  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.
