Infrared Optical Constants from Pressed Pellets of Powders: I. Improved n and k Values of (NH 4 ) 2 SO 4 from Single-Angle Reflectance

Abstract

In combination with other parameters, the real, n( $\stackrel{\sim }{ṽ}$ ), and imaginary, k( $\stackrel{\sim }{ṽ}$ ), components of the complex refractive index, $\hat{n}$  = n + ik, can be used to simulate the optical properties of a material in different forms, e.g., its infrared spectra. Ultimately, such n/k values can be used to generate a database of synthetic reflectance spectra for the different morphologies to which experimental data can be compared. But obtaining reliable values of the optical constants n/k for solid materials is challenging due to the lack of optical quality specimens, usually crystals, large enough to measure. An alternative to crystals is to press the powder into a uniform disk. We have produced pellets from ammonium sulfate, (NH₄)₂SO₄, powder and derived the pellets' n and k values via single-angle reflectance using a specular reflectance device in combination with a Fourier transform infrared spectrometer. The single-angle technique measures amplitude of light reflected from the material as a function of wavelength over a wide spectral domain; the optical constants are determined from the reflectance data using the Kramers–Kronig relationship. We investigate several parameters associated with the pellets and pellet formation and their effects upon delivering the most reliable n/k values. Parameters studied include pellet diameter, mass, and density (void space), drying, grinding, sieving, and particle size in the pellet formation, as well as pressing pressure and duration. Of these parameters, using size-selected mixtures of dried, small (<50 µm) particles and pressing at ≥10 tons for at least 30 min were found key to forming highly reflective samples. Comparison of two sets of previous literature n( $\stackrel{\sim }{ṽ}$ ) and k( $\stackrel{\sim }{ṽ}$ ) values obtained from crystalline (NH₄)₂SO₄ both as crystal reflectance as well as extinction spectra of aerosols measured in a flow tube shows reasonable agreement, but suggests the present values, as confirmed from two independent techniques, represent a substantial improvement for n/k values for (NH₄)₂SO₄, also demonstrating promise to measure the optical constants of other materials.