Antimatter

Positron interactions with atoms and molecules

Positron is the simplest piece of antimatter. Its characteristic signature is usually in the form of two 511 keV gamma quanta produced when it annihilates with an electron. The annihilation signal is key to almost all positron studies and applications, from detecting antimatter in our galaxy to detecting the presence of tumors or onset of Alzheimer's desease in PET (Positron Emission Tomography) scans, or as a diagnostic in heavy-ion radiotherapy. Positron annihilation is used to map the shape and topology of Fermi surfaces in metals and probe intramolecular vibrational relaxation in polyatomic molecules. The simplest neutral matter-antimatter system, the atom of Positronium (Ps), which is a bound electron-positron pair, is employed to measure free volume or detect phase transitions in microvoids in various materials. Positrons or Ps is also one of the key ingredients (together with antiprotons) for producing antihydrogen, which is actively pursued at CERN.

The basic fundamental process of electron-positron annihilation is described by Quantum Electrodynamics (QED). Positronium annihilation and spectroscopic studies provide some of the stringent tests for testing of this quantum field theory. When positrons annihilate with electrons in the laboratory or in space, the electrons are usually not free, but bound in atoms or molecules, or occupy certain energy bands in solids. Prior to annihilation, the positrons normally experience many ionising and other inelastic collisions, and slow down to eV or lower, thermal, room-temperature energies.

The interaction of slow positrons with an electronic system, be this an atom or a molecule, is strongly affected by the collective response of the electronic "cloud" to the presence of the positron (so-called correlations). In molecules, excitation of vibrational, and in some case, rotational, degrees of freedom can also have a dramatic effect on positron annihilation. Understanding these interactions, both qualitatively and quantitatively, has been the main theme of our research.

For atoms, one of the best tools for understanding the interactions of slow positrons with atoms is many-body theory [1]. It enables one to achieve an excellent description of elastic scattering, annihilation rates and gamma-ray spectra in atoms, both simple and complex (e.g., noble-gas atoms) and positions ions [2-4]. A very interesting feature of the positron-atom interactions is the ability of electron-positron correlations to overcome the repulsive electrostatic positron-atom potential, and give rise to positron bound states. The initial predictions [5] made against the strong scepticism among the positron community, gave way to widely accepted acknowledgement of the importance of positron binding to neutral atomic and molecular species. Accurate predictions of positron-atom binding energies for most, especially open-shell species is still an open question, and we have recently used linearised couple-cluster many-body theory method to advance in this direction [6]. In spite of a wealth of theoretical predictions, positron-atom bound states have not been observed experimentally, because of the associated difficulties. We have proposed some schemes which should enable to do this using existing technologies [7,8]. Many-body theory also proved to be an excellent tool for calculating positron bound states with negative ions [9].

Our understanding in the area of positron annihilation in molecules, which remained a big puzzle for half-a-century, has seen a rapid advance over the past ten years. This came as a result of concerted efforts by the experimental group of Prof Cliff Surko (University of California in San Diego), and Queen's theorists, as well as other efforts worldwide. The main and largely unexpected and surprising feature of our findings is the fundamental role played by positron capture in vibrational Feshbach resonances (VFR) that occurs for most polyatomic molecules (see our review [10]). This process is underpinned by the strong positron-molecule attraction and binding, one hand, and the ability of positrons to effectively excite nuclear vibrations, in spite of the huge difference in masses. A further ingredient of the VFR annihilation mechanism that produces orders-of-magnitude enhancement in larger polyatomics, is the intramolecular vibrational energy redistributions (IVR). This process is a paradigm in most of the usual, electron-molecule collisions and chemistry. Positron annihilation through VFR serves as a unique timing signal, providing an additional means for studying this important process [11,12,13].

Encouraged by our near-complete understanding of the details of positron annihilation in atoms, and the corresponding gamma-ray spectra, we have analysed the key features which affect positron gamma-ray spectra in molecules [14]. This is a first step in the development of modern quantum chemistry approaches to the problem of positron-molecule annihilation spectra [15].

  1. G. F. Gribakin and J. Ludlow, Many-body theory of positron-atom interactions. Phys. Rev. A, 70, 032720 (2004).
  2. D. G. Green and G. F. Gribakin, Positron scattering and annihilation in hydrogenlike ions, Phys. Rev. A 88, 032708 (2013).
  3. D. G. Green, Ph.D. thesis, Queen's University Belfast, 2011.
  4. D. G. Green, J. A. Ludlow, and G. F. Gribakin [Phys Rev. A (to be published)].
  5. V. A. Dzuba, V. V. Flambaum, G. F. Gribakin, and W. A. King. Bound states of positrons and neutral atoms. Phys. Rev. A 52, 4541-6 (1995).
  6. V. A. Dzuba, V. V. Flambaum, G. F. Gribakin, and C. Harabati, Relativistic linearized coupled-cluster single-double calculations of positron-atom bound states, Phys. Rev. A 86, 032503 (2012).
  7. V. A. Dzuba, V. V. Flambaum and G. F. Gribakin, Detecting positron-atom bound states through resonant annihilation, Phys. Rev. Lett. 105, 203401 (2010).
  8. C. M. Surko, J. R. Danielson, G. F. Gribakin and R. E. Continetti, Measuring positron-atom binding energies through laser-assisted photorecombination, New J. Phys. 14, 065004 (2012).
  9. J. Ludlow and G. Gribakin, Many-body theory calculations of positron binding to negative ions, Int. Rev. At. Mol. Phys. 1, 73 (2010).
  10. G. F. Gribakin, J. A. Young and C. M. Surko, Positron-molecule interactions: Resonant attachment, annihilation, and bound states, Rev. Mod. Phys. 82, 2557-2607 (2010).
  11. J. A. Young, G. F. Gribakin, C. M. R. Lee and C. M. Surko, Role of combination vibrations in resonant positron annihilation, Phys. Rev. A 77, 060702 (2008).
  12. G. F. Gribakin and C. M. R. Lee, Positron annihilation in large polyatomic molecules. The role of vibrational Feshbach resonances and binding, Eur. Phys. J. D 51, 51 (2009).
  13. A. C. L. Jones, J. R. Danielson, M. R. Natisin, C. M. Surko and G. F. Gribakin, Ubiquitous nature of multimode vibrational resonances in positron-molecule annihilation, Phys. Rev. Lett. 108, 093201 (2012).
  14. D. G. Green, S. Saha, F. Wang, G. F. Gribakin and C. M. Surko, Effect of positron-atom interactions on the annihilation gamma spectra of molecules, New J. Phys. 14, 035021 (2012).
  15. F. Wang, X. G. Ma, L. Selvam, G. F. Gribakin and C. M. Surko, Effects of quantum chemistry models for bound electrons on positron annihilation spectra for atoms and small molecules, New J. Phys. 14, 085022 (2012).