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。单电荷的氢分子离子（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，也可能在其他环境中短暂存在，包括活的有机体。结合两个原子的机制非常类似于一个卷曲的弹簧，允许原子相对于另一个移动而不会偏离轨道。根据量子力学的基础知识，双原子分子离子在不同温度下以一系列离散的量子态连续振动，这些量子态由整数定义，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，并且与温度计相当，可以读取冷物体的低温和热物体的高温。它还将提供数据，帮助理解振动状态在与其他粒子相互作用时如何影响它们的环境。这些缓慢的相互作用，技术上被称为低能相互作用，目前还没有得到很好的理解，它们在星际介质、核聚变等离子体的冷部分以及DNA链断裂等过程中发挥着重要作用。因此，这个项目将促进科学的进步，并可能对社会重要的广泛领域产生影响；与此同时，它将使参与的本科生有机会在自己的机构参与复杂的3d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许多学科。首先，它在基础物理学中非常重要，因为它涉及到最小的原子。在聚变托卡马克的冷转移等离子体区域或主要成分为中性氢、正氢离子和H分子的星际云等环境中，它也是主导反应之一。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，这些系统存在于生物物理学中，其中对生物分子（如DNA）的自由基攻击可能涉及极低能量的电荷转移。然而，将实验室测量的截面与现有的理论和计算进行比较通常几乎是不可能的，因为分子的振动状态分布是未知的。提出的三维成像设备将最终通过振动分辨来改进以前测量的绝对截面，从而使理论和实验结果之间进行更详细的比较。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量。探测子粒子的位置和它们的飞行时间差异使得探测器可以通过简单的动力学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片段的时间分辨快照图片（因此称为三维成像）。整个三维成像设备被设想为一种便携式设备，可以很容易地运输到各种研究机构并在其上使用。

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.