Plasmon Contribution to the Ultraviolet Absorbing Power of the Alkali Metals

The absorbing power due to a photon exciting both a plasmon and an intraband transition in a semi-infinite electron gas of metallic density has been found to be of the order of ${10}^{13}$ ${\sec }^{-1\mathrm{}}$ in the ultraviolet at approximately $\hslash {\omega }_p+{\epsilon }_F$. The plasmon absorption appears to give only a small contribution to the ultraviolet absorption spectrum of most metals, but for the alkali metals it is of the same order of magnitude as the previously calculated interband absorbing power. A linear superposition of the Drude, interband, and plasmon conductivities suggests that sodium and potassium may exhibit small broad peaks in their absorption spectra due to plasmon excitations near 8.8 and 5.8 eV, respectively. The calculated plasmon absorption for other free-electron-like metals in the ultraviolet and for an $n$-doped semiconductor in the infrared is briefly discussed.