On F‐ and F H ‐centres in cesium halides

Fg‐centres are formed in CsI doped with foreign halogen ions, by irradiation in the F‐band at 180 K. They are observed in CsI:Br − , CsI:Cl − , and CsI:F − . The lifetime of the relaxed excited state of the F‐centre in pure CsI and of the F H ‐centre is determined by measuring the repopulation of the ground state after depletion with laser light. The results are τ R = 3 μs for the F‐centre in CsI, 360 ns for CsI: Br − , 400 ns for CsI: Cl − , 290 ns for CsI: F − . The luminescence decay time of the F‐centre in CsCl is found to be 2 μs, the relaxed excited state lying 0.07 eV under the conduction band. The results on the F H ‐centre suggest that the lowest emitting level is 2s with a large admixture of 2p character. Simultaneously with the formation of F H ‐centres, new absorption bands appear in the M‐band region, possibly due to M H ‐centres.