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An embedded cluster study of the formation of water on interstellar dust grains

机译：星际尘埃颗粒上水形成的嵌入式聚类研究

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The formation of water in the interstellar medium from hydrogen and oxygen atoms on a pristine olivine surface (forsterite (010)) is investigated with an embedded cluster approach. The 55-atom quantum cluster is described at the density functional level while the remaining 1629 atoms of the surface cluster are described with atomistic potentials. Transition states are most easily calculated with our modified implementation of the climbing-image nudged elastic band method in ChemShell. With these computational techniques, we find that gas phase hydrogen atoms can chemisorb (-102 kJ mol~(-1)) without an activation barrier on the forsterite (010) surface, concomitantly creating a surface electron at the adjacent magnesium atom site. Subsequently, an oxygen atom chemisorbs strongly to this surface electron site (-432 kJ mol~(-1)). The rearrangement of the adjacently chemisorbed O and H to a chemisorbed OH-radical is endothermic by 4 kJ mol~(-1) and activated by 27 kJ mol~(-1). This chemisorbed OH can then react barrierlessly with a second hydrogen atom to yield adsorbed water (-511 kJ mol~(-1)). Alternatively, if O and H do not recombine to form OH, but instead thermally equilibrate, a second hydrogen atom can react with the chemisorbed oxygen atom (-501 kJ mol~(-1)) to yield dissociatively adsorbed water (OH~- and H~+), which then can rearrange to associatively adsorbed water (-5 kJ mol~(-1), ΔE~(not=) = 18 kJ mol~(-1)) or gas phase water ( + 91 kJ mol~(-1)). The formation of water on a bare dust grain from hydrogen and oxygen atoms is thus catalysed by an olivine surface by stabilising the reaction intermediates and product. Since the reaction proceeds via three chemisorbed intermediates, thermal equilibration is facilitated and back-dissociation of the freshly formed reaction products OH and H2O would not occur as frequently as it would in the gas phase or when the reactants are physisorbed on a surface.
机译：利用嵌入式簇方法研究了原始橄榄石表面（镁橄榄石（010））在星际介质中由氢和氧原子形成的水。 55个原子的量子簇在密度泛函能级上描述，而表面簇的其余1629个原子用原子势描述。通过我们在ChemShell中改进的爬升图像微移弹性带方法的改进实现，可以最轻松地计算过渡态。通过这些计算技术，我们发现气相氢原子可以化学吸附（-102 kJ mol〜（-1））而在镁橄榄石（010）表面上没有激活势垒，从而在相邻的镁原子位置上产生表面电子。随后，氧原子强烈地化学吸附到该表面电子位点（-432 kJ mol〜（-1））。相邻的化学吸附的O和H重排为化学吸附的OH自由基时吸热为4 kJ mol〜（-1），并被27 kJ mol〜（-1）激活。然后，这种化学吸附的OH可以与第二个氢原子无障碍地反应，生成吸附的水（-511 kJ mol〜（-1））。或者，如果O和H不重组形成OH，而是热平衡，则第二个氢原子可与化学吸附的氧原子（-501 kJ mol〜（-1））反应，生成解离吸附的水（OH〜-和H〜+），然后可以重新排列为缔合吸附的水（-5 kJ mol〜（-1），ΔE〜（not =）= 18 kJ mol〜（-1））或气相水（+ 91 kJ mol〜 （-1））。通过稳定反应中间体和产物，橄榄石表面催化了由氢和氧原子在裸露的尘埃颗粒上形成水。由于反应是通过三种化学吸附的中间体进行的，因此促进了热平衡，并且新形成的反应产物OH和H2O的后离解不会像在气相中或当反应物物理吸附在表面上时那样频繁发生。

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