Development and physical reason of IR spectral features during condensation and solid phase reactions of Fe-Mg silicate systems: IR studies focused on non-stoichiometric silicates

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Silicates are one of the most important dust components in astronomical environments. In fact, the existence of silicate grains has been revealed by means of high resolution infrared (IR) spectroscopic observations as well as by secondary ion mass-spectroscopic examinations of meteorites. However, the formation and evolution processes of silicates have not been well understood yet. It is expected that under the non-equilibrium conditions encountered in space the production of non-stoichiometric dust grains is most likely. Such non-stoichiometric dust grains show distinct IR spectral features as compared with the stoichiometric cases because IR band profiles from minerals are very sensitive to chemical and physical properties. Thus, chemical and physical variations of these compounds directly influence the complex optical constants of the systems. For the interpretation of observed spectra both experimentally determined and theoretically calculated spectra are employed. For the theoretical simulations the optical constants are the most important input parameters. Deriving optical constants of nonstoichiometric silicates from mid-IR spectra therefore is a pre-requisite for improving the accuracy of the model calculations as well as enhancing the understanding of chemical and physical properties of observed mineral species. Presently, mainly spectra of stoichiometric compounds are used for interpretation of observed spectra because of a lack of experimental data for non-stoichiometric compounds. Thus, we will investigate the role and effect that a disparity of metals (Mg, Fe, Ca, Al, Na) in silicates has on spectra and monitor how the IR spectral features undergo a change by use of FTIR spectrometer. The proportions of the abundant components such as Mg and Fe in silicates are a key to understand the possible chemical condensation paths of those minerals. Highly accurate complex optical constants will be determined by making use of IR ellipsometry. For this, high quality silicate samples with well-defined metal proportion in silicate systems will be prepared by pulsed laser deposition (PLD) at Ruhr University Bochum. This technique is highly versatile in producing thin films of refractory materials with various compositions. We will investigate chemical, physical, and optical properties of both stoichiometric and nonstoichiometric structures of silicates in detail for a deeper understanding of dust composition and dust processing mechanisms in protoplanetary and young debris disks.