Structure, Growth and Optical Properties of Si$_{n}$O$_{m}$ Nanoparticles: From SiO molecules to Silicates in Circumstellar Space*

Synergistic effort combining formation of clusters in molecular beams and first principles electronic structure studies within a gradient corrected density functional scheme are employed to examine the geometries, stability, vibrational frequencies and optical properties of Si$_{n}$O$_{m}$ clusters. It is proposed that the oxygen enrichment needed to form silicates in interstellar space, starting from SiO molecules can occur via two processes. (1) Chemically driven compositional separation in (SiO)$_{n}$ motifs resulting in oxygen rich and oxygen poor regions, and (2) reaction between Si$_{n}$O$_{m}$ clusters leading to oxygen richer and poorer fragments. Theoretically calculated optical and infrared spectra of Si$_{n}$O$_{m}$ clusters exhibit features observed in the extended red emissions and blue luminescence from interstellar medium indicating that the Si$_{n}$O$_{m}$ fragments could be contributing to these spectra.