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Laboratory measurements of unpolarized and polarized absorption spectra ofvarious samples and crystal stuctures of silicon carbide (SiC) are presentedfrom 1200--35,000 cm$^{-1}$ ($\lambda \sim$ 8--0.28 $\mu$m) and used to improvethe accuracy of optical functions ($n$ and $k$) from the infrared (IR) to theultraviolet (UV). Comparison with previous $\lambda \sim$ 6--20 $\mu$mthin-film spectra constrains the thickness of the films and verifies thatrecent IR reflectivity data provide correct values for $k$ in the IR region. Weextract $n$ and $k$ needed for radiative transfer models using a new``difference method'', which utilizes transmission spectra measured from twoSiC single-crystals with different thicknesses. This method is ideal fornear-IR to visible regions where absorbance and reflectance are low and can beapplied to any material. Comparing our results with previous UV measurements ofSiC, we distinguish between chemical and structural effects at high frequency.We find that for all spectral regions, 3C ($\beta$-SiC) and the $\vec{E}\bot\vec{c}$ polarization of 6H (a type of $\alpha$-SiC) have almost identicaloptical functions that can be substituted for each other in modelingastronomical environments. Optical functions for $\vec{E} \| \vec{c}$ of 6H SiChave peaks shifted to lower frequency, permitting identification of thisstructure below $\lambda \sim4\mu$m. The onset of strong UV absorption for pureSiC occurs near 0.2 $\mu$m, but the presence of impurities redshifts the riseto 0.33 $\mu$m. Optical functions are similarly impacted. Such largedifferences in spectral characteristics due to structural and chemical effectsshould be observable and provide a means to distinguish chemical variation ofSiC dust in space.

astrophysical journal/the astrophysical journal

Optical constants of silicon carbide for astrophysical applications. II.  Extending optical functions from IR to UV using single-crystal absorption  spectra