OVERTONE SPECTRA AND HYDROGEN POTENTIAL OF H2O AT HIGH-PRESSURE

The first overtones of the stretching and bending vibrations of H2O an d the first stretching overtone of D2O were measured to pressures of 3 7 GPa at 300 K, encompassing the stability fields of water, ice VI, an d ice VII. The successive disappearance of the overtone peaks in ice V II with increasing pressure indicates that the barrier height in the d ouble minimum hydrogen potential decreases at a rate of 230 cm(-1) GPa (-1) (0.029 eV GPa(-1)) to pressures of 32 GPa. The first overtone of the O-H stretching vibration splits and forms a doublet between 10 and 25 GPa, implying that the barrier height is near the energy of this o vertone and that proton tunneling may occur at this energy level betwe en 10 and 25 GPa. The frequency of the first stretching overtone of ic e VII is also strongly influenced by quantum effects near the barrier top, as its frequency is larger than harmonic values. The hydrogen bon ding potential in ice VII at high pressure is described using a semiem pirical potential model that incorporates the observed rate of barrier height reduction and available O-H and O-O bond length data of ice VI I under pressure. This model requires a rapid increase in O-H bond len gth between 40 and 60 GPa in order to reach a value of one half the O- O distance above 60 GPa, at which pressure the transition to ice X has been reported to occur. Up to 40 GPa, our model is consistent with th e trend of O-H bond length observed in neutron diffraction data to 10 GPa.