Optical Properties of Potassium for Photons of Energy 396 to 969 eV*

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Abstract
The refractive index of potassium has been measured by the critical-angle method for photon energies from 3.96 to 9.69 eV. The potassium films were deposited on the flat surface of a quartz or calcium fluoride semicylinder so that, viewed through the semicylinder, the potassium surface in contact with the substrate is uncontaminated by oxidation. The geometry used facilitated the calculation of the refractive index from the observed reflectance as a function of angle of incidence. These data have been interpreted in terms of a nearly-free-electron model with a volume plasma energy of 3.72±0.05, an optical effective mass of 1.06±0.03, and an ion-core polarization of 0.20±0.03.