Technique allowing the quantitative and qualitative detection of radiation-induced changes in the real, $\mathrm{\Delta }\text{Re}\sigma$, and imaginary, $\mathrm{\Delta }\text{Im}\sigma$, components of the conductivity of a medium by time-resolved measurement of changes in the microwave absorption resulting from the formation of mobile charges or from changes in the dipole moment or polarizability of molecules on excitation.