While it is true that free electrons in a metal do not absorb or emit photons directly, the behavior of light interacting with metals is more complex, and other factors come into play. The phenomenon you describe, where 100% of light is not reflected and some photons are absorbed, can be attributed to several reasons:
Electronic Transitions: While free electrons in metals do not absorb photons directly, when light interacts with a metal, it can cause electronic transitions within the metal's atoms. When light waves interact with the electrons in the metal, the electrons can absorb some of the energy from the photons, leading to electronic excitations. These excited electrons can then transfer their energy to the lattice of the metal through processes like electron-phonon interactions, resulting in energy dissipation and heat generation.
Interactions with Imperfections: Metals often contain imperfections such as impurities, defects, or surface roughness. These imperfections can introduce localized energy states within the metal's electronic structure. When light interacts with these imperfections, the energy of the photons can be absorbed by exciting these localized states, leading to partial absorption of light.
Surface Plasmons: Another important factor is the excitation of surface plasmons. Surface plasmons are collective oscillations of electrons at the surface of a metal when interacting with light. These oscillations can absorb and scatter light energy, leading to a partial loss of photons through absorption processes.
Thin Film Interference: When light interacts with thin films of metals, interference effects can occur. In thin films, such as those found in coatings or multilayer structures, constructive or destructive interference of light waves can result in certain wavelengths being selectively absorbed or transmitted, leading to a reduction in reflected light.
It's important to note that the reflective properties of metals can vary depending on the specific metal, its purity, surface conditions, and the characteristics of the incident light. Therefore, while metals have high reflectivity compared to other materials, some amount of light absorption does occur due to the factors mentioned above.