Fig. 2

a The anharmonic interaction between two degenerate vibrational levels, in this case the fundamental transition of the OH/OD stretching mode and the overtone of the OH/OD bending mode, causes a splitting of the energy levels into symmetric and asymmetric combination modes. b The broad, inhomogeneously broadened [22, 23] water O−D or O–H stretch mode density of vibrational states (DOS) couples strongly to the overtone of the bending mode. The bend overtone couples most strongly to those modes within the quasi-continuum of stretch modes that have the same frequency