Fine Structure of the Pygmy Dipole Resonance inXe136

The photoresponse of the semimagic $N=82$ nucleus ${}^{136}\mathrm{Xe}$ was measured up to the neutron separation energy $S_n$ using the $(\gamma ,{\gamma }^{\prime })$ reaction. A concentration of strong dipole excitations is observed well below $S_n$ showing a fragmented resonancelike structure. Microscopic calculations in the quasiparticle phonon model including complex configurations of up to three phonons agree well with the experimental data in the total integrated strength, in the shape and the fragmentation of the resonance, which allows us to draw conclusions on the damping mechanism of the pygmy dipole resonance.