Mixing of Longitudinal and Transverse Dynamics in Liquid Water

Molecular dynamics simulations of liquid water in the 1.3--35nm^-1 momentum transfer ( Q) region show two excitations with a Q dependent symmetry character. The symmetry analysis suggests that the observed anomalies in the high frequency collective dynamics originate from relaxation processes responsible for (i) the appearance of a propagating transverse dynamics at high frequency, (ii) the transition from normal to fast sound, and (iii) a mixed symmetry of the two modes at large Q.