Translational–rotational coupling in supercooled liquids: Heterodyne detected density induced molecular alignment

Abstract

We present a new time domain technique for studying molecular orientational relaxation in viscous liquids. A molecular velocity gradient (acoustic disturbance) associated with a density change induced by weak absorption of a 1.06 μm excitation pulse, causes molecular alignment through translational–rotational coupling. Using an optical heterodyne detection method, molecular orientational relaxation is monitored. An eightfold experimental cycle, analogous to phase cycles in NMR, is used to separate the DIHARD signal (density induced heterodyne amplified rotational dynamics) from optical Kerr effect (OKE) contributions and thermal lensing effects. Calculations combining the Navier–Stokes equation with translational–rotational coupling are presented that describe the nature of the method. The method is analyzed theoretically and demonstrated with experiments on supercooled salol (phenyl salicylate). DIHARD experiments on salol combined with heterodyne detected OKE experiments are used to examine long time scale orientational relaxation over a wide range of times and temperatures. While OKE experiments measure the time derivative of an orientational correlation function, it is shown that DIHARD directly measures the time dependence of an orientational correlation function. The experimental results are compared to those previously reported in the literature, which were obtained with other methods.