The critical contribution to vibrational and rotational relaxation

Abstract

A collective description of vibrational relaxation in a critical binary liquid mixture is presented. This relates the rate of dephasing (or vibrational linewidth) to the density and composition fluctuations occurring in the fluid. Although the amplitude and correlation length of the low-frequency composition fluctuations diverge at the critical point, no critical linewidth anomaly is found. The long-range ‘hydrodynamic’ composition fluctuations do not contribute significantly because of the short-range nature of the interaction giving rise to the relaxation. Furthermore, the critical ‘non-hydrodynamic’ composition fluctuations of shorter wavelength are found to give a linewidth increase characterized by a critical exponent, γ-2v, approximately equal to zero, in agreement with observation. A discussion of critical lineshape anomalies in other relaxation processes, such as rotation, is given.