Relaxation of 2p Excitons and F Center Formation in Picosecond Range in RbBr, RbI and KBr

Formation times of F centers (τ F ) and self-trapped excitons (τ S T E ) in RbBr crystals have been measured under the excitation by two photon absorption of second harmonics from a mode-locked ruby laser. Such excitation produces the 2 p hot exciton directly in the crystal. τ F is 27 ±8 ps at RT and 30 ±8 ps at LHeT. τ S T E is not explicitly observed, but appears to be around 30 ps. Absorption spectra measured at about 100 ps after the excitation coincide to that of the conventional F band at RT, and consist mainly of the F band and of a small contribution of the self-trapped excitons at LHeT. τ F at RT and τ S T E at LHeT in RbI are both 30 ±10 ps. Possibly τ F in KBr at RT was 20 ±10 ps, but the absorption decayed with a time constant of 800 ±200 ps. The relaxation process of the 2 p hot exciton in RbBr is discussed in connection with the formation of self-trapped excitons and F centers.