Low-temperature excitonic, electron-hole and interstitial-vacancy processes in LiF single crystals

The emission spectra and the excitation spectra of various emissions have been measured in LiF crystals at 9 K using VUV radiation of 10-33 eV. In contrast to the luminescence of self-trapped excitons (3.4 eV), the efficiency of several extrinsic emissions (4.2, 4.6 and 5.8 eV) is very low in the region of an exciton absorption (12.4-14.2 eV). A single exciting photon of 28-33 eV is able to create a primary electron-hole (e-h) pair and a secondary exciton. The tunnel phosphorescence has been detected after the irradiation of LiF by an electron beam or x-rays at 6 K, and several peaks of thermally stimulated luminescence (TSL) at 12-170 K appeared at the heating of the sample. It was confirmed that the TSL at 130-150 K is related to the diffusion of self-trapped holes (VK centres). The TSL peak at ∼160 K is ascribed to the thermal ionization of F' centres. The TSL at 20-30 K and 50-65 K is caused by the diffusion of interstitial fluorine ions (I centres) or H interstitials, respectively. The TSL peak at ∼13 K, the most intense after electron or x-irradiation, cannot be detected after LiF irradiation by VUV radiation, selectively forming excitons or e-h pairs. The creation of a spatially correlated anion exciton and an e-h pair is needed for the appearance of this peak: an exciton decays into an F-H pair, a hole forms a VK and an electron transforms H into I (an F-I-VK group is formed) or an F centre into a two-electron F' centre (an F'-H -VK group). The analysis of the elementary components of the 9-16 K TSL showed that a phonon-induced radiative tunnel recombination of F'-VK (5.6 eV), F-H (∼3 eV) and F-VK (3.4 eV) occurs within these groups