Low-temperature excitonic, electron–hole and interstitial-vacancy processes in LiF singlecrystals

Abstract

The emission spectra and the excitation spectra of various emissions have been measured inLiF crystals at 9 K using VUV radiation of 10–33 eV. In contrast to the luminescence ofself-trapped excitons (3.4 eV), the efficiency of several extrinsic emissions (4.2, 4.6 and5.8 eV) is very low in the region of an exciton absorption (12.4–14.2 eV). A single excitingphoton of 28–33 eV is able to create a primary electron–hole (e–h) pair and a secondaryexciton. The tunnel phosphorescence has been detected after the irradiation of LiF byan electron beam or x-rays at 6 K, and several peaks of thermally stimulatedluminescence (TSL) at 12–170 K appeared at the heating of the sample. It wasconfirmed that the TSL at 130–150 K is related to the diffusion of self-trapped holes(VK centres). TheTSL peak at ∼160 K is ascribed to the thermal ionization ofF′ centres. The TSL at 20–30 K and 50–65 K is caused by the diffusion of interstitialfluorine 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 LiFirradiation by VUV radiation, selectively forming excitons or e–h pairs. Thecreation of a spatially correlated anion exciton and an e–h pair is needed forthe appearance of this peak: an exciton decays into an F–H pair, a hole forms aVK and an electron transformsH into I (an F–I–VK group is formed) or an F centre into a two-electronF′ centre(an F′-H -VK group). The analysis of the elementary components of the 9–16 K TSLshowed that a phonon-induced radiative tunnel recombination ofF′–VK (5.6 eV),F–H (∼3 eV)and F–VK (3.4 eV) occurs within these groups.