State-resolved studies of the vibronic structure and reaction dynamics of molecular dications

带有两个正电荷的分子，即分子双阳离子，具有不寻常的性质。首先，由于两个电荷的排斥，分子具有很高的能量。这种高能量意味着它们对单电荷离子的反应不同。其次，两个正电荷之间的排斥意味着这些双电荷分子中的许多分子寿命很短，它们会分裂成一对单电荷离子。然而，对于大多数小分子来说，尽管正电荷对之间存在排斥力，但它们的电子的某些排列提供了足够强的化学键将分子保持在一起。这些电子的排列产生了所谓的长寿命（亚稳态）的分子双价电子态。在这些亚稳态中，许多小分子双价阳离子可以存活足够长的时间，以与中性分子碰撞并发生化学反应。本申请提出了对申请人开发的用于研究分子指示物的化学反应的一件装置的广泛升级。该装置通过识别通常在阳离子化学反应中形成的两种带电产物来操作。该仪器使用专门设计的质谱仪来检测和识别来自单个阳离子反应的两种带电产物;所谓的符合实验。在质谱仪中使用位置敏感检测器允许实验精确地确定两个产物离子在单个反应事件之后如何移动。这种对产物离子运动的研究，即所谓的反应过程动力学，为化学反应的机理提供了强有力的探索。在它目前的配置中，实验已经揭示了广泛的双阳离子的反应性，并表明双阳离子反应通过不寻常的途径进行。然而，该实验可以探测反应产物的能量的准确性目前是有限的，并且本申请对光谱仪提出了显著的发展以显著地提高该能量分辨率。我们建议安装一个更大面积的探测器和改进的定时电子设备，结合一个更受控制和约束的入口（分子束）的中性分子。这些改进，加上一个新的设计的质谱仪，它采用了速度成像方法，将提高能量分辨率的光谱仪，使能量和反应的个别振动水平的双原子dications可以确定。速度成像是为激光光谱学开发的技术，涉及质谱仪中电场的特殊布置，其允许精确地确定产物离子速度。为了在我们的实验中实现速度成像，该实验不使用激光来测量反应的产物，涉及我们已经开发的质谱仪的新设计，其仍然给出准确的产物离子速度，但不会降低质谱仪识别不同物种的能力。我们建议使用升级后的设备来研究现有技术无法探测的双原子dications的能级，并研究最近被牵连的行星电离层化学中的关键物种的分子dications的反应性和反应机制。

Molecules which carry two positive charges, molecular dications, possess unusual properties. Firstly, due to the repulsion of the two like-charges the molecules are highly energetic. This high energy means that they react differently to singly-charged ions. Secondly, the repulsion between the two positive charges means than many of these doubly charged molecules are very short-lived and they fragment to form a pair of singly-charged ions. However, for most small molecules, certain arrangements of their electrons give strong enough chemical bonds to hold the molecule together despite the repulsion between the pair of positive charges. These arrangements of the electrons generate so-called long-lived ( metastable ) electronic states of molecular dications. In these metastable states many small molecular dications can live for long enough to collide with a neutral molecule and undergo chemical reactions. This application proposes an extensive upgrade to a piece of apparatus developed by the applicant to study the chemical reactions of molecular dications. The apparatus operates by identifying both of the charged products that usually are formed in a dication chemical reaction. The apparatus uses a specially designed mass spectrometer to detect and identify both of the charged products from reaction of an individual dication; a so-called coincidence experiment. Using a position-sensitive detector in the mass spectrometer allows the experiment to precisely determine how both of the product ions are moving following an individual reactive event. Such studies of the motion of the product ions, the so-called dynamics of a reactive process, provide a powerful probe of the mechanism of the chemical reaction. In it's current configuration the experiment has revealed the reactivity of a wide-range of dications and shown that dication reactions proceed by unusual pathways. However, the accuracy with which the experiment can probe the energies of the reaction products is currently limited and this application proposes significant developments to the spectrometer to dramatically improve this energy resolution. We propose to install a larger area detector and improved timing electronics, together the incorporation of a more controlled and constrained inlet (a molecular beam ) for the neutral molecules. These improvements, together with a new design of mass spectrometer which incorporates a velocity imaging methodology, will improve the energy resolution of the spectrometer so that the energies and reactivity of individual vibrational levels of diatomic dications can be determined. Velocity imaging is a technique developed for laser spectroscopy, involving a special arrangement of electric fields in the mass spectrometer, which allows the product ion velocities to be determined accurately. To implement velocity imaging in our experiment, which does not use a laser to ionize the products of the reaction, involves a new design of mass spectrometer, which we have developed, which still gives accurate product ion velocities but does not degrade the ability of the mass spectrometer to identify different species. We propose to use the upgraded apparatus to study the energy levels of diatomic dications which cannot be probed by existing techniques and also to study the reactivity and reaction mechanisms of the molecular dications which have recently been implicated as key species in the chemistry of planetary ionospheres.