Reflectance Spectra of Real Metals

The optical constants derived for a free electron conductive media do not tell the whole story, however, as will become evident when we examine the reflection spectra of real metab. 

Observed values of (a) N and (b) K for vapor deposited Al under UHV conditions (solid lines) compared with theoretical free electron calculations (dashed lines). (N and K values taken from Smith, D.Y., Shiles, E. and Inokuti, M. in Palik, E.D., Ed., Handbook cf Optical Constants of Solids, Vols. 1-3, Academic Press, 1997.) 

Despite the small loss in reflectance in the visible region, pure Al is the best reflector for the far UV because its high electron density (three valence electrons per atom) extends its plasma frequency to shorter wavelengths and because there are no additional interband transitions in this region. Ag maintains its high reflectance throughout the visible spectrum, but imdergoes an interband transition at 4.0 eV (310 nm), which kills its UV reflectance. 

However, we virtually never see a pure Al surface. Al has a high affinity for oxygen and an oxide layer will quickly form when exposed to any O2 that may be in the atmosphere. 

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