Overtone spectra and hydrogen potential of H2O at high pressure

The first overtones of the stretching and bending vibrations of H2O and the first stretching overtone of D2O were measured to pressures of 37 GPa at 300 K, encompassing the stability fields of water, ice VI, and ice VII. The successive disappearance of the overtone peaks in ice VII with increasing pressure indicates that the barrier height in the double minimum hydrogen potential decreases at a rate of 230 cm21 GPa21 ~0.029 eV GPa21! 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 overtone and that proton tunneling may occur at this energy level between 10 and 25 GPa. The frequency of the first stretching overtone of ice VII is also strongly influenced by quantum effects near the barrier top, as its frequency is larger than harmonic values. The hydrogen bonding potential in ice VII at high pressure is described using a semiempirical potential model that incorporates the observed rate of barrier height reduction and available O-H and O-O bond length data of ice VII under pressure. This model requires a rapid increase in O-H bond length 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 the trend of O-H bond length observed in neutron diffraction data to 10 GPa. @S0163-1829~98!07837-0#