Short time structural relaxation processes in liquids: Comparison of experimental and computer simulation glass transitions on picosecond time scales

Recent Brillouin scattering studies of relaxation in the simple ion system Ca⁺⁺-K⁺/NO₃^- (two particles of argon structure and a small anion) provide the experimental basis for a direct comparison of glass transformation phenomenology in computer simulated LJ argon "glass," with that in laboratory substances studied on the same time scale. Most of the attenuated glass transition characteristics observed for LJ argon are found in the ionic system, and the same relation of the dispersion midpoint to the so-called "ideal" glass transition temperature is observed. Analysis of the real and imaginary parts of the complex longitudinal modulus shows that at high temperatures the relaxation function for Ca ⁺⁺-K⁺-NO₃^-, in strong contrast with that at normal low temperature behavior, closely approaches a simple exponential decay with Arrhenius form for the relaxation time. Furthermore, the high temperature Arrhenius plot extrapolates naturally to the reciprocal quasilattice vibration frequency determined by far infrared absorption studies on the glassy solid. Because of the low activation energy in the high temperature regime the glass "transition" observed with decreasing temperature is smeared out almost beyond recognition. It is argued that this will be a rather general phenomenon for hyperquenched glasses and that for such cases, the fictive temperature concept which associates the glass structure with an internally equilibrated liquid structure (that at T_fictive) must collapse. Parallels between LJ argon and Ca⁺-K⁺-NO ₃-glasses are discussed.