A high-frequency electron nuclear double resonance study was made of the lowest triplet state of the self-trapped exciton in this halide. Direct information concerning the spatial distribution of the wave function was obtained, which confirmed that the exciton in its triplet state was built up from a strongly localized self-trapped hole and a very diffuse electron. The electron was contained mainly in a H-like 1s orbital with a Bohr radius of 1.51nm but, near to its center, the electron density deviated from spherical symmetry and reflected the D4h charge distribution of the hole. On the basis of a comparison with the results of an electron nuclear double resonance study of the self-trapped hole, it was concluded that the microscopic and electronic structures of a self-trapped hole were not significantly altered by the attraction of a shallow electron. No evidence was found for the formation of Frenkel pairs during ultra-violet irradiation at low temperatures.

M.T.Bennebroek, A.Arnold, O.G.Poluektov, P.G.Baranov, J.Schmidt: Physical Review B, 1996, 53[23], 15607-16