecules. In the infrared range, well-known internal modes nu(1), nu(2), and nu(3) 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 1000 cm(-1) reveal a rich set of highly anisotropic features in the low-energy response of H2O molecule in a crystalline nano-cavity. While for E parallel to c only two absorption peaks are detected, at similar to 90 cm(-1) and similar to 160 cm(-1), several absorption bands are discovered for E perpendicular to c, each consisting of narrower resonances. The bands are assigned to librational (400-500 cm(-1)) and translational (150 -200 cm(-1)) vibrations of water-I molecule that is weakly coupled to the nano-cavity %26quot;walls.%26quot; A model is presented that explains the %26quot;fine structure%26quot; of the bands by a splitting of the energy levels due to quantum tunneling between the minima in a six-well potential relief felt by a molecule within the cavity.

• 出版日期2014-6-14