Experimental effects on IR reflectance spectra: Particle size and morphology

For geologic and extraterrestrial samples it is known that both particle size and morphology can have strong effects on a species' infrared reflectance spectra. Due to such effects, the reflectance spectra cannot be predicted from the absorption coefficients alone. This is because reflectance is both a surface as well as a bulk phenomenon, incorporating both dispersion as well as absorption effects. The same spectral feature can even be observed as either a maximum or minimum. The complex effects depend on particle size and preparation, as well as the relative amplitudes of the optical constants n and k, i.e. the real and imaginary components of the complex refractive index. While somewhat oversimplified, upward-going amplitude in the reflectance spectrum usually results from surface scattering, i.e. rays that have been reflected from the surface without penetration, whereas downward-going peaks are due to either absorption or volume scattering, i.e. rays that have penetrated or refracted into the sample interior and are not reflected. While the effects are known, we report seminal measurements of reflectance along with quantified particle size of the samples, the sizing obtained from optical microscopy measurements. The size measurements are correlated with the reflectance spectra in the 1.3 - 16 micron range for various bulk materials that have a combination of strong and weak absorption bands in order to understand the effects on the spectral features as a function of the mean grain size. We report results for both anhydrous sodium sulfate Na₂SO₄ as well as ammonium sulfate (NH₄)₂SO₄; the optical constants have been measured for (NH₄)₂SO₄. To go a step further from the laboratory and into the field we explore our understanding of particle size effects on reflectance spectra using standoff detection at distances of up to 160 meters in a field experiment. The studies have shown that particle size has a strong influence on the measured reflectance spectra of such materials; successful identification requires sufficient, representative reflectance data to include the particle sizes of interest.