Persistent spectral hole burning is observed for the 413 nm 4f(7)(S-8(
7/2)) --> 4f(6)5d(Gamma(8)) zero-phonon transition of Eu2+ in MBE-grow
n superlattices of CaF2:Eu2+/CdF2 on Si(111). Possible mechanisms for
the trapping of the photoionized electron are discussed in terms of th
e band offsets of the superlattice layers.