Minissale M, Dulieu F
LERMA, Université de Cergy Pontoise et Observatoire de Paris, UMR 8112 du CNRS. 5, mail Gay Lussac, 95031 Cergy Pontoise, France.
J Chem Phys. 2014 Jul 7;141(1):014304. doi: 10.1063/1.4885847.
In cold astrophysical environments, some molecules are observed in the gas phase whereas they should have been depleted, frozen on dust grains. In order to solve this problem, astrochemists have proposed that a fraction of molecules synthesized on the surface of dust grains could desorb just after their formation. Recently the chemical desorption process has been demonstrated experimentally, but the key parameters at play have not yet been fully understood. In this article, we propose a new procedure to analyze the ratio of di-oxygen and ozone synthesized after O atoms adsorption on oxidized graphite. We demonstrate that the chemical desorption efficiency of the two reaction paths (O+O and O+O2) is different by one order of magnitude. We show the importance of the surface coverage: for the O+O reaction, the chemical desorption efficiency is close to 80% at zero coverage and tends to zero at one monolayer coverage. The coverage dependence of O+O chemical desorption is proved by varying the amount of pre-adsorbed N2 on the substrate from 0 to 1.5 ML. Finally, we discuss the relevance of the different physical parameters that could play a role in the chemical desorption process: binding energy, enthalpy of formation, and energy transfer from the new molecule to the surface or to other adsorbates.
在寒冷的天体物理环境中，一些分子在气相中被观测到，然而它们本应已被耗尽并冻结在尘埃颗粒上。为了解决这个问题，天体化学家提出，在尘埃颗粒表面合成的一部分分子可能在形成后立即解吸。最近，化学解吸过程已通过实验得到证实，但其中起作用的关键参数尚未完全被理解。在本文中，我们提出了一种新的程序来分析氧原子吸附在氧化石墨上后合成的双氧和臭氧的比例。我们证明，两条反应路径（O+O和O+O₂）的化学解吸效率相差一个数量级。我们展示了表面覆盖率的重要性：对于O+O反应，化学解吸效率在零覆盖率时接近80%，在单分子层覆盖率时趋于零。通过将衬底上预吸附的N₂量从0变化到1.5 ML，证明了O+O化学解吸对覆盖率的依赖性。最后，我们讨论了可能在化学解吸过程中起作用的不同物理参数的相关性：结合能、生成焓以及新分子向表面或其他吸附质的能量转移。
