Absorption of Light Due to Interband Transitions in Sodium

The intrinsic absorbing power due to interband transitions in Na is calculated, using orthogonalized plane waves to evaluate the matrix elements appearing in the oscillator strengths, and a realistic band structure, obtained from a pseudopotential interpolation scheme, to perform the sum over $k$ space. It is found that for a pseudopotential form factor $V_{110}$ in reasonable agreement with the results of Fermi-surface measurements and band calculations, the calculated absorbing power is an order of magnitude smaller than that indicated by the experimental data of Mayer and co-workers, or by the data of Ives and Briggs.