MRI: Development of a 3-D Imaging for Vibrationally Resolved Cross Section Measurements

该项目的目标是开发一种多功能设备，用于探测小双原子分子离子（由失去一个或多个电子的两个原子组成的粒子）的量子特性。单电荷氢分子离子（两个质子，由一个电子结合在一起）是这种离子的一个简单但基本的例子。双原子分子离子很重要，因为它们在太空中大量存在，也可能短暂地存在于其他环境中，包括生物体。结合两个原子的机制非常类似于螺旋弹簧，允许原子相对于另一个移动而不会迷路。根据量子力学的基础知识，双原子分子离子在不同温度下在一系列离散量子态中持续振动，这些量子态由整数定义，这些整数表示给定尺度上的振动程度，整数越大，脉动越强。所提出的设备探测这些离散状态，相当于温度计阅读低温的冷物体和高温的热的。它还将提供有助于理解振动状态在与其他粒子相互作用期间如何影响其环境的数据。这些缓慢的相互作用，技术上称为低能相互作用，人们还不太了解，但已知它们在星际介质、核聚变等离子体的冷部分以及DNA链断裂过程等方面发挥着重要作用。同时，参与计划的本科生亦有机会在自己的院校参与先进的三维成像设备的开发。氢与双原子分子离子碰撞中的电荷转移涉及从物理科学到生命科学的多个学科。首先，它在基础物理学中是最重要的，因为它涉及最小的原子。它也是环境中的主要反应之一，例如聚变托卡马克的冷偏滤器等离子体区域或星际云中的主要成分是中性H，正氢离子和H分子。此外，理解这个最简单的基本系统是掌握更复杂系统的关键，例如，在生物物理学中，对生物分子如DNA的自由基攻击可能涉及能量非常低的电荷转移。然而，通常几乎不可能将实验室测量的横截面与现有的理论和计算进行比较，因为分子的振动态分布是未知的。拟议的三维成像设备将最终改善以前测量的绝对横截面测量，使他们振动解决，使理论和实验结果之间的更详细的比较。在这种三维成像技术中，分子离子与碱原子进行共振解离电荷交换，并以两个碎片的动能形式释放其振动能。子粒子的位置和它们的飞行时间差的检测使得检测器能够通过简单的动力学来重建分子离子的初始振动能量。 这种检测技术相当于拍摄分子离子碎片的时间分辨快照（因此称为3D成像）。 整个3-D成像装置被设想为便携式设备，其可以容易地运输到各种研究设施并在各种研究设施中使用。

The goal of this project is to develop a versatile device that probes quantum properties of small diatomic molecular ions (particles composed of two atoms that have lost one or more electrons). The singly charged hydrogen molecular ion (two protons, bound together by a single electron) is a simple but fundamental example of such an ion. Diatomic molecular ions are important because they are abundantly found in space, and might also briefly exist in other settings, including living organisms. The mechanism binding the two atoms is very similar to a coiled spring, allowing the atoms to move one with respect to the other without going astray. According to the basics of Quantum Mechanics, a diatomic molecular ion is continually vibrating in a range of discrete quantum states at different temperatures, which are defined by integers that state the degree of vibration on a given scale for which the bigger the integer, the stronger the pulsation. The proposed device probes these discrete states and is comparable to a thermometer reading a low temperature for a cold object and high temperature for hot one. It will also provide data that will help to understand how the vibrational states affect their environment during interaction with other particles. Not well understood, these slow interactions, technically called low energy interactions, are known to play important roles in the interstellar medium, in the cold part of nuclear fusion plasmas, and in the processes of DNA strand breaks, etc. Thus, this project will promote the progress of science and may have implications to a broad spectrum of areas of importance to society; at the same time, it will give participating undergraduate students the opportunity to take part in the development of a sophisticated 3-D imaging device at their own institution.The charge transfer in collisions between hydrogen and diatomic molecular ions touches a variety of disciplines spreading from physical science to life science. First of all, it is of foremost importance in fundamental physics because it involves the smallest atom. It is also one of the dominant reactions in environments such as the cold divertor plasma regions of a fusion tokamak or in interstellar clouds where the main constituents are neutral H, the positive hydrogen ion, and H-molecules. Moreover, understanding of this simplest fundamental system is a key for mastering more complex systems which exist in, e.g., biophysics where radical attacks on biomolecules such as DNA potentially involve charge transfer at very low energy. However, it is often almost impossible to compare laboratory measured cross sections to existing theories and calculations because the vibrational state distribution of the molecules is not known. The proposed 3-D imaging device will ultimately improve previously measured absolute cross section measurements by making them vibrationally resolved, enabling a more detailed comparison between theoretical and experimental results. In this 3-D imaging technique, the molecular ion undergoes a resonant dissociative charge exchange with an alkali atom and releases its vibrational energy in the form of kinetic energy of the two fragments. The detection of the positions of the daughter particles and their flight time differences made possible with the detectors allows the reconstruction of the molecular ion's initial vibrational energy via simple dynamics. This detection technique is equivalent to taking a time resolved snapshot picture of the molecular ion fragments (thus the name 3-D imaging). The whole 3-D imaging apparatus is envisioned to be a portable device which could be easily transported to and used at a variety of research facilities.