Terahertz-field-induced second-harmonic generation measurements of liquid dynamics

A new third-order non-linear THz pump-optical probe measurement of liquid d ynamics is reported. Terahertz-field-induced second-harmonic generation (TF ISH) measures the correlations between the hyperpolarizability and dipole m oment fluctuations at equilibrium. This method is analyzed in the Born-Oppe nheimer approximation, and is shown to measure diffusive orientational dyna mics as [P-1(cos theta)]. Measurements of water and acetonitrile yield rela xation time constants of 13 and 9.6 ps, respectively. Signals were also obt ained from thin polymer films and surfaces, suggesting a number of applicat ions for this method. (C) 1999 Elsevier Science B.V. All rights reserved.