Enhanced electric dipole strength below particle-threshold as a consequence of nuclear deformation

Photoabsorption cross sections σ_γ up to the neutron-separation energy Sₙ were measured for the stable even-mass isotopes ^92-100Mo in photon-scattering experiments. The photon-scattering data were analyzed in a novel way by taking into account the intensity of unresolved levels at high excitation energy and high level density. Simulations of γ-ray cascades were performed to estimate the intensity distribution of inelastic transitions to low-lying levels and, hence, to deduce intensities and branching ratios of the ground-state transitions needed for the determination of σ_γ. The present (γ,γ') data can be combined for the first time with (γ,n) data, which allows us to obtain σ_γ in the energy range from about 4 MeV up to the giant dipole resonance for a series of isotopes. The σ_γ values below Sₙ increase with the number of neutrons above the neutron shell closure at $N=50$. Calculations using a quasiparticle random-phase approximation in a deformed Woods-Saxon potential describe this effect as a consequence of the increasing nuclear deformation.