标题： 硅酸盐尘埃表面H₂的快速正交到平行核自旋转化 显示英文标题

标题： Rapid ortho-to-para nuclear spin conversion of H2 on a silicate dust surface

摘要： H₂分子有两种核自旋异构体，即所谓的正交和顺式异构体。这些状态之间的核自旋转化（NSC）在气相中是禁戒的。最低能量的正交态与顺式态之间的能量差高达14.7 meV，相当于约170 K。因此，H₂的每一个状态不仅影响化学反应，还影响空间中的宏观气体动力学，所以H₂的正交-顺式丰度比（OPR）对各种天文现象有重大影响。 长期以来，人们认为在尘埃颗粒上形成的新生H₂的OPR具有统计值三，并且会在环境温度下逐渐在气相中达到平衡。 最近，实验揭示了H₂的NSC可以在极低温度下的水冰上发生，因此被纳入气-尘化学模型。 然而，在尘埃颗粒被水冰覆盖之前，H₂分子应该已经形成。关于H₂在冰壳形成之前的裸露硅酸盐尘埃上的OPR行为的信息尚缺乏。 了解即使在天文学意义上可测量的时间尺度内，H₂的OPR在裸露硅酸盐表面上发生变化时的影响是有必要的。 我们报告了在高达18K的温度下，H₂在无定形硅酸盐（Mg₂SiO₄）上物理吸附的NSC的第一个实验室测量结果。发现这种转化发生得非常迅速。 显示英文摘要

摘要： The H2 molecule has two nuclear spin isomers, the so-called ortho and para isomers. Nuclear spin conversion (NSC) between these states is forbidden in the gas phase. The energy difference between the lowest ortho and para states is as large as 14.7 meV, corresponding to ~170 K. Therefore, each state of H2 differently affects not only the chemistry but also the macroscopic gas dynamics in space, and thus, the ortho-to-para abundance ratio (OPR) of H2 has significant impacts on various astronomical phenomena. For a long time, the OPR of nascent H2 upon formation on dust grains has been assumed to have a statistical value of three and to gradually equilibrate in the gas phase at the temperature of the circumstances. Recently, NSC of H2 was experimentally revealed to occur on water ice at very low temperatures and thus incorporated into gas-dust chemical models. However, H2 molecules should form well before dust grains are coated by water ice. Information about how the OPR of H2 behaves on bare silicate dust before ice-mantle formation is lacking. Knowing the influence of the OPR of H2 if the OPR changes even on a bare silicate surface within an astronomically meaningful time scale is desirable. We report the first laboratory measurements of NSC of H2 physisorbed on amorphous silicate (Mg2SiO4) at temperatures up to 18 K. The conversion was found to occur very rapidly.