The reflectance spectra of solid and liquids can be complicated since they depend not only on absorption, but also on the refraction, reflection, and scattering of light, all of which are wavelength dependent. The physical form and morphological effects associated with solid and liquid samples are thus known to affect their reflectance spectra in a non-linear fashion, particularly in the infrared. Measuring the optical constants n(ν) and k(ν) represents an alternative approach, allowing one to model these many effects and thus requiring fewer laboratory measurements. In this paper an overview is presented of the protocols used to measure the n/k optical constants, particularly for liquids. For the liquids, a multiple path length measurements approach is employed, and in this paper we demonstrate the method to determine the complex optical constants n(ν) and k(ν) of squalene. The resultant calculated spectra of 1 μm and 100 μm thick layers of squalene on an aluminum substrate as derived from the experimental n(ν) and k(ν) values are shown to demonstrate such effects. The public availability of the n(ν)/k(ν) data as well as solids hemispherical reflectance data are also discussed.
Other than open bodies of water, bulk liquids are rarely encountered in the environment. Rather, liquids are typically found as aerosols, liquid droplets, or liquid layers on sundry substrates: glass, concrete, metals, etc. The layers can be of varying thicknesses, from micron-level to millimeter thick deposits. The infrared (IR) reflectance spectra of such deposits vary greatly, approximating the bulk reflectance for thicker deposits and for thinner layers on reflective surfaces, producing “transflectance” spectra that more closely replicate simple transmission of the IR light twice traversing the absorptive medium. Rather than recording large numbers of such spectra to serve as endmembers of a spectral reflectance library, we have recognized that the spectra can be modeled so long as the complex optical constants n(ν) and k(ν) are known as a function of frequency, ν. Here n is the real (dispersion) part and k is the imaginary (absorption) component of the complex index of refraction. However, in many cases the bands in the longwave IR (7 to 13 μm) can become saturated, and better signal-to-noise and specificity can be realized at shorter wavelengths. In earlier studies, we obtained the n/k values from 1.28 to 25 μm for a series of liquids, but are now expanding those measurements to include additional liquid species and extending the spectral range to lower wavelengths. In this paper we describe the methodologies for compiling and fusing the two data sets collected to provide better and more complete spectral coverage from 1 to 25 μm (10,000 to 400 cm-1 ). The broad spectral range means that one needs to account for both strong and weak spectral features, all of which can be useful for detection, depending on the scenario. To account for the large dynamic range, both long and short path length transmission cells are required for accurate measurements.
Reflectance (emittance) spectroscopy, especially at infrared wavelengths, continues to grow in utility as an analytical technique for contact, standoff, and remote sensing. The reflectance spectra of solids, however, are complex, depending on many parameters, even for the same material. Granule or powder particle size, crystal morphology, layer thickness, and substrate material all affect the spectral distribution of reflected light. However, such phenomena can all be modeled if the optical constants n(ν) and k(ν) are available. If the quantitative absorption coefficient K(ν) is known, the k value can be obtained via the relation k(ν) = 2.303K(ν)/4πν. The absorption coefficient can in turn be derived from a simple KBrpellet infrared absorption measurement, provided the pellet mass ratio is prepared quantitatively. The method requires the pellet’s mass and diameter, along with the analyte mass fraction and density. In this paper we demonstrate the requisite experimental details in preparing the pellets, as well as methods to reduce light scattering in order to obtain more quantitative values. Theoretical methods to derive the related optical constants will also be detailed, in particular the assumptions used to obtain the scalar refractive index n. Ideally, this value is known or measured separately, but in some cases we have found that it can be approximated (first approximation) for most organic chemicals by n~1.5 at the shortest wavelength. The results are presented for a couple of species.
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