From SiO Molecules to Silicates in Circumstellar Space: Atomic Structures, Growth Patterns, and Optical Signatures of Si_nO_m Clusters

SiO is the dominant silicon bearing molecule in the circumstellar medium; however, it agglomerates to form oxygen-rich silicates. Here we present a synergistic effort combining experiments in beams with theoretical investigations to examine mechanisms for this oxygen enrichment. The oxygen enrichment may proceed via two processes, namely, (1) chemically driven compositional separation in (SiO)(n) motifs resulting in oxygen-rich and silicon-rich or pure silicon regions, and (2) reaction between Si(n)O(m) clusters leading to oxygen richer and poorer fragments. While SiO(2) molecules are emitted in selected chemical reactions, they readily oxidize larger Si(n)O(n) clusters in exothermic reactions and are not likely to agglomerate into larger (SiO(2))(n) motifs. Theoretically calculated optical absorption 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.