Optical Constants of Germanium in the Extreme Ultraviolet Region

The optical constants n and k of a germanium single crystal in the extreme ultraviolet region, from 10.2 to 23.4 eV, have been determined by the reflectance measurements. Energies of the plasma oscillation estimated from the real part of the dielectric constant, and the imaginary part of the reciprocal dielectric constant are 14.7 and 16.4 eV, respectively, suggesting that the characteristic energy loss of fast electrons observed at 16.4 eV is just caused by the collective excitation of electrons as expected from the free-electron approximation. The tangential plasmon energy is also estimated at about 13 eV, corresponding to the value of subsidiary electronic loss observed by Powell at 11 eV.