Energy-resolved positron annihilation for molecules

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 (ΔE~25meV, 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, CₙH₂ₙ₊₂, for $n=1--9$ and 12. Data for other molecules are also presented: C₂H₂, C₂H₄; CD₄; isopentane; partially fluorinated and fluorinated methane (CHₓF_4-x); 1-fluorohexane (C₆H₁₃F) and 1-fluorononane (C₉H₁₉F). 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 (C₂H₆), the position of these peaks is shifted downward by an amount ~20meV 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.