OPTICAL CONSTANTS OF SILICON CARBIDE FOR ASTROPHYSICAL APPLICATIONS. II. EXTENDING OPTICAL FUNCTIONS FROM INFRARED TO ULTRAVIOLET USING SINGLE-CRYSTAL ABSORPTION SPECTRA

Laboratory measurements of unpolarized and polarized absorption spectra of various samples and crystal structures of silicon carbide (SiC) are presented from 1200-35000 cm(-1) (lambda similar to 8-0.28 mu m) and used to improve the accuracy of optical functions (n and k) from the infrared (IR) to the ultraviolet (UV). Comparison with previous lambda similar to 6-20 mu m thin-film spectra constrains the thickness of the films and verifies that recent IR reflectivity data provide correct values for k in the IR region. We extract n and k needed for radiative transfer models using a new "difference method," which utilizes transmission spectra measured from two SiC single-crystals with different thicknesses. This method is ideal for near-IR to visible regions where absorbance and reflectance are low and can be applied to any material. Comparing our results with previous UV measurements of SiC, we distinguish between chemical and structural effects at high frequency. We find that for all spectral regions, 3C (beta-SiC) and the (E) over right arrow perpendicular to (c) over right arrow polarization of 6H (a type of alpha-SiC) have almost identical optical functions that can be substituted for each other in modeling astronomical environments. Optical functions for (E) over right arrow parallel to(c) over right arrow of 6H SiC have peaks shifted to lower frequency, permitting identification of this structure below lambda similar to 4 mu m. The onset of strong UV absorption for pure SiC occurs near 0.2 mu m, but the presence of impurities redshifts the rise to 0.33 mu m. Optical functions are similarly impacted. Such large differences in spectral characteristics due to structural and chemical effects should be observable and provide a means to distinguish chemical variation of SiC dust in space.