Binding energies and sticking coefficients of H2 on crystalline and amorphous CO ice
Binding energies and sticking coefficients of H2 on crystalline and amorphous CO ice
复制标题
H2 在结晶和非晶态 CO 冰上的结合能和粘附系数
DOI：
10.1051/0004-6361/202040023
复制
发表时间：
2021
影响因子：
6.5
通讯作者：
Kastner J.
中科院分区：
文献类型：
--
作者：
Molpeceres G.;Zaverkin V.;Watanabe N.;Kastner J.
ContextMolecular hydrogen (H2) is the most abundant interstellar molecule and plays an important role in the chemistry and physics of the interstellar medium. The interaction of H2with interstellar ices is relevant for several processes (e.g., nuclear spin conversion and chemical reactions on the surface of the ice). To model surface processes, quantities such as binding energies and sticking coefficients are required.AimsWe provide sticking coefficients and binding energies for the H2/CO system. These data are absent in the literature so far and could help modelers and experimentalists to draw conclusions on the H2/CO interaction in cold molecular clouds.MethodsAb initio molecular dynamics simulations, in combination with neural network potentials, were employed in our simulations. Atomistic neural networks were trained against density functional theory calculations on model systems. We sampled a wide range of H2internal energies and three surface temperatures.ResultsOur results show that the binding energy for the H2/CO system is low on average, − 157 K for amorphous CO and −266 K for crystalline CO. This carries several implications for the rest of the work. H2binding to crystalline CO is stronger by 109 K than to amorphous CO, while amorphous CO shows a wider H2binding energy distribution. Sticking coefficients are never unity and vary strongly with surface temperature, but less so with ice phase, with values between 0.95 and 0.17. With the values of this study, between 17 and 25% of a beam of H2molecules at room temperature would stick to the surface, depending on the temperature of the surface and the ice phase. Residence times vary by several orders of magnitude between crystalline and amorphous CO, with the latter showing residence times on the order of seconds at 5 K. H2may diffuse before desorption in amorphous ices, which might help to accommodate it in deeper binding sites.ConclusionsBased on our results, a significant fraction of H2molecules will stick on CO ice under experimental conditions, even more so under the harsh conditions of prestellar cores. However, with the low H2–CO binding energies, residence times of H2on CO ice before desorption are too short to consider a significant population of H2molecules on pure CO ices. Diffusion is possible in a time window before desorption, which might help accommodate H2on deeper binding sites, which would increase residence times on the surface.
