Positron bound states and annihilation in polyatomic molecules

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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.