title: Vibrational states of a water molecule in a nano-cavity of beryl crystal lattice.
abstract: 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 σ(ν) and permittivity ɛ'(ν) spectra are recorded in terahertz and infrared ranges, at frequencies from several wavenumbers up to ν = 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 σ(ν) and ɛ'(ν) caused exclusively by water molecules. In the infrared range, well-known internal modes ν1, ν2, and ν3 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∥c only two absorption peaks are detected, at ~90 cm(-1) and ~160 cm(-1), several absorption bands are discovered for E⊥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 "walls." A model is presented that explains the "fine structure" 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.
authors.affiliation: Moscow Institute of Physics and Technology, 9 Institutskiy per., 141700 Dolgoprudny, Moscow Region, Russia.
authors.forename: Elena S
authors.initials: ES
authors.lastname: Zhukova
authors.affiliation: Faculty of Physics, Southern Federal University, 5 Zorge St., 344090 Rostov-on-Don, Russia.
authors.forename: Victor I
authors.initials: VI
authors.lastname: Torgashev
authors.affiliation: Moscow Institute of Physics and Technology, 9 Institutskiy per., 141700 Dolgoprudny, Moscow Region, Russia.
authors.forename: Boris P
authors.initials: BP
authors.lastname: Gorshunov
authors.affiliation: Moscow Institute of Physics and Technology, 9 Institutskiy per., 141700 Dolgoprudny, Moscow Region, Russia.
authors.forename: Vladimir V
authors.initials: VV
authors.lastname: Lebedev
authors.affiliation: Kazan Physical-Technical Institute, Russian Academy of Sciences, 10/7 Sibirsky Trakt, 420029 Kazan, Russia.
authors.forename: Gil'man S
authors.initials: GS
authors.lastname: Shakurov
authors.affiliation: Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany.
authors.forename: Reinhard K
authors.initials: RK
authors.lastname: Kremer
authors.affiliation: Institute of Laser Physics, Russian Academy of Sciences, 13/3 Ac. Lavrentyev's Prosp., 630090 Novosibirsk, Russia.
authors.forename: Efim V
authors.initials: EV
authors.lastname: Pestrjakov
authors.affiliation: Institute of Geology and Mineralogy, Russian Academy of Sciences, 3 Ac. Koptyug's Prosp., 630090 Novosibirsk, Russia.
authors.forename: Victor G
authors.initials: VG
authors.lastname: Thomas
authors.affiliation: Institute of Geology and Mineralogy, Russian Academy of Sciences, 3 Ac. Koptyug's Prosp., 630090 Novosibirsk, Russia.
authors.forename: Dimitry A
authors.initials: DA
authors.lastname: Fursenko
authors.affiliation: Moscow Institute of Physics and Technology, 9 Institutskiy per., 141700 Dolgoprudny, Moscow Region, Russia.
authors.forename: Anatoly S
authors.initials: AS
authors.lastname: Prokhorov
authors.affiliation: 1. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, 70550 Stuttgart, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Dressel