The basal-plane reflectance of single crystals of high-purity (99.999%) Zn, cleaved and maintained in a vacuum chamber at pressures less than ${10}^{-9\mathrm{}}$ Torr, has been measured in the wavelength region $1150\le \lambda \le 5600$ Å. All measurements were made at 96°K. The shape of the reflectance curve for the ultraviolet region is quite accurately calculated on the basis of a free-electron model using the relaxation time, ${\tau }_c=3.2\mathrm{}\times {10}^{-14\mathrm{}}$ sec, calculated from the dc electrical conductivity at 96°K, and an optical mass $m_a=2.04\mathrm{}$. The magnitude of the reflectance, about 85% just below the plasma frequency, is less than that calculated for the free-electron model. The difference is presumably the effect of interband transitions. The plasma resonance occurs at 10.1 eV. The experiments clearly demonstrate the necessity of an atomically clean surface for meaningful reflectance measurements in the vacuum ultraviolet.

• Received 25 January 1968

DOI:https://doi.org/10.1103/PhysRev.171.743