Low-temperature infrared properties of cosmic dust analogues

Cosmic dust analogue materials including amorphous and crystalline silicates, ferrous oxide, and iron sulfide have been studied by reflectance spectroscopy of bulk surfaces as well as by transmission spectroscopy of particulates at temperatures down to 10 K. From the reflectance measurements optical data have been derived. The spectra show a sharpening and strengthening of the vibrational bands with decreasing temperature. This effect is weak for the silicates, surprisingly also for the crystalline one, with the exception of the far-infrared metal ion vibrations found in a pyroxene glass of approximately cosmic composition. For the iron compounds, the strengthening 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. Part of the temperature effects are masked in the transmission spectra by the complicated particle morphology.