Low-temperature infrared properties of cosmic dust analogues

Abstract

Cosmic dust analogue materials including amor- phous and crystalline silicates, ferrous oxide, and iron sulde have been studied by reflectance spectroscopy of bulk surfaces as well as by transmission spectroscopy of particulates at tem- peratures down to 10 K. From the reflectance measurements optical data have been derived. The spectra show a sharpen- ing and strengthening of the vibrational bands with decreasing temperature. This effect is weak for the silicates, surprisingly alsoforthecrystallineone,withtheexceptionofthefar-infrared metalionvibrationsfoundinapyroxeneglassofapproximately cosmic composition. For the iron compounds, the strengthen- ing of the bands is considerably stronger. The FIR continuum absorption caused by free charge carriers in the semiconducting materials and by phonon difference processes in the crystalline silicate, respectively, decreased with decreasing temperature as expected.Partofthetemperatureeffectsaremaskedinthetrans- mission spectra by the complicated particle morphology.