Positron-molecule interactions: Resonant attachment, annihilation, and bound states

Abstract

This article presents an overview of current understanding of the interaction of low-energy positrons with molecules with emphasis on resonances, positron attachment, and annihilation. Measurements of annihilation rates resolved as a function of positron energy reveal the presence of vibrational Feshbach resonances ͑VFRs͒ for many polyatomic molecules. These resonances lead to strong enhancement of the annihilation rates. They also provide evidence that positrons bind to many molecular species. A quantitative theory of VFR-mediated attachment to small molecules is presented. It is tested successfully for selected molecules ͑e.g., methyl halides and methanol͒ where all modes couple to the positron continuum. Combination and overtone resonances are observed and their role is elucidated. Molecules that do not bind positrons and hence do not exhibit such resonances are discussed. In larger molecules, annihilation rates from VFR far exceed those explicable on the basis of single-mode resonances. These enhancements increase rapidly with the number of vibrational degrees of freedom, approximately as the fourth power of the number of atoms in the molecule. While the details are as yet unclear, intramolecular vibrational energy redistribution ͑IVR͒ to states that do not couple directly to the positron continuum appears to be responsible for these enhanced annihilation rates. In connection with IVR, experimental evidence indicates that inelastic positron escape channels are relatively rare. Downshifts of the VFR from the vibrational mode energies, obtained by measuring annihilate rates as a function of incident positron energy, have provided binding energies for 30 species. Their dependence upon molecular parameters and their relationship to positron-atom and positron-molecule binding-energy calculations are discussed. Feshbach resonances and positron binding to molecules are compared with the analogous electron-molecule ͑negative-ion͒ cases. The relationship of VFR-mediated annihilation to other phenomena such as Doppler broadening of the gamma-ray annihilation spectra, annihilation of thermalized positrons in gases, and annihilation-induced fragmentation of molecules is discussed. Possible areas for future theoretical and experimental investigation are also discussed.

Ubiquitous Nature of Multimode Vibrational Resonances in Positron-Molecule Annihilation

“…The experimental procedures are described in [1]. An energy tunable, trap-based positron beam is used that has a total energy spread $40 meV (FWHM).…”

“…A theory has been developed to explain quantitatively VFR-enhanced annihilation rates in selected small molecules [2]. However the details of the IVR enhancements remain sketchy [1]. Described here are new data for positron annihilation resolved as a function of positron energy.…”

“…DOI: 10.1103/PhysRevLett.108.093201 PACS numbers: 34.80.Uv, 34.50.Às, 34.80.Lx Positron annihilation on atoms and molecules is important in many areas of science and technology including positron emission tomography in medicine, astrophysics, and studies of material properties. For many molecules, the annihilation rates as a function of incident positron energy " exhibit vibrational Feshbach resonances (VFRs) (e.g., populated via excitation of infrared-active modes) [1]. These resonances occur at specific values of incident positron energy given by…”

“…For many molecules, the annihilation rates as a function of incident positron energy " exhibit vibrational Feshbach resonances (VFRs) (e.g., populated via excitation of infrared-active modes) [1]. These resonances occur at specific values of incident positron energy given by…”

Characterization of the NEPOMUC primary and remoderated positron beams at different energies

“…Experiments using positrons are of broad, interdisciplinary interest for example in materials science [1], atomic and molecular physics [2], positronium (Ps) physics [3], fundamental research on matter-antimatter symmetry [4,5] and surface physics [6]. Another novel topic is the creation of an electronpositron plasma in the laboratory, as suggested by some of the current authors [7,8].…”