The F Center in Cesium Fluoride [Thesis]

A number of properties of the F center in cesium fluoride have been investigated. These include the optical absorption F band lineshape and its temperature, magnetic field and stress dependence, and the photoconductivity, luminescence, and ground-state repopulation behavior. The F band possesses a well-resolved triplet structure. The magnetic circular dichroism measurements show that the P states are split by a large spin-orbit coupling. The coupling of the center to cubic modes of vibration is relatively weak and the coupling to non-cubic modes of vibration relatively strong, their contributions to the second moment of the F band being in the ratio of one to four. The high-energy F-band component is due to transitions to the P_1/2 state and the other two components to transitions to the P_3/2 state. The latter is split by a dynamic Jahn-Teller effect, as suggested by Moran, The temperature-and stress-induced behavior is consistant with this model. An unusual feature in the F-band lineshape is the presence of structure due to optical transitions in which zero, one or more optical phonons of cubic symmetry are created. The photoconductivity and certain optical bleaching experiments show that the lattice relaxation processes which follow the absorption of a photon depend upon whether the electron is excited to the P_1/2 or P_3/2 state. For P_1/2 excitation the electron reaches an intermediate metastable "relaxed" state before decaying to the lowest relaxed excited state. Luminescence and ground-state repopulation measurements have revealed an unusual discrepancy between the luminescence lifetime, 50 nsec., and the time required for the electron to return to its ground state after excitation, 1 μsec. The bottleneck is in the return to the ground state after the emission and is of unknown origin. Because of this behavior the interpretation of the luminescence behavior is somewhat tentative. If the effect is intrinsic to the F center then the luminescence experiments suggest that the usual picture of a highly diffuse relaxed excited state does not apply to the F center in cesium fluoride.