We investigated the absorption spectra of KI under excitation by an ArF excimer laser at 11 K. The laser light excited the sample through a one-photon process. Stable F centres were formed with excitation at a power level above ~ 2 mJ/cm2. From the time-resolved absorption spectrum, it is evident that the F centres are generated within 50 ns after the time of initiation of the laser pulse. The width of the F band at 50 ns was broader than that of the stationary F band at 11 K, suggesting the temperature increase of the crystal. We discuss the relaxation process in KI under dense electronic excitation.