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

Abstract

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.

Theoretical prediction of atomic and electronic structure of neutral Si6Om (m=1–11) clusters

“…The minimum energy fragmentation contains Si 2 O 2 , Si 3 O 3 , and SiO clusters, so these fragmentation products are considered to be relatively stable clusters. These results are in good agreement with the experimental data reported by Rebert et al 28 We define RE as the energy required to remove one oxygen atom, that is RE= E͑Si 6 …”

“…12,26,27 Experiments showed that SiNWs can be obtained by thermal evaporation or laser ablation of Si powder mixed with SiO 2 or SiO, 12,26,27 but there is no SiNWs growth from pure SiO 2 materials. 12 Reber et al 28 demonstrated that SiO units assemble to form bigger clusters, with silicon rich agglomerations at the center and oxygen rich structures at the periphery.…”

Fume Formation from Oxidation of Liquid SiMn Alloy

“…SiO clusters [22] may form inside the SiO boundary layer and then collect Mn and O 2 atoms and/or MnO clusters to form larger clusters which may or may not crystallize upon solidification. The shape and amorphous nature of these particles indicate that they go through a liquid aerosol phase before they solidify into solid particulate matter.…”

Stardust silicate nucleation kick-started by SiO+TiO 2

“…For TiO 2 -SiO 2 , a recent theoretical study showed how stoichiometric (TiO 2 ) N (SiO 2 ) M mixing could conceivably occur in very small clusters [59]. Finally, we note that silicon monoxide (SiO), a stable molecule that has been postulated to nucleate homomolecularly in stellar outflows [32], already at the small cluster scale shows signs of disproportionation (segregation in Si and SiO 2 domains) [60].…”