which have very far reaching effects in term of refraction (the real part of the permittivity decays as 1/w).
In term of resonances, it's important to note that the nuclei themselves are not neutral either. In the IR, the time dependence of the field is slow enough that the nuclei can follow it resonantly. Those fields are much below electronic resonances so the electrons aren't even contributing much beyond making the crystal stable!
https://en.wikipedia.org/wiki/Kramers%E2%80%93Kronig_relatio...
which have very far reaching effects in term of refraction (the real part of the permittivity decays as 1/w).
In term of resonances, it's important to note that the nuclei themselves are not neutral either. In the IR, the time dependence of the field is slow enough that the nuclei can follow it resonantly. Those fields are much below electronic resonances so the electrons aren't even contributing much beyond making the crystal stable!