FCenter in Cesium Fluoride: Properties of the Optically Excited Center

Studies of the luminescence, photoconductivity, Schottky ionization, bleaching, and ground-state repopulation of the optically excited $F$ center in CsF are reported, with these main results: (a) Excitation in the $F$ absorption band centered at 1.88 eV gives rise with high efficiency to a luminescence band (at 1.43 eV) whose unusual properties include the small Stokes shift (0.45 eV), narrow low-temperature width (0.10 eV), and short lifetime (50 nsec). (b) Photoconductivity, Schottky ionization, and optical bleaching show a wavelength dependence in the $F$-band region. Thermal ionization of the optically excited center is found to be more efficient for excitation in the high-energy component of the triplet $F$ absorption band, which corresponds to the $P_{\frac{1}{2}}$ level of the spin-orbit-split $P$-like excited states. This implies that the lattice relaxation processes are influenced by the initial state. (c) The characteristic time for return to the ground state for an optically excited $F$ center is found to be 1.0 μsec, much longer than the luminescence lifetime. This suggests the occurrence of a bottleneck in the lattice relaxation following the emission.