Proton dynamics in the hydrogen bond. Inelastic neutron scattering by single crystals of CsH2PO4 at 20 K

Abstract

Inelastic neutron scattering spectra (INS) of the powder and of oriented single crystals of cesium dihydrogen phosphate (CsH2PO4, or CDP) at 20 K have been investigated. For single crystals the incident neutron beam was perpendicular to either the [100] or [001] crystal planes in order to distinguish between the short and long hydrogen bonds. The proposed assignments are based on previous infrared and Raman data and on the INS band intensities and polarisation. The optical and INS OH stretching band profiles are compared. Their shapes are described in terms of mechanical and electrical coupling of the two stretching modes of a OH…O hydrogen bond. For the longer bond a Fermi resonance of the OH stretching mode with an overtone of the bending mode is observed. Finally, a broad central mode observed in the INS at very low frequency has been tentatively assigned to the relaxation of the proton transfer along the hydrogen bond.