Copyright © 2014 CC-BY-3.0 and by the respective authors. Pulse radiolysis, which utilizes a pump electron beam and a probe pulse, is a powerful tool that can be used for the time-resolved observation of ultrafast radiation-induced phenomena. Recently, double-decker pulse radiolysis using visible probe pulses were demonstrated based on a photocathode RF gun driven by two UV pulses, which enabled synchronized pump electron beam and visible probe pulses. In this study, pulse radiolysis using terahertz (THz) probe pulses which were realized by the "double-decker" electron beams and dynamics of transient quasi-free electrons in semiconductors are presented.