Uniform properties ofJπ=1−two-phonon states in the semimagic even-even tin isotopes116,118,120,122,124Sn

Systematic nuclear resonance fluorescence experiments including model-independent parity determinations provided clear evidence for strong, isolated $E1\mathrm{}$ excitations at excitation energies of 3.3–3.5 MeV in the spherical, semimagic nuclei ${}^{116,118,120,122,124}\mathrm{Sn}.$ The corresponding $J^{\pi }{=1\mathrm{}}^{-}$ states are interpreted as two-phonon excitations ${(2\mathrm{}}^{+}\otimes 3^{-}).\mathrm{}$ The excitation energies $E_x{(1\mathrm{}}^{-})$ and reduced excitation probabilities $B\left(E1\mathrm{}\right)\mathrm{}\uparrow$ are nearly constant in the entire isotopic chain $A=116\mathrm{}–124.$ The results are described in the framework of quasiparticle-phonon-model calculations.