Optical Constants of Silver Sulfide Tarnish Films*

The optical constants of rapidly grown silver sulfide films approximately 400 Å thick have been determined in the wavelength range 0.28–2.5 μ using three independent methods. A fourth method was used to obtain values of n and k for thin (40 Å), naturally grown silver sulfide films on opaque silver in the wavelength range 0.34–0.60 μ. Although these results are probably not representative of bulk silver sulfide, they should be approximately correct for tarnish films on silver. Thus, they can be used to calculate the reflectance as a function of wavelength for silver covered with tarnish films of various thicknesses. The measured reflectances of two such films were in reasonable agreement with the calculations, and the reflectance of heavily tarnished silver was essentially the same as that of fresh silver in the infrared. The measured optical constants of silver sulfide can also be used to calculate the thickness of thin tarnish films from ellipsometric measurements, and to calculate the shift in the surface plasma-resonance frequency when silver is covered with a thin tarnish film.