Methods for Determining the Infrared Complex Refractive Indices n (λ) and k (λ) from Organic Solid Powders: Comparison of a Sucrose Single Crystal and a Pressed Pellet

Yokosuk, Michael O.; Saunders, Danielle L.; Resch, Charles Tom; Erickson, Jeremy D.; Lonergan, Charmayne E.; Burton, Sarah D.; Bowden, Mark E.; Peterson, Kelly A.; Johnson, Timothy J.; Myers, Tanya L.

doi:10.1021/acsaom.2c00004

DOE OSTI · 2574411

Methods for Determining the Infrared Complex Refractive Indices n (λ) and k (λ) from Organic Solid Powders: Comparison of a Sucrose Single Crystal and a Pressed Pellet

Yokosuk, Michael O.·Saunders, Danielle L.·Resch, Charles Tom·Erickson, Jeremy D.·Lonergan, Charmayne E.·Burton, Sarah D.·Bowden, Mark E.·Peterson, Kelly A.·Johnson, Timothy J.·Myers, Tanya L.

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Abstract

There exists a need to measure optical constants n and k for many materials, especially organic solids in the infrared spectral region, but at present, there are only two established methods for making such measurements: single-angle (SA) reflectance and infrared spectroscopic ellipsometry (IRSE). The former derives the n/k vectors via measurement of the change in the amplitude of light reflected from the sample and then uses the Kramers–Kronig transform, while the latter measures the change in polarization of the reflected light from the sample to derive n/k. Here we make a direct comparison between the n/k vectors derived from SA measurements of a sucrose single crystal and the n/k values obtained after the crystal had been crushed, ground to powder, and pressed into a pellet form. The pellet was measured not only by SA but also via the IRSE method, and we found that pressing specular-quality pellets was critical, especially for the SA data. The n/k values of this typical organic molecule all agree to within 0.04 root-mean-square error, confirming that a pressed pellet can be used instead of a crystal to derive the intrinsic n/k vectors. In conclusion, this result is important for the many organic materials that, unlike sugar, do not have centimeter-sized crystals readily available.

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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY Carbohydrates Crystal structure Crystals Granular materials Optical constants Optical properties extinction coefficient pressed pellet refractive index single crystal single-angle reflectance spectroscopic ellipsometry sucrose

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Yokosuk, Michael O., Saunders, Danielle L., Resch, Charles Tom, Erickson, Jeremy D., Lonergan, Charmayne E., Burton, Sarah D., Bowden, Mark E., Peterson, Kelly A., Johnson, Timothy J., Myers, Tanya L.. 2022-09-06. Methods for Determining the Infrared Complex Refractive Indices n (λ) and k (λ) from Organic Solid Powders: Comparison of a Sucrose Single Crystal and a Pressed Pellet. https://doi.org/10.1021/acsaom.2c00004

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