Ultraviolet and infrared refractive indices of amorphous silicates

Thin films of amorphous magnesium silicates with compositions similar to those of enstatite (MgSiO3) and forsterite (Mg2SiO4) have been deposited using excimer laser ablation of the parent materials. Refractive indices n and k for these materials have been derived from optical transmission and reflection spectra together with Kramers-Kronig analysis. Measured refractive indices are reported for the range 0.12-17.5 mu m and are extended to shorter and longer wavelengths by fitting to data reported from other experiments and to theoretical predictions. The overall n, k data set is consistent with Kramers-Kronig. We find that the strength of the 10 mu m band in these amorphous materials is comparable to that customarily assumed for astronomical silicates. In addition, the strength of absorption in the vacuum ultraviolet is similar to previous estimates. We find that the onset of strong UV absorption in our samples occurs near that of ''astronomical silicate'' and is only slightly dependent on composition. The emissivity of small particles of amorphous forsterite, calculated using our measured n, k, is found to peak at 9.6 mu m.