Temperature variation of the shape of the F band in undoped LiF

The optical absorption (OA) spectra between 3.1 and 6.2 eV of undoped X ray irradiated LiF measured during and after linear heating up to increasing end temperatures have been studied. The temperature variation of the OA near 5 eV, which is usually attributed to F centres has been analysed by direct comparison between different OA spectra and by deconvolution of the OA spectra. It was found that the OA consists of three overlapping bands belonging to three different centres. The bands, which are Gaussian with maxima at 4.4, 4.78 and 5.08 eV (at 300 K), have different behaviour under thermal treatment. The most stable band is the 5.08 eV band, which corresponds to the pure F centres. The 4.78 eV band disappears completely after heating up to 600 K and was supposed to be connected with F₁ centres. The 4.4 eV band (absent after irradiation) starts to increase and decrease after passing a maximum with temperature increasing. The 4.78 and 5.08 eV bands show strong linear temperature dependencies of the maximum position and the width.