  The formation of the electronic structure of small Kr_m clusters (m<150)
embedded inside Ne_N clusters (1200<N<7500) has been investigated with the help
of fluorescence excitation spectroscopy using synchrotron radiation.
Electronically excited states, assigned to excitons at the Ne/Kr interface, 1i
and 1'i were observed. The absorption bands, which are related to the lowest
spin-orbit split atomic Kr 3P1 and 1P1 states, initially appear and shift
towards lower energy when the krypton cluster size m increases. The
characteristic bulk 1t and 1't excitons appear in the spectra, when the cluster
radius exceeds some critical value, R_cl>Delta_1i . Kr clusters comprising up
to 70 atoms do not exhibit bulk absorption bands. We suggest that this is due
to the penetration of the interface excitons into the Kr_m cluster volume,
because of the negative electron affinity of surrounding Ne atoms. From the
energy shift of the interface absorption bands with cluster size an
unexpectedly large penetration depth of delta_1i =7.0+/-0.1 A is estimated,
which can be explained by the interplay between the electron affinities of the
guest and the host cluster.
