Explanation

A typical color centers formed by the capture of an electron by a negative ion vacancy in an alkali halide. For example, when sodium chloride crystal is heated in sodium vapor, the original transparent crystal is yellow under transmission light, that is, absorption occurs at the blue end close to the visible spectrum, which is called F-band. These optical absorption centers are F-centre, which are composed of an electron captured by a negative halogen ion vacancy. The width and precise position of the F-band vary with temperature. This is the result of the thermal movement of ions around the F-centre. The detailed study of the shape of the F-band shows that the F-centre has several excited states below the conduction band of the crystal. Some detailed information about F-centre comes from electron spin resonance and electron nuclear double resonance experiments. From this, the range and properties of the captured electron wave function are obtained. The results indicate that even if the electron is in the ground state, its wave function will extend to the adjacent six positive ions or even further regions,

Classification