Higher vibrational states of OH/OD in the bulk of congruent LiNbO₃and in proton/deuteron exchanged layers at the surface of LiNbO₃

The fundamental vibrational transition nu 01 of the higher vibrational transitions nu 02 , nu 03 and nu 04 and the OH and the OD oscillator in the bulk of congruent LiNbO 3 and in proton/deuteron exchanged LiNbO 3 :PE/DE surface layers are investigated with respect to their spectroscopic properties and their temperature dependence. All observed vibrational transitions are completely polarized perpendicular with respect to the ferroelectric z direction. The spectral positions of the vibrational transitions follow closely a Morse type potential with D e =4.75 eV and a=22.23 nm -1 ( omega e,OH =3684 cm -1 and Omega e x e,OH =88.5 cm -1 for LiNbO 3 :PE). No hint of possible hydrogen bonding was found. The integrated absorption intensity ratios of the higher vibrational transitions exhibit the existence of an electrical anharmonicity, i.e. a non-linearity of the dipole moment versus O-H separation. The electrical anharmonicity reveals a noticeable temperature dependence whereas the vibrational transition energies and therefore the mechanical anharmonicity shows no temperature dependence in the range from room temperature to liquid helium temperature.