  By means of first-principles methods we analyze the optical response of
transparent dense sodium as a function of applied pressure. We discover an
unusual kind of charge-transfer exciton that proceeds from the interstitial
distribution of valence electrons repelled away from the ionic cores by the
Coulomb interaction and the Pauli repulsion. The predicted absorption spectrum
shows a strong anisotropy with light polarization that just at pressures above
the metal-insulator transition manifests as sodium being optically transparent
in one direction but reflective in the other. This result provides a key
information about the crystal structure of transparent sodium, a new
unconventional inorganic electride.
