Spectroscopy of even Sn nuclei and generalized-seniority breaking

Properties of even Sn nuclei are described in a broken-pair or generalized-seniority (vsub(g)) scheme. Special attention is paid to the degree of vsub(g) mixing in various types of low-lying states. A finite-range interaction as well as a surface-delta interaction (SDI) are employed and single-particle energies are deduced from spectra of odd Sn isotopes. It appears that up to 3 MeV no experimental indications for states with vsub(g) > 4 exist. The only low-lying states which are not included in the model are the members of the well-known two-proton-hole band. With the SDI the vsub(g) mixing is in general a factor two less than with the finite-range force. For the latter an improvement of the description of energy spectra as well as electromagnetic decay is obtained due to about 20% vsub(g) = 4 admixtures in predominantly vsub(g) = 2 states. Only ground states, 2+1 and 3-1 states have less than 10% of vsub(g) = 4 admixtures. We argue that the main origin of vsub(g) mixing is a particle-phonon coupling mechanism. A strong fragmentation of two-phonon 0+, 2+, 4+ states by vsub(g) mixing emerges from the calculations. For the 0+ states the total two-phonon E2 transition strength is much less than predicted by phenomenological phonon or boson models. Excitation strengths for unnatural-parity states are reduced by 30-40% by vsub(g) mixing. For natural-parity states this reduction is less; for 2+1 and 3-1 enhancements by ground-state correlations overcompensate the reduction by fragmentation. (orig.)