Experimental effects on IR reflectance spectra: particle size and morphology

Abstract

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 Na 2 SO 4 as well as ammonium sulfate (NH 4 ) 2 SO 4 ; the optical constants have been measured for (NH 4 ) 2 SO 4 . 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.

Identification of Uranium Minerals in Natural U-Bearing Rocks Using Infrared Reflectance Spectroscopy

“…While this paper does not explicitly focus on particle size effects, one must bear in mind that such effects can strongly influence spectral manifestation and therefore the spectral analysis. 22,46,48…”

“…While this paper does not explicitly focus on particle size effects, one must bear in mind that such effects can strongly influence spectral manifestation and therefore the spectral analysis. 22,46,48 The interferent materials used in this study contain chemical compositions that are commonly found in the natural environment, i.e., in good abundance on the earth's surface. In Fig.…”

“…In the LWIR, the strong oscillator strengths of the fundamentals often result in upward-going reststrahlen bands with adjacent Christiansen minima, and transparency peaks are often manifest at longer wavelengths, but only for the finest particle sizes. 21,22,45,46,48 Conversely, the SWIR/MWIR region, as discussed in the section ''Multivariate Curve Analysis Using Shortwave and Midwave Infrared,'' has been found to exhibit few reststrahlen features, more often displaying a simple broadband increase in reflectance (i.e., increased albedo) across the spectrum as the particles become smaller, with the weaker and moderately strong overtone/combination bands then manifesting themselves as downward-going bands in the reflectance spectra.…”

Radiative transfer modeling of surface chemical deposits

“…As a result, the same chemical can exhibit vastly different spectra, or "signatures," depending on the size of particles composing the deposit. 3,4 There is strong interest in understanding the impact of morphology on such chemical signatures, and in developing reflectance models which can compute signatures displaying behavior consistent with observed dependencies. Here we present such a model, which leverages computational tools developed within the planetary and atmospheric science communities for evaluating the scattering and extinction properties of individual particles based on their size and shape, and the complex refractive index n+ik of the material itself.…”

Synthesis and Application of Nano structured Bi layer YSZ LZ Thermal Barrier Coating

“…The reduction in particle size increases the inter particle boundaries and therefore the number of reflections at the boundaries increases. Thus, reflectivity increases at most of the wavelengths when the particle size decrease 23,24 .…”