Infrared properties of SiC particles

TL;DR

This study provides comprehensive infrared spectral data on SiC particles, revealing how grain morphology, impurities, and free charge carriers influence spectral features relevant to carbon star observations.

Contribution

It offers new laboratory infrared spectra of SiC particles, highlighting the effects of morphology, impurities, and free charge carriers on spectral features, aiding astrophysical interpretation.

Findings

Wide variety of SiC phonon features observed

Free charge carriers cause strong plasmon absorption

No systematic dependence on crystal type

Abstract

We present basic laboratory infrared data on a large number of SiC particulate samples, which should be of great value for the interpretation of the 11.3 micron feature observed in the spectra of carbon-rich stars. The laboratory spectra show a wide variety of the SiC phonon features in the 10-13 micron wavelength range, both in peak wavelength and band shape. The main parameters determining the band profile are morphological factors as grain size and shape and, in many cases, impurities in the material. We discovered the interesting fact that free charge carriers, generated e.g. by nitrogen doping, are a very common characteristics of many SiC particle samples. These free charge carriers produce very strong plasmon absorption in the near and middle infrared, which may also heavily influence the 10-13 micron feature profile via plasmon-phonon coupling. We also found that there is no systematic dependence of the band profile on the crystal type (alpha- vs. beta-SiC). This is proven both experimentally and by theoretical calculations based on a study of the SiC phonon frequencies. Further, we give optical constants of amorphous SiC. We discuss the implications of the new laboratory results for the interpretation of the spectra of carbon stars.