Improving Models of Molecular Clouds and Planetary Atmospheres: Dissociative Recombination Measurements for Molecular Ions of Astronomical Interest

PI和他的团队将在寒冷的星际云中测量解离重组分支比率和重要化学分子离子反应的速率系数。这项关于解离重组反应速率和横截面的工作对于了解星际介质的分子库存和冷却，以及将天体化学模型建立在坚实的基础上是必要的。分子是漫射、半透明和致密分子云、热核、光子主导区、原恒星盘、原行星盘、行星和卫星电离层、彗星、彗发和垂死恒星周围的星周包层的关键组成部分。各种天文环境中的热结构依赖于控制这些性质的潜在分子碰撞的可靠知识。在解离复合反应中，宇宙等离子体中分子的初级中和反应尤为重要。涉及离子-分子反应的解离重组通常是化学网络中特定合成途径的终止步骤。人们需要知道最终产物的速率和分支比，以便理解反应途径，并了解化合物是否可以在气相中产生，或者是否必须调用颗粒表面化学。如果解离重组的最终产物是高能的，它们可以碰撞加热等离子体；如果它们处于激发态，它们可以通过辐射松弛冷却气体。实验是在德国海德堡马克斯-普朗克研究所独特的重离子测试存储环（TSR）设施中完成的。研究的离子有HF+、H2F+、CF+、16O16O+、18O16O+、HSiO+、CH2O+和ch30 +。它们的选择是基于它们对地面光谱观测和天体化学建模研究的重要性。所选择的分子可以在TSR中储存足够长的时间，以冷却到最低的电子和振动水平，除了16O16O+，它会电子松弛，但缺乏偶极矩，不会振动辐射松弛。PI和他的团队将生成每个分子的最低振动水平的总解离重组速率系数与温度的关系，以及各种可能的输出最终通道的分支比率。这样和相关的数据也将在天文学的其他领域，如行星科学中找到应用，例如，对于16O16O+，他们可能能够生成这些数据作为振动水平的函数，这是火星大气研究所需要的。这个项目有一个持续的国际合作和对教育的坚定承诺，包括与哥伦比亚大学科学教师暑期研究项目合作，为史坦顿岛的高中教师提供研究经验。

The PI and his team will measure dissociative recombination branching ratios and rate coefficients for important chemical molecular ion reactions in cold interstellar clouds. This work on dissociated recombination reaction rates and cross-sections is needed to understand the molecular inventory and cooling of the interstellar medium, and to put astrochemical models onto a firm base.Molecules are key components of diffuse, translucent and dense molecular clouds, hot cores, photon dominated regions, protostellar disks, protoplanetary disks, planetary and satellite ionospheres, cometary comae, and circumstellar envelopes around dying starsModels and interpretations of the chemical composition, charge balance, emission and/or absorption spectra, and thermal structure in the various astronomical environments depend on reliable knowledge of the underlying molecular collisions which control these properties. Among the dissociative recombination reactions the primary neutralizing reactions for molecules in cosmic plasmas are particularly important. Dissociative recombination involving ion-molecule reactions is often the terminating step for particular synthesis pathways in chemical networks. One needs to know rates and branching ratios for final products in order to understand reaction pathways, and to understand whether a compound can be produced in the gas phase or if grain surface chemistry must be invoked. If the end products of dissociative recombination are energetic, they can collisionally heat the plasma; if they are in excited states, they can cool the gas through radiative relaxation.The experiments are done at the unique heavy-ion Test Storage Ring (TSR) facility at the Max-Planck Institute in Heidelberg, Germany. The ions to be studied are HF+, H2F+, CF+, 16O16O+, 18O16O+, HSiO+, CH2O+, and CH3O+. They are selected based on their importance for ground-based spectroscopic observations combined with astrochemical modeling studies. The selected molecules can be stored long enough in TSR to cool to their lowest electronic and vibrational levels, except for 16O16O+ which will electronically relax but lacks a dipole moment and will not vibrationally radiatively relax. The PI and his team will generate total dissociative recombination rate coefficients versus temperature for the lowest vibrational level of each molecule as well as branching ratios for the various possible outgoing final channels. Such and related data will also find applications in other areas of astronomy such as planetary sciences, e.g., for 16O16O+, they may be able to generate these data as a function of vibrational level, which is needed for martian atmosphere studies. This project has an ongoing international collaboration and strong commitment to education and involves offers a research experience to high school teachers in Staten Island in collaboration with the Columbia Summer Research Program for Science Teachers.