Positron bound states and annihilation in polyatomic molecules

正电子是最简单和最丰富的反物质粒子，也是第一个被发现的粒子。1930年，狄拉克在分析统一了量子力学和相对论的著名方程的解时，发现了“反电子”，预言了它的存在。1932年，安德森在宇宙射线实验中发现了正电子，两位科学家都在不久后获得了诺贝尔奖。正电子和其他反粒子是基本粒子理论(如标准模型)的基石，也是大爆炸理论和我们对宇宙的理解的重要组成部分。它们也是我们周围世界物质-反物质不对称这一根本未解问题的根源。反物质最显著的特征是它与物质相遇时能够湮灭。在正电子和电子的情况下，它们的湮灭导致伽马射线的爆发。这种独特的信号使人们能够探测到我们银河系中正电子的存在，并支撑着地球上许多关键的正电子技术。例如，正电子寿命和湮没伽马射线光谱仪被广泛用于材料表征，以及正电子发射断层扫描(PET)，这是一种用于癌症的医学成像和功能诊断，神经成像(例如，阿尔茨海默病)等。在银河源或加速器或放射性同位素产生的瞬间，正电子速度很快。在湮灭之前，它们通常会经历一系列快速的电离碰撞，导致它们的减速和热化。S在1950年对正电子湮没的早期研究发现，正电子与多原子分子的湮灭发生得比基于可用电子计数的预期要快得多。这一现象一直是正电子物理学的一个未解之谜，直到本世纪初，由于理论(主要是由于Pi的工作)和实验的共同努力，这种现象的真实物理图景开始显现。我们现在知道，分子中的正电子湮没是一个分两步进行的过程，在这个过程中，正电子首先通过将其能量转移到分子骨架的振动而经历共振俘获进入束缚态。尽管时间短暂，但这种“陷阱”极大地增加了正电子与电子相遇及其湮灭的几率。正电子湮没共振的实验研究已经提供了70多个分子的正电子束缚态能量的信息。然而，理论上对正电子与大多数多原子分子结合的描述还很少，正电子与多原子分子结合是一个难题。到目前为止，标准的量子化学方法甚至无法定量解释最明显的趋势，如烷烃的结合能随分子尺寸的线性增加。这里的主要困难是由正电子和原子电子的相关运动驱动的微妙的束缚。在这个项目中，我们基于我们以前在正电子-原子相互作用方面的经验，计划发展一种新的方法来研究正电子-分子相互作用。我们将构建正电子-分子关联势，并使用一小部分实验数据对其进行“校准”。这应该能够计算目前所研究的大多数分子的正电子束缚态，并使我们能够对许多其他分子进行预测。束缚态的计算也将使我们能够计算湮没伽马射线谱，这仍然是一个悬而未决的问题。研究不同分子电子对正电子湮没的选择性将使我们进一步了解分子离子中的能量沉积，以及分子湮灭后分子碎裂的特征模式，这将是对正电子-分子湮灭的轻描淡写的重大进展。

The positron is the simplest and most abundant antimatter particle, and the first to have been found. Its existence was predicted in 1930 by Dirac who discovered "anti-electrons" while analysing solutions of his famous equation which unified Quantum Mechanics and Relativity. In 1932 positrons were discovered experimentally discovery in cosmic rays by Anderson, both scientists awarded the Nobel prize soon afterwards.Positrons and other antiparticles are cornerstones of elementary particle theories, such as the Standard Model, and an essential part of the Big Bang theory and our understanding of the Universe. They are also at the root of a fundamental unanswered question of matter-antimatter asymmetry of the world around us.The most striking feature of antimatter is its ability to annihilate upon encounters with matter. In the case of positrons and electrons, their annihilation results in a burst of gamma rays. This unique signal enables one to detect the presence of positrons in our Galaxy, and underpins a number of crucial positron technologies on Earth. Examples of these are positron-lifetime and annihilation gamma-ray spectroscopies used widely for material characterisation, and positron-emission tomography (PET), a type of medical imaging and functional diagnostic for cancer, neuroimaging (e.g., for Alzheimer's disease), etc.At the instant of production by Galactic sources or in accelerators or radioactive isotopes, the positrons are fast. Before annihilation they typically undergo a quick succession of ionising collisions with matter that leads to their slowing down and thermalisation. The early studies of positron annihilation in 1950's uncovered that positron annihilation with polyatomic molecules occurred much more quickly than could be expected based on counting the available electrons. This phenomenon remained a unsolved puzzle of positron physics until early this century, when the true physical picture of this phenomenon began to emerge, due to a concerted effort of theory (chiefly, due to PI's work) and experiment.We now know that positron annihilation in molecules proceeds as a two-step process, in which the positron first undergoes resonant capture into a bound state by transferring its energy into the vibrations of the molecular framework. Although temporal, such "trapping" dramatically increases the probability of positron encounters with electrons and its annihilation. Experimental studies of positron annihilation resonances have provided information on positron bound-state energies for more than 70 molecules. However, theoretical understating of positron binding to most polyatomic molecules is lacking.Positron binding to polyatomic molecules is a difficult problem. So far standard quantum-chemistry methods have been unable to explain quantitatively even the most obvious trends, such as the linear increase of the bind energy with molecular size for alkanes. The main difficulty here is in the subtlety of binding, which is driven by correlated motion of the positron and atomic electrons.Based on our previous experience in positron-atom interactions, in this project we plan develop a new approach to positron-molecule interactions. We will construct positron-molecule correlation potentials and "calibrate" them using a small subset of experimental data. This should enable calculation of positron bound-states for the majority of molecules studied so far, and will allow us to make predictions for many other molecules. Calculation of the bound states will also allow us to compute the annihilation gamma-ray spectra, which remains an unsolved problem. Studying the selectivity of positron annihilation with various molecular electrons will further allow us to understand the energy deposition in molecular ions that are formed, and of characteristic patterns of molecular fragmentation that follows annihilation.This will represent a major advance in understating positron-molecule annihilation.