  The 112,120Sn(gamma,gamma') reactions below the neutron separation energies
have been studied at the superconducting Darmstadt electron linear accelerator
S-DALINAC for different endpoint energies of the incident bremsstrahlung
spectrum. Dipole strength distributions are extracted for 112Sn up to 9.5 MeV
and for 120Sn up to 9.1 MeV. A concentration of dipole excitations is observed
between 5 and 8 MeV in both nuclei. Missing strength due to unobserved decays
to excited states is estimated in a statistical model. A fluctuation analysis
is applied to the photon scattering spectra to extract the amount of the
unresolved strength hidden in background due to fragmentation. The strength
distributions are discussed within different model approaches such as the
quasiparticle-phonon model and the relativistic time blocking approximation
allowing for an inclusion of complex configurations beyond the initial
particle-hole states. While a satisfactory description of the fragmentation can
be achieved for sufficently large model spaces, the predicted centroids and
total electric dipole strengths for stable tin isotopes strongly depend on the
assumptions about the underlying mean field.
