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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