THz-IR spectroscopy of single H2O molecules confined in nanocage of beryl crystal lattice

We have measured the terahertz–infrared (3–7000 cm−1) spectra of the optical conductivity of iron-doped single crystals of beryl, (Mn,Fe):Be3Al2Si6O18, that contain lone water molecules isolated within nanometer-sized cages formed by the ions of beryl crystal lattice. By comparing the spectra with those of dehydrated crystals, we exclude phonon resonances and reconstruct the spectra determined exclusively by vibrations of the water molecules. At liquid-helium temperatures, well-known intramolecular H2O modes are observed above 1000 cm−1 and accompanied with satellite resonances that are combinations of intramolecular and external vibrations of H2O molecules. At terahertz frequencies, a broad bump centred around 20 cm−1 (at 5 K) is observed with three rather narrow resonances at its high-frequency shoulder (38, 42 and 46 cm−1). The origin of these low-energy excitations is discussed.