Formation of Silicon Oxide Grains at Low Temperature

Abstract

The formation of grains in the interstellar medium, i.e., at low temperature, has been proposed as a possibility to solve the lifetime problem of cosmic dust. This process lacks a firm experimental basis, which is the goal of this study. We have investigated the condensation of SiO molecules at low temperature using neon matrix and helium droplet isolation techniques. The energies of SiO polymerization reactions have been determined experimentally with a calorimetric method and theoretically with calculations based on the density functional theory. The combined experimental and theoretical values have revealed the formation of cyclic (SiO) k (k = 2-3) clusters inside helium droplets at T = 0.37 K. Therefore, the oligomerization of SiO molecules is found to be barrierless and is expected to be fast in the low-temperature environment of the interstellar medium on the surface of dust grains. The incorporation of numerous SiO molecules in helium droplets leads to the formation of nanoscale amorphous SiO grains. Similarly, the annealing and evaporation of SiO-doped Ne matrices lead to the formation of solid amorphous SiO on the substrate. The structure and composition of the grains were determined by infrared absorption spectroscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Our results support the hypothesis that interstellar silicates can be formed in the low temperature regions of the interstellar medium by accretion through barrierless reactions.Subject headings: ISM: dust formation -ISM: silicate dust -low-temperature chemistry

Cosmic dust formation at cryogenic temperatures

“…Moreover, reaction energies of 178 and 291 kJ mol −1 have been experimentally determined for the reactions SiO + SiO → Si 2 O 2 and SiO + Si 2 O 2 → Si 3 O 3 , respectively. The values are consistent with the results of theoretical calculations in which a cyclic structure was given to the oligomers [4].…”

“…In the UV domain, isolated SiO molecules were identified by measuring the A 1 Π ← X 1 Σ + band system [6]. Other features at UV wavelengths were tentatively attributed to Si x O x compounds as their intensity increased at the beginning of the annealing procedure, before decreasing [4]. In fact, the presence of SiO oligomers, formed during deposition, could be expected, as reported by Khanna et al in a similar study with N 2 matrices [7].…”

“…1 (see also Ref. [4]). In the UV domain, isolated SiO molecules were identified by measuring the A 1 Π ← X 1 Σ + band system [6].…”

“…[4]. Briefly, the formation of Si x O x compounds was observed and attributed to reactions that took place inside the nanodroplets, not only between SiO molecules, but also between SiO molecules and oligomers after the latter were formed.…”

“…[4], they are only briefly presented here. This work focuses on our experiments with doped Ne matrices, more specifically on the characterization of the SiO grains produced in these experiments.…”