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 . 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  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 . 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 .
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 ). 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 . This is a first step in the development of modern quantum
chemistry approaches to the problem of positron-molecule annihilation
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