Cross section measurements for molecular reactions of fundamental and astrophysical relevance

该项目旨在加深我们对星际介质中致密分子云的理解，这些致密分子云孕育了恒星和行星系统。为此，有必要提高我们对气相天体化学基础的理解。分子云的气相化学演化的模拟直接依赖于单个分子反应的热速率系数的知识。这些反应通常包括一个带电的伙伴和一个中性的伙伴，因为分子云的低温大多不足以克服中性-中性反应的潜在障碍。天体物理相关分子反应的理论模型可能很容易产生高达两倍或更多的不确定性，因此准确的实验确定关键反应速率系数可以显著提高分子云模拟的可靠性。我的目标是实验研究与氢的关键反应之一，其当前的不确定性显著阻碍了我们可靠地模拟和分析稠密分子云中的恒星形成过程的能力。这项实验将在纽约市哥伦比亚大学丹尼尔·萨文博士团队的合并波束装置上进行。该仪器是目前世界上唯一能够测量设想反应的设备。在这里，带正电的分子和中性原子之间的分子反应可以作为相对碰撞速度的函数来研究。利用两个相对速度较小的快束合并技术，可以再现与分子云中的反应相关的低温。通过测量两束的强度和计算反应产物，在绝对尺度上准确地确定了作为相对碰撞速度的函数的合束率系数。具体地说，我将测量相对碰撞能量从3.5 meV到20 eV的反应D+H3+-&GT；H2D++H的合并光束速率系数。与麦克斯韦-玻尔兹曼分布的卷积将从数据中得到热速率系数。将分析这些结果对天体物理模型的影响。

The project aims at deepening our understanding of dense molecular clouds in the interstellar medium, which give birth to stars and planetary systems. For this purpose, it is essential to improve our understanding of the underlying gas-phase astrochemistry. Simulations of the gas-phase chemical evolution of molecular clouds rely directly on the knowledge of thermal rate coefficients for the individual molecular reactions. These reactions typically involve a charged and a neutral partner, since the low temperatures of molecular clouds are mostly not sufficient to overcome the potential barrier of neutral-neutral reactions. Theoretical models for astrophysically relevant molecular reactions may easily reach uncertainties up to a factor of two or more, and hence accurate experimental determination of key reaction rate coefficients can significantly enhance the reliability of molecular cloud simulations.My objective is to experimentally study one of the key reactions with deuterium, whose current uncertainty significantly hinders our ability to reliably model and analyze the star formation process in dense molecular clouds. The experiment will be performed at the merged-beams apparatus of the group of Dr. Daniel Savin at Columbia University, New York City. This apparatus is the only facility world-wide, which is currently capable of measuring the envisaged reaction. Here, molecular reactions between positively charged molecules and neutral atoms can be studied as a function of the relative collision velocity. Using the technique of merging two fast beams with small relative velocities, the low temperatures relevant for the reactions in molecular clouds can be reproduced. By measuring the intensities of both beams and counting the reaction products, the merged-beams rate coefficient as a function of the relative collision velocity is determined accurately on an absolute scale. Specifically, I will measure the merged-beams rate coefficient for the reaction D + H3+ -> H2D+ + H for relative collision energies from 3.5 meV to 20 eV. The convolution with a Maxwell-Boltzmann distribution will yield the thermal rate coefficient from the data. Implications of the results on astrophysical models will be analyzed.