  The low-lying electric dipole (E1) strength in the semi-magic nucleus 136Xe
has been measured which finalizes the systematic survey to investigate the
so-called pygmy dipole resonance (PDR) in all stable even N=82 isotones with
the method of nuclear resonance fluorescence using real photons in the entrance
channel. In all cases, a fragmented resonance-like structure of E1 strength is
observed in the energy region 5 MeV to 8 MeV. An analysis of the fragmentation
of the strength reveals that the degree of fragmentation decreases towards the
proton-deficient isotones while the total integrated strength increases
indicating a dependence of the total strength on the neutron-to-proton ratio.
The experimental results are compared to microscopic calculations within the
quasi-particle phonon model (QPM). The calculation includes complex
configurations of up to three phonons and is able to reproduce also the
fragmentation of the E1 strength which allows to draw conclusions on the
damping of the PDR. Calculations and experimental data are in good agreement in
the degree of fragmentation and also in the integrated strength if the
sensitivity limit of the experiments is taken into account.
