EPR optical detection of F centre pairs in alkali halides. - II : Spin-lattice relaxation of the F centre in its relaxed excited state

Like the EPR line of the fundamental state of the F centre in most of the alkali halide crystals, the line attributed to the relaxed excited state, detected optically for F-centre pairs is also Gaussian in shape. That is explained by an inhomogeneous broadening due to hyperfine interactions with the surrounding nuclei. The measurements of these resonances as a function of the microwave power allowed one to determine the saturation parameter and the broadening of the elementary line (homogeneous). On the other hand the measurement of the resonance signal as a function of temperature, the other parameters remaining constant, shows that it stays constant for T < 10°K and decreased for increasing temperature. This is attributed to the spin-lattice relaxation time T1 of the defect in its relaxed excited state. The relaxation mechanism is characterized by an Orbach process