Time-resolved optical pump-terahertz (THz) probe experiments are currently used to obtain information about the ultrafast dynamics of photoexcited carriers in semiconductors and about the far-infrared nonlinear response during solvation in liquids. The THz dynamics in such photoexcited systems is fully characterized by a two-dimensional nonlinear susceptibility. We have developed a frequency-domain analytical method for the direct extraction of this susceptibility from the experimental data. Following effects are taken into account: dispersive propagation of radiation in a photoexcited medium, refraction on its surfaces, THz sensor responsivity, and spatio-temporal transformations of the THz pulses. Strategies for possible experiments are discussed.