[论文解读] On the relevance of the H2 + O reaction pathway for the surface formation of interstellar water - A combined experimental and modeling study

Thanja Lamberts, H. M. Cuppen+4 more|arXiv (Cornell University)|Sep 10, 2014

Astrophysics and Star Formation Studies参考文献 31被引用 15

一句话总结

本研究结合超高真空程序升温脱附实验与动力学蒙特卡罗模拟，评估H₂ + O反应在星际水形成中的作用。结果表明，该路径在冰冻尘埃颗粒上生成OH和水的贡献最多仅为11%，其速率常数的保守上限为1.68 × 10³³ · exp(−960/T) s⁻¹，表明在星际条件下该路径不太可能是主要途径。

ABSTRACT

The formation of interstellar water has been commonly accepted to occur on the surfaces of icy dust grains in dark molecular clouds at low temperatures (10-20 K), involving hydrogenation reactions of oxygen allotropes. As a result of the large abundances of molecular hydrogen and atomic oxygen in these regions, the reaction H2 + O has been proposed to contribute significantly to the formation of water as well. However, gas phase experiments and calculations, as well as solid-phase experimental work contradict this hypothesis. Here, we use precisely executed temperature programmed desorption (TPD) experiments in an ultra-high vacuum setup combined with kinetic Monte Carlo simulations to establish an upper limit of the water production starting from H2 and O. These reactants are brought together in a matrix of CO2 in a series of (control) experiments at different temperatures and with different isotopological compositions. The amount of water detected with the quadrupole mass spectrometer upon TPD is found to originate mainly from contamination in the chamber itself. However, if water is produced in small quantities on the surface through H2 + O, this can only be explained by a combined classical and tunneled reaction mechanism. An absolutely conservative upper limit for the reaction rate is derived with a microscopic kinetic Monte Carlo model that converts the upper limit into a maximal possible reaction rate. Incorporating this rate into simulations run for astrochemically relevant parameters, shows that the upper limit to the contribution of the reaction H2 + O in OH, and hence water formation, is 11% in dense interstellar clouds. Our combined experimental and theoretical results indicate however, that this contribution is likely to be much lower.

研究动机与目标

• 评估H₂ + O反应路径在星际水表面生成中的重要性。
• 解决关于H₂ + O在低温下是否能显著贡献于水形成的实验与理论争议。
• 利用受控的超高真空实验与动力学建模，建立该反应速率的严格上限。
• 评估该上限对星际化学网络模型及水中氘分馏的影响。

提出的方法

• 在超高真空（UHV）腔室中进行程序升温脱附（TPD）实验，将H₂与O共沉积于CO₂基质中。
• 使用四极质谱检测脱附的水，并与腔室污染区分开。
• 通过同位素标记实验（如H₂、D₂、O-18）追踪反应路径并确认产物来源。
• 开发微观动力学蒙特卡罗（kMC）模型，将实验上限转化为最大可能的反应速率。
• 将推导出的上限纳入星际条件（10–20 K，相关密度）下的天体化学模拟。
• 通过量子力学表达式比较反应能垒与速率，评估隧穿与经典贡献的作用。

实验结果

研究问题

• RQ1 在星际条件（10–20 K）下，H₂ + O反应是否能在冰冻尘埃颗粒表面产生可检测量的OH和H₂O？
• RQ2 在实验约束与理论模型下，H₂ + O反应的最大可能速率是多少？
• RQ3 在致密星际云中，H₂ + O路径相对于其他水形成路径（如O + H、O₂ + H、OH + H）的重要性如何？
• RQ4 隧穿或经典活化在H₂ + O反应中贡献多大程度，能否克服较大的吸热性（960 K）？
• RQ5 H₂ + O路径对星际水中氘分馏有何影响？

主要发现

• 实验中观测到的水脱附信号主要源自腔室污染，而非H₂ + O反应。
• 任何可检测的H₂O生成必须通过经典与隧穿机制的协同作用实现，因为该反应为吸热反应且具有高能垒。
• 推导出H₂ + O反应速率常数的绝对上限为1.68 × 10³³ · exp(−960/T) s⁻¹。
• 在天体化学模拟中，H₂ + O路径在致密星际云中对OH和水形成的贡献最多为11%。
• 由于模型中采用保守假设（如低吸附概率和热化效应），实际贡献可能低得多。
• 结果表明，H₂ + O路径并非水形成的主要途径，不应在星际冰化学模型中假设其具有显著作用。