  Nuclear collective motion provides valuable constraint on the size of
neutron-skin thickness and the properties of nuclear matter symmetry energy. By
employing relativistic nuclear energy density functional (RNEDF) and covariance
analysis related to $\chi^2$ fitting of the model parameters, relevant
observables are identified for dipole excitations, which strongly correlate
with the neutron-skin thickness $(r_{np})$, symmetry energy at saturation
density $(J)$ and slope of the symmetry energy $(L)$. Using the RNEDF framework
and experimental data on pygmy dipole strength ($^{68}$Ni, $^{132}$Sn,
$^{208}$Pb) and dipole polarizability ($^{208}$Pb), it is shown how the values
of $J$, and $L$, and $r_{np}$ are constrained. The isotopic dependence of
moments associated to dipole excitations in $^{116-136}$Sn shows that the
low-energy dipole strength and polarizability in neutron-rich nuclei display
strong sensitivity to the symmetry energy parameter $J$, more pronounced than
in isotopes with moderate neutron-to-proton number ratios.
