Abstract
The influence of the quadrupole shape fluctuations on the dipole vibrations in transitional nuclei is investigated in the framework of the instantaneous shape sampling model, which combines the interacting boson model for the slow collective quadrupole motion with the random phase approximation for the rapid dipole vibrations. Coupling to the complex background configurations is taken into account by folding the results with a Lorentzian with an energy-dependent width. The low-energy portion of the $\ensuremath{\gamma}$-absorption cross section, which is important for photonuclear processes, is studied for the isotopic series of Kr, Xe, Ba, and Sm. The experimental cross sections are well reproduced. The low-energy cross section is determined by the Landau fragmentation of the dipole strength and its redistribution caused by the shape fluctuations. Collisional damping only wipes out fluctuations of the absorption cross section, generating the smooth energy dependence observed in experiment. In the case of semimagic nuclei, shallow pygmy resonances are found, in agreement with experiment.
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