DataSheet1_Effect of Binding Energies on the Encounter Desorption.ZIP

The abundance of interstellar ice constituents is usually expressed with respect to the water ice because, in denser regions, a significant portion of the interstellar grain surface would be covered by water ice. The binding energy (BE) or adsorption energy of the interstellar species regulates the chemical complexity of the interstellar grain mantle. Due to the high abundance of water ice, the BE of surface species with the water is usually provided and widely used in astrochemical modeling. However, the hydrogen molecules would cover some part of the grain mantle in the denser and colder part of the interstellar medium. Even at around ∼10 K, few atoms and simple molecules with lower adsorption energies can migrate through the surface. The BE of the surface species with H2 substrate would be very different from that of a water substrate. However, adequate information regarding these differences is lacking. Here, we employ the quantum chemical calculation to provide the BE of 95 interstellar species with H2 substrate. These are representative of the BEs of species to a H2 overlayer on a grain surface. On average, we notice that the BE with the H2 monomer substrate is almost ten times lower than the BE of these species reported earlier with the H2O c-tetramer configuration. The encounter desorption of H and H2 was introduced [with ED(H,H2)=45 K and ED(H2,H2)=23 K] to have a realistic estimation of the abundances of the surface species in the colder and denser region. Our quantum chemical calculations yield higher adsorption energy of H2 than that of H [ED(H,H2) = 23–25 K and ED(H2,H2) = 67–79 K]. We further implement an astrochemical model to study the effect of encounter desorption with the present realistic estimation. The encounter desorption of the N atom [calculations yield ED(N,H2)=83 K] is introduced to study the differences with its inclusion.星际冰（interstellar ice）组分的丰度通常以水冰为参照进行表述，这是因为在致密区域中，星际尘埃颗粒表面的相当一部分区域会被水冰覆盖。星际物种的结合能（binding energy，BE）或吸附能，调控着星际尘埃颗粒表面覆盖层的化学复杂性。由于水冰的丰度极高，目前天体化学建模（astrochemical modeling）中通常采用并广泛使用的是星际物种与水基底的结合能数据。然而在星际介质（interstellar medium）更为致密且寒冷的区域，氢分子会覆盖尘埃覆盖层的部分区域。即使在约10 K的温度下，少数吸附能较低的原子与简单分子仍可在尘埃表面发生迁移。星际物种与氢基底的结合能，与物种与水基底的结合能存在显著差异。但目前针对这类差异的有效信息仍较为匮乏。本研究采用量子化学计算（quantum chemical calculation），给出了95种星际物种与氢基底的结合能数据，这些数据对应了星际尘埃表面氢覆盖层（H₂ overlayer）上物种的结合能。平均而言，我们发现这些物种以氢单体（H₂ monomer）为基底的结合能，相较于此前文献中报道的、以水环状四聚体（H₂O c-tetramer）为构型的结合能，几乎低了一个数量级。为了对寒冷致密区域的星际表面物种丰度进行更贴合实际的估算，研究中引入了氢原子与氢分子的遭遇解吸（encounter desorption）模型，其参数设定为ED(H,H₂)=45 K、ED(H₂,H₂)=23 K。我们的量子化学计算结果显示，氢分子的吸附能高于氢原子的吸附能，对应ED(H,H₂)=23–25 K、ED(H₂,H₂)=67–79 K。我们进一步搭建了天体化学模型，基于本次研究得到的贴合实际的参数，探究遭遇解吸效应对表面物种丰度的影响。研究中还引入了氮原子的遭遇解吸模型（计算得到ED(N,H₂)=83 K），以对比考量该模型纳入前后的差异。