  The $^{112,120}$Sn$(\gamma,\gamma')$ reactions have been studied at the
S-DALINAC. Electric dipole (E1) strength distributions have been determined
including contributions from unresolved strength extracted by a fluctuation
analysis. Together with available data on $^{116,124}$Sn, an experimental
systematics of the pygmy dipole resonance (PDR) in stable even-mass tin
isotopes is established. The PDR centroid excitation energies and summed
strengths are in reasonable agreement with quasiparticle-phonon model
calculations based on a nonrelativistic description of the mean field but
disagree with relativistic quasiparticle random-phase approximation
predictions.
