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 detecting 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 terahertz spectroscopy experiments has generally relied on a collapse of the information available in the fs-THz pulse to give a one-dimensional time-dependent function, whose physical significance is not well defined. We describe a method to exploit the full electric field profile of the pulse as a function of pump/probe delay time to obtain a two-dimensional function that describes the system’s time-dependent dielectric response. We discuss the physical 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.