Mid-infrared reflectance spectra (23–22 μm) of sulfur, gold, KBr, MgO, and halon

Abstract

Biconical diffuse reflectance spectra in the mid-infrared are presented for powder and other solid forms of sulfur, gold, potassium bromide, magnesium oxide, and Halon. Comparisons are made with previously published results of other investigators, and recommendations are made regarding the relative usefulness of these materials as reflectance standards in the mid-IR. Sulfur has strong intrinsic bands at wavelengths >7 μm that must be taken into account for its use as a reflectance standard. Some sulfur samples have hydrocarbon contaminants and in powder form may have adsorbed water, both of which produce bands in the 3–4-μm region. Potassium bromide has several weak intrinsic bands and is very sensitive to adsorbed water contamination; otherwise it is a good IR reference material. Magnesium oxide and Halon have major band structure and low reflectivity at wavelengths >2.6 μm and thus are unsuitable as reference materials in the mid-IR. Vapor-deposited gold on fine sandpaper (600 grit) is very bright, spectrally flat, and fairly diffuse, so it is the superior material (of those examined) for reflectance reference material throughout the IR. Fine gold powder, on the other hand, is much less bright than evaporated gold, and its reflectivity at wavelengths greater than its particle size is highly sensitive to particle packing density.