Electron and photon induced dynamics of astrochemical molecules

自从75年前第一个分子在星际介质中被发现以来，我们对天体化学的理解有了显著的提高。然而，在我们完全了解星际气体云内的复杂环境之前，还有很长的路要走。由于在太空中研究分子是不现实的，因此必须使用实验室研究来获取发展和完善天体化学模型所需的数据为了准确地模拟电离，需要总电离和部分电离截面（概率），以及对碎片动力学的理解，即碎片动能释放和角分布。我们的实验室条件很好，可以详细研究电子和光电离。该项目的主要目标是对各种在地球条件下稳定的天体化学分子中的电子诱导和光子诱导过程进行实验和理论研究。这些研究的结果，连同小组其他成员的研究结果，将被用来为所研究的分子建立一个电离截面、分支比和碎片动能释放分布的在线数据库。总电子电离截面将被测量为电子能量的函数使用束流气体仪器已经在我们的实验室运行。这些横截面量化了母离子电离的概率，但没有给出形成的碎片离子的信息。部分电离截面给出了形成特定片段离子的概率，并且可以在我们的实验室中使用配备速度图成像检测的电子-分子交叉束仪器作为电子能量的函数进行测量。在这些测量中获得的部分电离截面只是相对值，但可以通过归一化到束气仪中测量的总电离截面来转换为绝对值。在开始测量部分电离截面和碎片动能释放之前，我们的电子-分子交叉束仪器需要进行一些重新调试工作。这将包括优化电子束和分子束，以及安装一个像素成像质谱（PImMS）相机。PImMS相机是一种具有12.5 ns时间分辨率的快速成像传感器，当用于飞行时间实验时，允许在每次采集期间对所有碎片进行成像。这将大大减少采集时间，并允许在同一过程中形成的不同离子的运动之间的相关性被检测。离子成像探测器将配备一个新的超快闪烁体来取代现有的P47荧光粉。新闪烁体衰减时间的缩短将提高实验的质量分辨率。使用Kim和Rudd的二元相遇-贝特（BEB）模型[3,4]的理论研究也将进行，该模型用于预测总电子电离截面，最初特别关注优化含有第三排原子的分子所需的校正。还将尝试扩展该模型来预测部分电子电离截面。还将考虑其他模型，并将结果与BEB模型进行比较。我们的目标是在理论和实验之间取得良好的一致性，这样我们就可以预测分子的电离截面，这将是一个实验研究的挑战。该项目属于EPSRC化学反应动力学和机理、计算和理论化学、分析科学以及等离子体和激光研究领域。

Since the first molecules were identified in the interstellar medium 75 years ago,[1] our understanding of astrochemistry has increased significantly. However, there is still a long way to go before we are even close to achieving a full understanding of the complex environment within an interstellar gas cloud. Due to the impracticality of studying molecules in space, laboratory-based studies must be used to acquire the data required to develop and refine astrochemical models.[2] To model ionisation accurately, both total and partial ionisation cross sections (probabilities), as well as an understanding of the fragmentation dynamics, i.e. the fragment kinetic energy release and angular distribution, are required. Our laboratory is well placed to study both electron and photoionisation in detail.The primary objective of this project will be the experimental and theoretical study of electron-induced and photon-induced processes in a variety of astrochemical molecules that are stable under terrestrial conditions. The results of these studies, along with those carried out by other members of the group, will be used to produce an online database of ionisation cross sections, branching ratios, and fragment kinetic energy release distributions for the molecules studied.Total electron ionisation cross sections will be measured as a function of electron energy using a beam-gas instrument already operational within our laboratory. These cross sections quantify the probability of ionising the parent ions, but give no information on the fragment ions formed. Partial ionisation cross sections give the probability of forming a particular fragment ion, and can be measured in our laboratory as a function of electron energy using an electron-molecule crossed beam instrument equipped with velocity-map imaging detection. The partial ionisation cross-sections obtained in these measurements are only relative values, but can be converted to absolute values by normalising to the total ionisation cross section measured in the beam-gas instrument.Before partial ionisation cross section and fragment kinetic energy release measurements can begin, some recommissioning work is needed on our electron-molecule crossed beam instrument. This will include optimising both the electron and molecular beams, as well as installing a Pixel Imaging Mass Spectrometry (PImMS) camera. The PImMS camera is a fast imaging sensor with a 12.5 ns time resolution which, when used in a time of flight experiment, allows all fragments to be imaged during each acquisition. This will significantly reduce acquisition time, and allow correlations between the motions of different ions formed in the same process to be detected. The ion imaging detector will be fitted with a new ultrafast scintillator to replace the existing P47 phosphor. The reduced decay time of the new scintillator will improve the mass resolution of the experiment. Theoretical studies using Kim and Rudd's binary-encounter-Bethe (BEB) model,[3,4] used to predict total electron ionisation cross-sections, will also be carried out, with a particular focus initially on optimising a correction required for molecules containing third-row atoms. An attempt will also be made to extend the model to predict partial electron ionisation cross sections. Other models will also be considered, and the results compared with the BEB model. Our aim is to achieve good agreement between theory and experiment, so that we can predict ionisation cross sections of molecules which would be a challenge to study experimentally.This project falls within the EPSRC chemical reaction dynamics and mechanisms, computational and theoretical chemistry, analytical science and plasma and lasers research areas.