  The rate of photon absorption in warm dense matter (WDM) induced by free-free
electron-ion collisions is derived from Sommerfeld's cross-section for
non-relativistic bremsstrahlung emission, making use of detailed balance
relations. Warm dense matter is treated as a metal-like state in the
approximation of a uniform degenerate electron gas and a uniform ion
background. Total absorption rates are averaged over the electron Fermi
distribution. A closed expression is obtained for the absorption rate,
depending on temperature, density, and photon energy, that scales with ion
charge Z. It is evaluated numerically for the full parameter space, which
requires different representations of the hypergeometric functions involved.
The results are valid for photon frequencies larger than the plasma frequency
of the medium. They are compared with approximate formulas in various
asymptotic regions.
