Raymond and Beverly Sackler Laboratory for Astrophysics, Leiden Observatory, University of Leiden, P.O. Box 9513, NL 2300 RA Leiden, The Netherlands.
Phys Chem Chem Phys. 2013 Jun 7;15(21):8287-302. doi: 10.1039/c3cp00106g. Epub 2013 Apr 24.
Water is the most abundant molecule found in interstellar icy mantles. In space it is thought to be efficiently formed on the surfaces of dust grains through successive hydrogenation of O, O2 and O3. The underlying physico-chemical mechanisms have been studied experimentally in the past decade and in this paper we extend this work theoretically, using Continuous-Time Random-Walk Monte Carlo simulations to disentangle the different processes at play during hydrogenation of molecular oxygen. CTRW-MC offers a kinetic approach to compare simulated surface abundances of different species to the experimental values. For this purpose, the results of four key experiments-sequential hydrogenation as well as co-deposition experiments at 15 and 25 K-are selected that serve as a reference throughout the modeling stage. The aim is to reproduce all four experiments with a single set of parameters. Input for the simulations consists of binding energies as well as reaction barriers (activation energies). In order to understand the influence of the parameters separately, we vary a single process rate at a time. Our main findings are: (i) The key reactions for the hydrogenation route starting from O2 are H + O2, H + HO2, OH + OH, H + H2O2, H + OH. (ii) The relatively high experimental abundance of H2O2 is due to its slow destruction. (iii) The large consumption of O2 at a temperature of 25 K is due to a high hydrogen diffusion rate. (iv) The diffusion of radicals plays an important role in the full reaction network. The resulting set of 'best fit' parameters is presented and discussed for use in future astrochemical modeling.
水是星际冰幔中最丰富的分子。在太空中，人们认为它是通过 O、O2 和 O3 的连续氢化过程在尘埃颗粒表面高效形成的。过去十年中，人们已经对其潜在的物理化学机制进行了实验研究，而在本文中，我们通过连续时间随机漫步蒙特卡罗模拟从理论上扩展了这项工作，以分离在分子氧氢化过程中起作用的不同过程。CTRW-MC 为比较模拟表面不同物种的丰度与实验值提供了一种动力学方法。为此，选择了四个关键实验的结果——连续氢化以及在 15 和 25 K 下的共沉积实验——作为整个建模阶段的参考。目的是用一组参数重现所有四个实验。模拟的输入包括结合能和反应势垒（活化能）。为了单独理解参数的影响，我们一次只改变一个过程的速率。我们的主要发现是：（i）从 O2 开始的氢化途径的关键反应是 H + O2、H + HO2、OH + OH、H + H2O2、H + OH。（ii）H2O2 的相对高实验丰度是由于其缓慢的破坏。（iii）25 K 时 O2 的大量消耗是由于氢扩散率高。（iv）自由基的扩散在整个反应网络中起着重要作用。提出并讨论了一套“最佳拟合”参数，以供未来的天体化学建模使用。
