Colloquia 2006
Positron
annihilation on molecules
Dr.
Gleb Gribakin,
Department of Applied Mathematics and Theoretical Physics
Queen's Univesity, Belfast
Date
4pm,
Thursday, 6 April
Location
School of Physics
Common Room
Room 64 Old Main Building
The University of New South Wales
Abstract
Positron annihilation
in atomic or molecular gases is usually expressed in terms of Zeff -
the effective number of electrons per target atom or molecule, that
contribute to annihilation. Naively, one expects that this number is
comparable to the actual number of target electrons Z. However, it has
been known for half a century that in some polyatomic gases positrons
annihilate very rapidly, with Zeff exceeding Z by several orders of
magnitude! For example, while Zeff of methane (CH4) is 150, the Zeff
of hexane (C6H14) is 120,000. Incidentally, some of the earliest experiments
in this field were performed in 1960's by S. J. Tao at UNSW. Thus he
found that Zeff of molecular chlorine is about 1500.
In
the talk I will describe how this puzzle can be resolved by considering
positron capture into molecular vibrational Feshbach resonances. I will
also show the latest experimental data on positron-molecule annihilation
obtained with a high-resolution positron beam, which give some answers
and pose new questions about this phenomenon.
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