Quantitative reflectance spectra of solid powders as a function of particle size

Abstract

We have recently developed vetted methods for obtaining quantitative infrared directional-hemispherical reflectance spectra using a commercial integrating sphere. In this paper, the effects of particle size on the spectral properties are analyzed for several samples such as ammonium sulfate, calcium carbonate, and sodium sulfate as well as one organic compound, lactose. We prepared multiple size fractions for each sample and confirmed the mean sizes using optical microscopy. Most species displayed a wide range of spectral behavior depending on the mean particle size. General trends of reflectance versus particle size are observed such as increased albedo for smaller particles: for most wavelengths, the reflectivity drops with increased size, sometimes displaying a factor of 4 or more drop in reflectivity along with a loss of spectral contrast. In the longwave infrared, several species with symmetric anions or cations exhibited reststrahlen features whose amplitude was nearly invariant with particle size, at least for intermediate and large size sample fractions: that is, ≳150 μm. Trends of other types of bands (Christiansen minima, transparency features) are also investigated as well as quantitative analysis of the observed relationship between reflectance versus particle diameter.

Synthesis, electronic transport and optical properties of Si:α-Fe2O3 single crystals

“…A Tensor 37 FT-IR spectrometer (Bruker Optics) equipped with an A562 integrating sphere at Pacific Northwest National Laboratory was used to measure quantitative reflectance from 1.3 to 16.6 mm. 25,26 All optical measurements were performed at room temperature. Later in this article we compare the optical properties of our Si-doped hematite to those of Ti-doped hematite (Section 3.4).…”

Dehydration of uranyl nitrate hexahydrate to uranyl nitrate trihydrate under ambient conditions as observed via dynamic infrared reflectance spectroscopy

“…On the wavelength axis, even the peak positions observed in reflectance cannot necessarily be directly compared to the positions of absorption and reflectance peak positions as reflectance bands involve both the real and imaginary component of the refractive index, whereas for absorption bands it is only the imaginary component that gives rise to the signal. 15,17,25,26 In general, in the visible and shortwave infrared surface scattering tends to dominate reflectance spectra of Lambertian samples which show high %R values for small particles when there are no or few absorption bands present. When there are weak to medium absorption bands (i.e.…”

“…with small but non-zero k values), they usually manifest themselves as downward-going peaks due to light extinction. [15][16][17]26 Especially in the longwave infrared, however, there can be exceptions to this general trend due to certain phenomena known as reststrahlen bands: These are characterized by large values for both the optical constants n and k, and typically result in surface scattering of the light, which manifests as upward-going peaks. 15,26 For many different types of powders in which volume scattering dominates, such reflectance spectra tend to be strongly particle-size dependent; smaller particle sizes result in higher albedo, i.e., higher overall reflectivity.…”

“…The sample or a reference material sits at the bottom port; a mirror on a lever arm can be used to direct the incoming horizontal light beam from the FTIR either down towards this sample position, or upwards to a beam exit port, or to a position on the sphere's front wall, just below the light entrance port. [13][14][15] As seen at left in Figure 1, with the mirror pointing down towards the sample port, the angle between the incident light on the sample surface and the surface normal is 14.8°. Via use of the upper specular exclusion port with removable dome cap, two sets of spectra can be recorded for each sample: either the diffuse-only reflectance spectra (cap removed) or the total (also called hemispherical) reflectance spectra.…”

“…The total reflectance measurement captures both the diffuse and specular components that are scattered into the 2π steradians of a hemisphere. [13][14][15][16][17][18] For this paper all plots represent the total hemispherical spectra. The experiment places a few grams of solid powder into an open 1.9-cm diameter sample cup at the bottom of the sphere.…”