The processes leading to the widespread presence of crystalline silicates throughout the galaxy and the origin of silicon nanoparticles thought to be responsible for the observed extended red emission in diffuse galactic background are still far from being understood. One of the most abundant oxygen bearing species in molecular astronomical regions is SiO. It has been conjectured that silicate formation probably proceeds via the agglomeration of these molecular species; however there are no studies to reveal the microscopic mechanism. We have used a synergistic approach combining experiments in molecular beams and first principles theoretical calculation to demonstrate that the passage from SiO to Si02 proceeds via gradual oxygen enrichment of SinOm clusters and that the smallest cascade involves Si203, S304, Si405, Si506 as the intermediate products. We also demonstrate that as the SiO molecules cluster together, the chemistry drives the agglomerates towards configurations such that the central core are pure Sin clusters while the outer shell are SiO2 molecules. The gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital range from 0.84 to 3.84 eV and hence can contribute to the observed extended red emission and blue luminescence. The findings are of general interest in Astrophysics but are also critical to a fundamental understanding of the interstellar extinction.
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