Low-energy excitonic resonances in metals. I. Experiments on divalent atoms

We report differential ellipsometric measurement of optical constants for impurities in dilute alloys. For Mg, Zn, and Ca impurities in Li metal an impurity-specific absorption feature occurs in the (2–3)-eV range, superposed on an impurity-induced Drude background. The peaks are the ${\mathrm{ns}}^2$→nsnp excitations of the free divalent atoms, red-shifted by metallic screening. These are identified as low-energy excitonic resonances in which the photoexcited electron and hole linger together in the impurity cell under their mutual interaction. Single-particle approximations to the optical properties cannot reproduce these explicit two-carrier effects. The three species exhibit different behaviors with increasing composition. For Mg impurities, the resonance can be followed across the entire Li-Mg phase diagram, where in pure Mg metal it becomes the ‘‘interband transition.’’ We infer that an excitonic resonance enhances the interband process. The following paper explores the explicit consequences of the electron-hole interaction, using a many-electron approach to small clusters, which appears to confirm the experimental deductions.