THE ANHARMONIC FEATURES OF THE SHORT-TIME DYNAMICS OF FLUIDS - THE TIME EVOLUTION AND MIXING OF INSTANTANEOUS NORMAL-MODES

Despite the obvious role of sharply varying repulsive forces in determ ining the structure of most liquids, for short periods of time, motion in liquids looks remarkably harmonic. That is, there seem to be well- defined collective, but independent, harmonic modes governing the ultr afast dynamics launched from any given liquid configuration. Because l iquids are not truly harmonic, however, these modes cannot last foreve r. In particular, ''instantaneous'' modes of this sort eventually have to give way to new instantaneous modes-ones more appropriate to whate ver new configuration the liquid has evolved into. In this paper we in vestigate just this process of mode evolution. By concentrating on sol ely the highest frequency modes, it is possible to formulate analytica l models for both the modes and the anharmonic interactions that affec t them. We can therefore begin to understand the mechanisms by which m odes change in time and the kinds of time scales on which the specific anharmonic processes occur in liquids. What we find is that there are several rather distinct signatures of anharmonicity: we see first tha t the anharmonicity within a mode itself continually causes the mode f requency to fluctuate. More sporadically, we find that two different b ut nearly resonant modes will sometimes interact strongly enough with one another to cause a temporary-though not a permanent-mixing between the modes. Of course, both of these processes are, in some sense, bre akdowns of instantaneous-normal-mode theory, but neither of them affec ts the basic identity and existence of instantaneous modes. The eventu al destruction-of the modes turns out to be an even less frequent even t precipitated by an even stronger mixing between a mode and the motio n of surrounding atoms. It is precisely this longer time scale that ma y mark the first point at which diffusive motion plays an essential ro le in liquid dynamics. (C) 1998 American Institute of Physics.