Photostimulated Transformations of Complex Colour Centres in Lithium Hydride Single Crystals

Optical absorption, luminescence, and EPR are applied to study the nature of complex aggregate colour centres which arise in nonstoichiometric lithium hydride single crystals. Electron-anionic disorder induced by the lack of hydrogen is found to create F-centres which interact with the lattice through electron-phonon coupling. Photostimulated transformation of F-centres into F₃-centres is studied. The vibronic structure of the F-centres absorption spectra is due to the interaction of the electronic transition with acoustic phonons TA(X), TA(L), LA(X), LA(X) + TA(X) and to the excitement of a quasilocal mode; the structure of F₃-centres spectra is due to the interaction of the electronic transition with acoustic TA(X), TA(L), LA(X), LA(X) + TA(W), and optical TO(L) phonons. The considerable deviation from the stoichiometry is shown to give rise to the formation of metallic lithium colloids.