Theory for determination of the low-frequency time-dependent response function in liquids using time-resolved terahertz pulse spectroscopy

With the current rapid improvement in techniques for generating and detecti ng femtosecond terahertz (fs-THz) pulses, it is now possible to detect time -dependent perturbations to low-frequency intermolecular far-infrared modes during solvation events. The interpretation of these time-resolved teraher tz spectroscopy experiments has generally relied on a collapse of the infor mation available in the fs-THz pulse to give a one-dimensional time-depende nt function, whose physical significance is not well defined. We describe a method to exploit the full electric field profile of the pulse as a functi on of pump/probe delay time to obtain a two-dimensional function that descr ibes the system's time-dependent dielectric response. We discuss the physic al significance of this response function in terms of the time evolution of solute/solvent modes following photoexcitation, and give examples to show how its qualitative features relate to the measurable signal. (C) 1999 Amer ican Institute of Physics. [S0021-9606(99)51417-7].