OPTICAL DETERMINATION OF THE SPATIAL EXTENT OF DEFECTS IN INSULATORS

We have studied the size of common electron-excess defects using the V inti sum rule that relates the mean-square radius of a defect's ground state to its optical absorption spectrum. Here we present the single- electron sum-rule theory and consider its many-electron extension. We show that numerous electron-excess defects in highly ionic systems, in cluding F centers and substitutional s(2) impurity anions, are highly localized, primarily to a single lattice site. Similar defects in alka line-earth oxides and sulfides are more diffuse, presumably because of greater covalent mixing between quantum states of the defect and the host. (C) 1998 Published by Elsevier Science B.V. All rights reserved.