  Motivated by arguments of the nuclear core-layer model formulated in [S.I.
Bastrukov, J.A. Maruhn, Z.Phys. A 335 (1990) 139], the macroscopic excitation
mechanism of electric pygmy dipole resonance (PDR) is considered as owing its
origin to perturbation-induced effective decomposition of nucleus into two
spherical domains - undisturbed inner region treated as static core and
dynamical layer undergoing elastic shear vibrations. The focus is placed on the
imprinted in the core-layer model mechanism of emergence of the low-energy
dipole electric resonant excitation as Goldstone soft mode of translation
layer-against-core oscillations. To accentuate this attitude we regain the
obtained in the above paper spectral equation for the frequency of spheroidal
elastic vibrations trapped in the finite-depth layer, but by working from
canonical equation of elastic dynamics of continuous medium. The obtained
analytic equations for the frequency of dipole vibrational state in question
and its excitation strength exhibit fundamental character of this soft dipole
mode of nuclear resonant response.
