Shallow electron centers were studied by means of pulsed electron nuclear double resonance at 95GHz. Information concerning the geometrical structure and the ground-state wave function was obtained, and this confirmed that these centers consisted of an electron that was loosely bound to a Coulombic trap. The microscopic models which were proposed for these Coulombic traps differed from those which had been suggested in previous optical studies. It appeared that the optical transition energies could be reasonably well accounted for by using simple H-like and quantum-defect methods. Due to the similarity between shallowly trapped and conduction electrons, information concerning the lowest state of the conduction band could be deduced. This revealed that this state was not principally Ag s-like (as had frequently been assumed), but also included an appreciable degree of Cl s-character.

M.T.Bennebroek, A.Arnold, O.G.Poluektov, P.G.Baranov, J.Schmidt: Physical Review B, 1996, 54[16], 11276-89