PNNL

Abstract

Abstract. Since solids are only sometimes seen en masse in a pure bulk form, and for liquids other than water almost never, a capability to model reflectance spectra from analytes deposited on various substrates would be highly advantageous. If available, the real, n(?), and imaginary, k(?), components of the complex refractive index, , can be used to simulate infrared spectra, accounting for reflection, refraction and absorption phenomena as a function of wavelength. In this paper we focus on using the PNNL derived n/k vectors for solid and liquid analytes deposited as thin layers on different types of substrates including conductors such as aluminum, and inorganic dielectrics such as glass. The model is an adaptation of the Monte Carlo ray trace modeling program, TracePro, extended through use of its macro language. The model is tested using thin films of organic liquids including silicone oil and No. 2 diesel fuel, as well as organic solids such as caffeine and acetaminophen on aluminum and glass. The predicted spectra for the solid films were compared to experimental hemispherical reflectance data measured using a Fourier transform spectrometer with an integrating sphere. The thickness of the calculated layer is a parameter for predicting the (transflectance) spectra and is obtained using the areal density measured from gravimetric methods to generate the thin-layer samples. Comparison of the calculated spectra with experimental hemispherical reflectance data shows excellent agreement, indicating promise for the use of measured n/k data to synthesize reference spectral data. In particular, theoretical modeling shows that for thicker layers (ca. 20 to 100 µm) of typical organics possessing moderately strong k values, the longwave infrared features are often saturated and better spectral contrast is obtained from the overtone/combination bands in the shortwave IR. The research was based upon work supported in part by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA), via DOE DE-AC05-76RL01830. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the ODNI, IARPA, DRDC, or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for governmental purposes notwithstanding any copyright annotation thereon. Pacific Northwest National Laboratory is operated for the Department of Energy by Battelle Memorial Institute under Contract No. DE-AC05-76RL01830. PNNL wishes to acknowledge scientists from John Hopkins University – Applied Physics Laboratory, including Drs. Leslie Hamilton and Natasha Zimmerman for providing the air-brushed solid samples used for model validation.