The spherical semimagic ${}^{116,118,120,122,124}\mathrm{Sn}$ nuclei have been investigated intensively using nuclear resonance fluorescence techniques. The measurement of the photon scattering cross sections, angular distributions, and linear polarization or azimuthal asymmetries of the resonantly scattered (unpolarized and polarized) photons enabled a model independent determination of reduced transition probabilities, level spins, and parities. Besides the enhanced dipole excitations to the well-known ${(2\mathrm{}}_1^{+}\otimes 3_1^{-}{)}_{1^{-}}$ two-phonon states several electric quadrupole transitions were detected in the investigated energy region below 4 MeV. Quasiparticle phonon model calculations reveal several collective and noncollective $2^{+}$ states in this energy region. In contrast to the known two-phonon ${(2\mathrm{}}_1^{+}\otimes 3_1^{-}{)}_{1^{-}}$ states, the wave functions of the observed $2^{+}$ states are dominated by one-phonon components. However, the fragmentation of the $B\left(E2\mathrm{}\right)\mathrm{}\uparrow$ strength is influenced by two-phonon $2^{+}$ admixtures.

• Received 11 August 1999

DOI:https://doi.org/10.1103/PhysRevC.61.024309