The optically active process of higher-order bands (HOBs) in fast-neutron-irradiated float-zone silicon has been investigated at low temperature using a Fourier-transform infrared spectrometer with optical excitation. It is found that the photoexcitation process follows an exponential time dependence and that the decay is logarithmic with a decay time constant of 10⁵ s at 7 K. The saturation value of the absorption coefficients depends on the logarithm of the illumination-light intensity. These characteristics are associated with the slow relaxation of photoexcited carriers originating mainly from defect clusters. The relaxation behaviour of photoexcited carriers has been analysed using the macroscopic-barrier model, which is in good agreement with the observations.