The optical constants of molybdenum, tantalum, tungsten, and rhenium have been measured at wavelengths from 537 through 5770 Å by the reflectance vs angle of incidence method. The spectra for the two parts of the complex dielectric constant exhibit structural similarities among the four metals, consisting of three broad, partially overlapping absorption bands situated in the approximate photon-energy regions between 13 and 26 eV, between 7 and 15 eV, and below 10 eV. The latter band generally has one or more narrower absorption resonances superimposed on it. The observed structure bears a resemblance to the level separations calculated by Mattheiss for tungsten and rhenium, but correlation with specific transitions has not been pursued in detail. The f-sum rule applied to the imaginary part of the dielectric constant yields appropriate values of 5 and 7 electrons/atom for tantalum and rhenium. For molybdenum and tungsten the corresponding values are near 8, rather than the anticipated 6, electrons/atom.
D. W. JUENKER, L. J. LEBLANC, and C. R. MARTIN, "Optical Properties of Some Transition Metals," J. Opt. Soc. Am. 58, 164-171 (1968)