PNNL

Abstract

There exists a need to measure the optical constants n and k for many materials, especially organic solids, but at present there are only two established methods to make such measurements: single-angle (SA) reflectance and infrared spectral ellipsometry (IRSE). The former determines the n/k vectors via measurement of the amplitude change of reflected light from the sample while the latter measures the polarization change of light reflected from the sample. Here we make direct comparison between the n/k vectors derived from SA measurements of a sucrose single crystal and the n/k values obtained from a pellet after the crystal had been crushed, ground into a powder, and pressed into a pellet. The pellet was measured not only by SA but also via the IRSE method. Pressing specular-quality pellets was found to be critical, but the two methods both measured the n/k of this typical organic molecule with results agreeing to within 0.04 root-mean-squared error (RMSE). The agreement confirms equivalency for both pellet and single crystal forms to derive the intrinsic n/k vectors, the results especially important for those materials that, unlike sugar, do not have cm-sized crystals readily available.