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Directed Gas Phase Formation of Silicon Dioxide and Implications for the Formation of Interstellar Silicates

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Directed Gas Phase Formation of Silicon Dioxide and Implications for the Formation of Interstellar Silicates ARTICLE DOI: 10.1038/s41467-018-03172-5 OPEN Directed gas phase formation of silicon dioxide and implications for the formation of interstellar silicates Tao Yang 1,2, Aaron M. Thomas1, Beni B. Dangi1,3, Ralf I. Kaiser 1, Alexander M. Mebel 4 & Tom J. Millar 5 1234567890():,; Interstellar silicates play a key role in star formation and in the origin of solar systems, but their synthetic routes have remained largely elusive so far. Here we demonstrate in a combined crossed molecular beam and computational study that silicon dioxide (SiO2) along with silicon monoxide (SiO) can be synthesized via the reaction of the silylidyne radical (SiH) with molecular oxygen (O2) under single collision conditions. This mechanism may provide a low-temperature path—in addition to high-temperature routes to silicon oxides in circum- stellar envelopes—possibly enabling the formation and growth of silicates in the interstellar medium necessary to offset the fast silicate destruction. 1 Department of Chemistry, University of Hawai’iatMānoa, Honolulu, HI 96822, USA. 2 State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai, 200062, China. 3 Department of Chemistry, Florida Agricultural and Mechanical University, Tallahassee, FL 32307, USA. 4 Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199, USA. 5 Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast, BT7 1NN, UK. Correspondence and requests for materials should be addressed to R.I.K. (email: [email protected]) or to A.M.M. (email: mebela@fiu.edu) or to T.J.M. (email: [email protected]) NATURE COMMUNICATIONS | (2018) 9:774 | DOI: 10.1038/s41467-018-03172-5 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-03172-5 — 28 + 28 + he origin of interstellar silicate grains nanoparticles ( SiO2 ), and 44 ( SiO ). No signal was observed at m/z 62, consisting primarily of olivine-type ((Mg,Fe)2SiO4) suggesting that under single collision conditions the lifetime of T 28 fl — refractory minerals has remained a controversial topic for the SiDO2 adduct is shorter than its ight time to the electron- more than half a century, since interstellar silicates are faster impact ionizer. Reactive scattering signal was detected at m/z 60 28 + destroyed by sputtering than formed during the late stages of ( SiO2 ) (Fig. 1). The scattering signal is relatively weak, and at stellar evolution through nucleation in circumstellar envelopes of each angle up to 6 × 106 time-of-flight (TOF) spectra (60 h col- oxygen-rich Asymptotic Giant Branch (AGB) and Red Supergiant lection time) had to be averaged to obtain a reasonable signal-to- – (RSG) stars1 5. These nanoparticles have been associated with the noise ratio. Signal detection at m/z 60 alone provides conclusive 28 prebiotic evolution of the interstellar medium (ISM) through the evidence on the formation of a molecule with the formula SiO2 synthesis of molecular building blocks of life such as amino acids via a single collision event of two neutral reactants. Taking into and sugars on their ice-coated surfaces by ionizing radiation6. account the data accumulation time, the signal-to-noise ratio Interstellar silicates also play a critical role in star formation and obtained at m/z 60 and the abundances of naturally occurring in the origin of solar systems contributing to the radiation balance silicon isotopes of 30Si(3.10 %), 29Si (4.67 %), and 28Si(92.23 %), and acting as a molecular feedstock, both through the formation we would not expect—as confirmed experimentally—any reactive 30 + of complex organics through the release of icy mantles that cover scattering signal at m/z 62 ( SiO2 ). Finally, the background them and through disruption of grains in interstellar shocks3,7.In counts at m/z 44 originating from singly ionized carbon dioxide molecular clouds, they absorb light and hence shield complex (CO ) in the detector precludes an identification of any reactive 2 + organic molecules (COMs)—organics containing carbon, hydro- scattering signal at m/z 44 (28SiO ). To summarize, the labora- 28 gen, nitrogen, and oxygen like glycolaldehyde (HCOCH2OH) and tory data indicate that a molecule with the formula SiO2 — formamide (HCONH2) from the destructive interstellar ultra- (hereafter: SiO2) along with atomic deuterium is formed under violet radiation field8. Therefore, the elucidation of the origin of single collision conditions in the reaction of the SiD radical with interstellar silicates is of vital importance to the astrochemistry, O2. astrobiology, and astrophysics communities to eventually understand the fundamental processes that create a visible galaxy Crossed molecular beam studies in the center-of-mass frame. including our own. We transformed the experimental data from the laboratory to the A crucial point of concern is that the mass of dust ejected center-of-mass (CM) reference frame32 to gain information on during the late stages of stellar evolution is produced at a rate that the underlying reaction dynamics, which yields the CM transla- is significantly slower than the dust destruction time in the ISM, tional energy flux distribution P(E ) and the CM angular flux implying that grains also form in the lower density environment T – distribution T(θ) as depicted in Fig. 2. Best fits of the laboratory of the ISM5,9 14. Current astrochemical models of circumstellar data are achieved with a single-channel fit forming products with envelopes propose that dust formation in AGB stars is driven a mass combination of 60 amu (SiO ) and 2 amu (D) (Figs. 1 and eventually by clustering and reactions of silicon oxides along with 2 – 2). A detailed inspection of the CM functions affords crucial magnesium-type and iron-type oxides2,15 20. There does appear information on the pertinent reaction channel(s) and dynamics. to be, however, a severe discrepancy between the formation rates First, P(E ) assists in the identification of the product isomer(s). of silicate grains in circumstellar envelopes of 3 × 109 years and T For the reaction products formed without internal excitation, the their destruction via sputtering once dispersed into the ISM that − – high energy cutoff of 493 ± 57 kJ mol 1 in P(E ) denotes the sum limits their lifetime to only a few 108 years5,21 23. This dis- T of the absolute value of the reaction exothermicity plus the col- crepancy24 may eventually be resolved through a better under- − lision energy E (33.2 ± 2.0 kJ mol 1). A subtraction of the colli- standing of the processes of dust destruction5,23, but it remains c sion energy reveals that the reaction is highly exothermic with the possible that significant formation of dust needs to occur in the − – energy of -460 ± 59 kJ mol 1. This finding agrees nicely with our interstellar as opposed to the circumstellar medium12 15. Indeed, − computed value of -441 ± 5 kJ mol 1 (Fig. 3) and the energetics silicate grains may grow in the ISM by accreting and incorpor- − obtained from NIST Webbook (-464 kJ mol 1)33 to form the ating silicon oxide molecules25,26. linear SiO molecule (l-SiO ) along with atomic hydrogen (H). Here we show that the silicon dioxide molecule (SiO ) along 2 2 2 This shows for the very first time that a SiO molecule is observed with silicon monoxide (SiO) can be efficiently formed via a low- 2 in the gas phase as a result of a reaction between two neutral temperature gas phase chemistry even at 10 K. We report the species under controlled experimental conditions. Previously, results of a combined crossed molecular beam study and of Ahmed et al. generated SiO via laser ablation of silicon (Si) and electronic structure calculations on the reaction of the D1- 2 − two successive oxygen abstractions from the seeding and reactant silylidyne radical (SiD; X2Π) with molecular oxygen (O ,X3Σ ) − 2 g gas—CO ;34 Wang et al. formed gas phase SiO through SiO leading to the formation of SiO and SiO through a barrierless 2 2 2 2 electron photodetachment35. A complex forming reaction reaction27. This system represents a proxy for the reaction of the mechanism is evident from T(θ) which depicts the flux over the silylidyne radical (SiH) generated via photolysis of silane (SiH ) – 4 complete angular range36. This distribution reveals further a 28 30 with O to synthesize silicon oxides via a single collision 2 forward-backward symmetry proposing that the lifetime of the event. In the ISM, the reaction of SiH with O may represent a 2 decomposing intermediate is longer than its rotational period37. potential pathway to SiO and silicon oxide formation in those 2 Alternatively, a ‘symmetric’ reaction intermediate can account for molecular clouds, where gas phase chemistry follows ice mantle these findings by emitting a deuterium atom with equal prob- sublimation; these silicon oxides might drive an exothermic abilities into θ and π–θ38. chemistry that possibly produces larger silicon oxides25,31 leading ultimately to silicates at low temperatures. Electronic structure calculations and reaction mechanism.We combined the experimental findings with electronic structure Results calculations on the reaction of SiH with O2 to elucidate the Crossed molecular beam studies in the laboratory frame. The underlying dynamics (Fig. 3). These calculations were performed reactive scattering experiments were performed using a crossed at a level of theory high enough to predict relative energies of the molecular beam apparatus (Methods). We monitored the scat- transition states and local minima as well as reaction energies 28 + −1 tering signal at mass-to-charge ratio (m/z)62( SiDO2 ), 60 within 5 kJ mol (Methods). Based on the difference in zero 2 NATURE COMMUNICATIONS | (2018) 9:774 | DOI: 10.1038/s41467-018-03172-5 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-03172-5 ARTICLE a 1000 800 600 distribution 400 CM angle Normalized experimental 200 0 0 10 20 30 40 50 60 90 Laboratory angle (°) b 100 10.25° 16.25° 22.25° 80 60 40 20 0 100 28.25° 34.25° 40.25° 80 Normalized counts 60 40 20 0 0 200 400 600 0 200 400 600 0 200 400 600 Laboratory time-of-flight (µs) + Fig.
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