Excitation of the electric pygmy dipole resonance by inelastic electron scattering^⋆

Abstract.

To complete earlier studies of the properties of the electric pygmy dipole resonance (PDR) obtained in various nuclear reactions, the excitation of the 1^- states in ¹⁴⁰Ce by $(e,e')$ scattering for momentum transfers $q = 0.1\mbox{--}1.2$ fm^-1 is calculated within the plane-wave and distorted-wave Born approximations. The excited states of the nucleus are described within the Quasiparticle Random Phase Approximation (QRPA), but also within the Quasiparticle-Phonon Model (QPM) by accounting for the coupling to complex configurations. It is demonstrated that the excitation mechanism of the PDR states in $(e,e')$ reactions is predominantly of transversal nature for scattering angles $\theta_e \approx 90^{\circ}\mbox{--}180^{\circ}$ . Being thus mediated by the convection and spin nuclear currents, the $(e,e')$ like the $(\gamma,\gamma')$ reaction, may provide additional information to the one obtained from Coulomb and hadronic excitations of the PDR in $(p,p')$ , $(\alpha,\alpha')$ , and heavy-ion scattering reactions. The calculations predict that the $(e,e')$ cross sections for the strongest individual PDR states are in general about three orders of magnitude smaller as compared to the one of the lowest 2⁺₁ state for the studied kinematics, but that they may become dominant at extreme backward angles.