Two-electron F' centers in alkali halides: A negative-U analysis
  The existence of bound excited states of two-electron centers loosely trapped in an anion vacancy, the F' centers, is a long-standing problem of color center physics. Optical absorption bands attributed to F' centers in NaI and NaBr as observed by Baldacchini et al. seem to confirm the existence of F' bound snglet states. Nevertheless, a paper by Zhang et al. published not long ago reports extended ion calculations predicting bound excited states for F' centers in NaI alone while the evidence for NaBr is only marginal. For the first time we apply the negative-U mechanism in order to see whether bound excited F' states do not arise by virtue of the lattice polarization produced by the two trapped charge carriers. The result is positive for NaI and less so for NaBr, in agreement with Zhang et al. Other hosts, such as CsI and RbI also show a tendency towards sustaining F' bound excited states. However, the lack of entry data for LiI makes it presently too hard to obtain a complete negative-U picture for the iodides. 
Mladen Georgiev (Institute of Solid State Physics, Bulgarian Academy   of Sciences, Sofia, Bulgaria)