  The electromagnetic dipole strength below the neutron-separation energy has
been studied for the xenon isotopes with mass numbers A = 124, 128, 132, and
134 in nuclear resonance fluorescence experiments using the ELBE bremsstrahlung
facility at Helmholtz-Zentrum Dresden-Rossendorf and the HIgS facility at
Triangle Universities Nuclear Laboratory Durham. The systematic study gained
new information about the influence of the neutron excess as well as of nuclear
deformation on the strength in the region of the pygmy dipole resonance. The
results are compared with those obtained for the chain of molybdenum isotopes
and with predictions of a random-phase approximation in a deformed basis. It
turned out that the effect of nuclear deformation plays a minor role compared
with the one caused by neutron excess. A global parametrization of the strength
in terms of neutron and proton numbers allowed us to derive a formula capable
of predicting the summed E1 strengths in the pygmy region for a wide mass range
of nuclides.
