  Interactions mediated by electron-phonon coupling are responsible for
important cooperative phenomena in metals such as superconductivity and
charge-density waves. The same interaction mechanisms produce strong collision
rates in the normal phase of correlated metals, causing sizeable reductions of
the dc conductivity and reflectivity. As a consequence, low-energy excitations
like phonons, which are crucial for materials characterization, become visible
in optical infrared spectra. A quantitative assessment of vibrational
resonances requires the evaluation of dynamical Born effective charges, which
quantify the coupling between macroscopic electric fields and lattice
deformations. We show that the Born effective charges of metals crucially
depend on the collision regime of conducting electrons. In particular, we
describe, within a first principles framework, the impact of electron
scattering on the infrared vibrational resonances, from the undamped,
collisionless regime to the overdamped, collision-dominated limit. Our approach
enables the interpretation of vibrational reflectance measurements of both
super and bad conducting metals, as we illustrate for the case of strongly
electron-phonon coupled superhydride H$_3$S.
