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.
Other than water, pure bulk liquids and solids are rarely encountered in the environment, but more commonly exist as layers on various substrates, e.g. concrete, metals, glass, etc. Unlike gas-phase transmission spectra, condensed-phase reflectance spectra depend not only on absorption, but also on the material’s refraction and reflection at interfaces. Providing reference spectra to account for the plethora of morphological conditions (e.g. substrate, layer thickness, particle or droplet size) that may be encountered under different scenarios is a daunting challenge. An alternative approach is to derive the complex optical constants, n and k, which can be used to model the optical phenomena in media and at interfaces, minimizing the need for a vast number of laboratory measurements. The current status of obtaining such optical constants for both solids and liquids is briefly summarized.
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