Observation of Fano interference in high-pressure ice VII

Infrared absorption spectra of ice VII have been measured up to 100 GPa at room temperature. The OH stretching peaks, which showed a rapid shift to low frequency with an enormous line broadening with increasing pressure, were found to interfere with the OH bending and librational peaks at very high pressures above 40 GPa. Appearance of such Fano-type interference in absorption line shape indicates that the OH stretching modes form a continuous (or quasicontinuous) energy band in their excitation state. The formation of the energy continuum is closely related to the conversion from double- to single-minimum shape in the hydrogen-bond potential, and hence considered to be a signature of the onset of hydrogen-bond symmetrization.