Theory of profiles of hydrogen stretching infrared bands of hydrogen-bonded solids. Multi-Fermi resonance effects and strong coupling between the high-frequency hydrogen stretching vibration and low-frequency phonons

A theory is developed to describe the profiles of hydrogen stretching infra red bands of hydrogen-bonded solids taking into account the anharmonic coup ling between the high-frequency stretching vibration, nu(XH), and low-frequ ency lattice phonons, Ohm(i), as well as multi-Fermi resonances between sta tes involving the V(XH) stretching and overtones or combinations of some in ternal modes. The theory has been constructed in the framework of the exten ded molecular exciton Davydov approach. Model calculations show that the st rong couplings between the high frequency nu(XH) and low-lattice vibration frequencies, Ohm(i), generate the broadness of the nu(XH) band but the mult i-Fermi resonances between the V(XH) state and overtones or combinations of internal modes generate the complicated substructure band which is observe d experimentally. (C) 2000 Elsevier Science B.V. All rights reserved.