HIGH-RESOLUTION FTIR SPECTROSCOPY TO STUDY ISOTOPIC SUBSTITUTIONS OF OH- IN LIF AND NAF DOPED WITH MG2+ AND TI3+

High-resolution (0.04 cm(-1)) FTIR spectroscopy in the range 9-300 K i s applied to study the stretching modes of (OH-)-O-16 and of its isoto pic substitutions, as (OH)-O-18, (OD)-O-16 and (OT)-O-16, perturbed by neighbouring defects induced by Mg2+ and Ti3+ in LiF and NaF. The spe ctra are analysed in the framework of the Morse anharmonic oscillator model. The anharmonicity parameter (23 x 10(-2)) is evaluated either b y considering the isotopic replicas or the first overtone transition. The former is found slightly higher than the latter, since the hydroxy l ion vibrates as a ''nearly'' free diatomic molecule. The very weak i sotopic replica due to O-18 and D present in the samples as a result o f the isotope natural abundance are detected as well.