Accurate optical constants for liquids in the near-infrared: Improved methods for obtaining n and k from 1 to 4 μm

摘要

Other than open bodies of water, bulk liquids are rarely encountered in the environment. Rather, liquids are typicallyfound as aerosols, liquid droplets, or liquid layers on sundry substrates: glass, concrete, metals, etc. The layers can be ofvarying thicknesses, from micron-level to millimeter thick deposits. The infrared (IR) reflectance spectra of suchdeposits vary greatly, approximating the bulk reflectance for thicker deposits, but for thinner layers on reflectivesurfaces, producing “transflectance” spectra that more closely replicate simple transmission of the IR light twicetraversing the absorptive medium. Rather than recording large numbers of such spectra to serve as endmembers of aspectral reflectance library, we have recognized that the spectra can be modeled so long as the complex optical constantsn(ν) and k(ν) are known as a function of frequency. 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 to13 μm) can become saturated, and better signal-to-noise and specificity can be realized at shorter wavelengths. In earlierstudies, we obtained the n/k values from ~1.3 to 25 μm for a series of liquids, but are now expanding thosemeasurements to include additional liquid species and extending the spectral range to lower wavelengths. In this paperwe describe the methodologies for compiling and fusing the two data sets collected to provide better and more completespectral coverage from 1.0 to 25 μm (10,000 to 400 cm-1). The broad spectral range means that one needs to account forboth strong and weak spectral features, all of which can be useful for detection, depending on the scenario. To accountfor the larger dynamic range, both long and short path length transmission cells are required for accurate measurements.