Cascading Fermi Resonances and the Soft Mode in Dense Ice

Synchrotron infrared reflectance and absorbance spectroscopy reveals a cascading sequence of pressure-induced Fermi resonances in compressed H₂O and D₂O ices up to and through the transition to the symmetric hydrogen-bonded phase. These resonances arise from the extremely large pressure shift of the O-H(D) stretching mode associated with the transition, which causes multiple avoided crossings between this and other infrared-active vibrational states. The highest frequency LO mode has a minimum at the transition, and the LO-TO splitting increases dramatically in the high-pressure phase. Fano-like features are present in absorbance even in the absence of mode coupling.