Optical Properties of Some Metallic Sulfides

Optical constants of molybdenite (MoS₂) and stibnite (Sb₂{S}₃$) in the visible region, 420 m{μ} to 740 m{μ}.---The reflectivity of a cleavage surface at 20^∘{} incidence was found to range from 0.35 to 0.60 (Figs. 2 and 3). At 45^∘{} incidence, the values are the same for MoS₂, but slightly less for Sb₂{S}₃. When cooled to about --- 160^∘ C., Mo{S}₂$ shows the selective bands enhanced and shifted toward the violet (Fig. 6), while ${Sb}₂S₃ showed no change. Transmission of two thin specimens of MoS₂, 6.3 and 9.6 {μ} thick, increases rapidly as the infra-red is approached (Fig. 7). From these measurements the extinction modulus nκ is computed. Cooling by liquid air was found to shift the transmission curve about 40 m{μ} toward the violet. Indices of refraction and of extinction were determined by allowing plane polarized light of azimuth 135^∘{} to fall on the crystal surface and analyzing the elliptically polarized light reflected. The method and apparatus are described in detail. For MoS₂, n and κ were obtained for light in the direction of the optic axis (Fig. 12). Anomalous dispersion is shown, n having a maximum value at 620 m{μ} (5.3) and minima at 595 and 655 m{μ}. The extinction index lies between 0.2 and 0.4, with two maxima indicating selective absorption bands at 603 and 665 m{μ}. For Sb₂{S}₃$, two principal indices were obtained for light incident normal to the cleavage plane. Both refractive indices reach maximum values at 510 m{μ} (5.5 and 4.8), while both extinction indices increase steadily with decreasing wave-length. Reflectivities computed from these values of n and κ agree in general with those observed directly, but are somewhat higher throughout the middle of the spectrum. In the case of MoS₂ no variation of the optical properties with the age of the surface was observed, but with Sb₂{S}₃$, fresh cleavage surfaces were used as the surface soon changed after exposure to the air. The metallic luster of these sulfides is due to the high refraction, the absorption being considerably lower than for metals.Reflectivity of cleavage surfaces of pyrite (FeS₂) and of galena (PbS) in the visible region, 420 m{μ} to 740 m{μ}, was measured for an incidence of 20 ^∘{}(Figs. 4 and 5). For 45^∘{} incidence, pyrite gives slightly lower values. Cooling with liquid air did not change the results for pyrite; galena was not tried.