Optical constants of silicon carbide for astrophysical applications. II.
Extending optical functions from IR to UV using single-crystal absorption
spectra
Laboratory measurements of unpolarized and polarized absorption spectra of
various samples and crystal stuctures of silicon carbide (SiC) are presented
from 1200--35,000 cm−1 (λ∼ 8--0.28 μm) and used to improve
the accuracy of optical functions (n and k) from the infrared (IR) to the
ultraviolet (UV). Comparison with previous λ∼ 6--20 μm
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 (β-SiC) and the E⊥c polarization of 6H (a type of α-SiC) have almost identical
optical functions that can be substituted for each other in modeling
astronomical environments. Optical functions for E∥c of 6H SiC
have peaks shifted to lower frequency, permitting identification of this
structure below λ∼4μm. The onset of strong UV absorption for pure
SiC occurs near 0.2 μm, but the presence of impurities redshifts the rise
to 0.33 μm. 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.Comment: 46 pages inc. 8 figures and 2 full tables. Also 6 electronic-only
data files. Accepted by Ap