Relaxation Processes In Liquids Research Articles

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+/NO−3 (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+–NO−3, 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 Tfictive) must collapse. Parallels between LJ argon and Ca+–K+–NO3–glasses are discussed.

Picosecond relaxation processes in liquids

Recently there have been a number of important experimental and theoretical developments that focus on the dynamics of rotational and vibrational relaxation processes of molecules in liquids. This marks the emergence of a microscopic description of the forces that govern the structure of fluids. Ultra-short laser pulses permit us to observe directly the consequences of these molecular interactions by probing several causally related phenomena, namely electron localization, fluorescence depolarization and optical Kerr rotation in polar liquids. I present recent results from the picosecond laser spectroscopy of relaxation processes in liquids, and discuss their implications in terms of the influence of the molecular motion on reaction dynamics in liquids in general and on electron transfer processes specifically.

THE LONG TIME ASYMPTOTIC BEHAVIOUR OF THE CORRELATION FUNCTIONS——THE INFLUENCES OF THE WAVES ON THE RELAXATION PROCESSES

Based on the general theory of the non-equilibrium statistical mechanics, we have discussed the influences of waves on the relaxation processes in macroscopic systems, especially. about the non-exponential asymptotic behaviour of the correlation functions.Because, how to select the representation is important for discussing the dissipative processes in non-equilibrium statistical mechanics, it is desirable to start from the elementary excitation representation. Using the Basic idea of C* algebra theory, we have developed a method for operator expansion, and employed it to express the flow operators in second quantized elementary excitation representation. Taking the residual interaction between the elementary excitations as perturbation in the calculation of the resolvent of the Liouville operator, and using the method of projection operator, a general expression of the non-exponential decaying part of the correlation functions have been obtained.By analysing the procedure of treating the elementary excitation spectrum, we devided the residual interaction into two parts. For pure dissipative mode, taking into account the linear interaction, the main terms of the non-exponential decaying part(t(-d/2) term) were obtained. Detailed analysis shows that, the non-linear interaction (mode-mode coupling) is very small, and leads to a relatively faster decaying process (t(-d) or so), hence it can be neglected.This article emphasizes that, for the wave processes, the local fluctuation correlation, the particle self-correlation and the transport processes are different in nature. Their relative magnitudes (relative to the corresponding quasi-particle parts) may also be quite different.With these general results, we have analysed briefly the relaxation processes in liquid, and made some discussion about the so called mode-mode coupling theory.

Ultrasonic analysis of molecular relaxation processes in liquids

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Theory of Ultrasonic Thermal Relaxation in Liquids

The mechanism of energy transfer in the thermal relaxation process in liquids is considered theoretically as being due to a binary collision phenomena. The theory of Schwartz and Herzfeld, which has been successful in explaining the energy transfer process in the gas state, is applied to the liquid state. The main difficulty in application of the Schwartz-Herzfeld theory is in the evaluation of the mean free path of the molecule in the liquid. This problem is considered in terms of several different free volume theories of the liquid state. It is shown that by the use of the cubic cell model for liquid CS2 very good agreement is obtained between the Schwartz-Herzfeld theory and the experimental results for the pressure dependence of the relaxation time, the probability of transfer of energy per collision, and the temperature dependence of this probability. These results indicate that the mechanism of the thermal relaxation process is the same in the gas and liquid states.