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 hy drogen-bonded phase. These resonances arise from the extremely large p ressure shift of the O-H(D) stretching mode associated with the transi tion, which causes multiple avoided crossings between this and other i nfrared-active vibrational states. The highest frequency LO mode has a minimum at the transition, and the LO-TO splitting increases dramatic ally in the high-pressure phase. Fano-like features are present in abs orbance even in the absence of mode coupling.