Bulletin of the American Physical Society
52nd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 66, Number 6
Monday–Friday, May 31–June 4 2021; Virtual; Time Zone: Central Daylight Time, USA
Session Q01: New Physics with Positrons and PositroniumInvited Live
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Chair: Ilya Fabrikant, U. Nebraska - Lincoln |
Thursday, June 3, 2021 8:00AM - 8:30AM Live |
Q01.00001: Precision spectroscopy with positronium Invited Speaker: David B Cassidy
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Thursday, June 3, 2021 8:30AM - 9:00AM Live |
Q01.00002: Many-body theory of positron binding and annihilation in polyatomic molecules Invited Speaker: Dermot G Green Whilst positron binding energies have been measured via resonant annihilation spectra for over ~80 molecules in the past two decades [1], an accurate ab initio theoretical approach has remained elusive. Measured and calculated binding energies exist together for only 6 molecules. For these, standard quantum chemistry approaches have proved deficient, agreeing with experiment to at best 25% accuracy. The theoretical difficulty lies in the need to accurately describe strong positron-molecule many-body correlations including polarisation, screening of the positron-molecule Coulomb interaction, and the unique process of virtual-positronium formation (where a molecular electron temporarily tunnels to the positron). Their specific role in positron-molecule binding has not yet been elucidated and is not well understood. Accurately accounting for the correlations is a formidable task even for positron-atom interactions, but molecules bring additional formidable computational challenges. |
Thursday, June 3, 2021 9:00AM - 9:30AM Live |
Q01.00003: Progress in the study of the positronium negative ion Invited Speaker: YASUYUKI NAGASHIMA Electrons can bind to their anti-particles, positrons, to form positronium (Ps). It is the lightest atom composed of two leptons and used for atomic physics and material sciences. It also provides testing ground for quantum electrodynamics. |
Thursday, June 3, 2021 9:30AM - 10:00AM Live |
Q01.00004: Positron-Molecule Annihilation Resonances Beyond The Fundamental Vibrational Modes. Invited Speaker: Soumen Ghosh Low energy positrons ($< 0.5$ eV) bind to molecules through Feshbach resonant excitation of dipole- or quadrupole-active fundamental vibrational modes, and this leads to greatly enhanced annihilation rates. Recently, new annihilation resonances were observed involving vibrational modes beyond the fundamentals. In this talk, evidence for these resonances will be presented for several n-alkane and cycloalkane molecules. These new resonances are compared to infrared absorption spectra and they appear to be due to combinations and/or overtones of the fundamental vibrations. For molecules with binding energies larger than that of the highest lying fundamental mode, such multimode processes are expected to dominate the annihilation spectrum. An example is the polycyclic aromatic hydrocarbon (PAH) molecules, a potentially important component of annihilation in the interstellar medium. A room temperature (FWHM $\sim 36$ meV), as well as a cryogenically cooled (FWHM $\sim 20$ meV), positron beam is used in these studies. Implications of the energy distributions of these beams in determining the shape of the observed resonances and remaining open questions will be discussed. |
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