Energy-resolved positron annihilation for molecules
- Department of Physics, University of California, San Diego, California 92093-0319 (United States)
This paper presents an experimental study designed to address the long-standing question regarding the origin of very large positron annihilation rates observed for many molecules. We report a study of the annihilation, resolved as a function of positron energy ({delta}E{approx}25 meV, full width at half maximum) for positron energies from 50 meV to several eV. Annihilation measurements are presented for a range of hydrocarbon molecules, including a detailed study of alkanes, C{sub n}H{sub 2n+2}, for n=1-9 and 12. Data for other molecules are also presented: C{sub 2}H{sub 2}, C{sub 2}H{sub 4}; CD{sub 4}; isopentane; partially fluorinated and fluorinated methane (CH{sub x}F{sub 4-x}); 1-fluorohexane (C{sub 6}H{sub 13}F) and 1-fluorononane (C{sub 9}H{sub 19}F). A key feature of the results is very large enhancements in the annihilation rates at positron energies corresponding to the excitation of molecular vibrations in larger alkane molecules. These enhancements are believed to be responsible for the large annihilation rates observed for Maxwellian distributions of positrons in molecular gases. In alkane molecules larger than ethane (C{sub 2}H{sub 6}), the position of these peaks is shifted downward by an amount {approx}20 meV per carbon. The results presented here are generally consistent with a physical picture recently considered in detail by Gribakin [Phys. Rev. A 61, 022720 (2000)]. In this model, the incoming positron excites a vibrational Feshbach resonance and is temporarily trapped on the molecule, greatly enhancing the probability of annihilation. The applicability of this model and the resulting enhancement in annihilation rate relies on the existence of positron-molecule bound states. In accord with this reasoning, the experimental results presented here provide the most direct evidence to date that positrons bind to neutral molecules. The shift in the position of the resonances is interpreted as a measure of the binding energy of the positron to the molecule. Other features of the results are also discussed, including large, qualitative changes in the annihilation spectra observed when hydrocarbon molecules are fluorinated.
- OSTI ID:
- 20633842
- Journal Information:
- Physical Review. A, Vol. 67, Issue 3; Other Information: DOI: 10.1103/PhysRevA.67.032706; (c) 2003 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA); ISSN 1050-2947
- Country of Publication:
- United States
- Language:
- English
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