Feshbach-resonance-mediated annihilation in positron interactions with large molecules

Measurements of positron-on-molecule annihilation have established that positrons attach to a variety of molecules via vibrational Feshbach resonances VFRs . These resonances, in turn, result in greatly enhanced positron annihilation rates beyond those expected in simple collisions. A recent paper J. A. Young and C. M. Surko, Phys. Rev. Lett. 99, 133201 2007 described experimental studies and analysis of positron annihilation in large molecules. Presented here is a more complete description of those results as well as data and analysis. Positron-molecule binding energy data are presented for twenty-five molecules, including the relatively deeply bound species 1-chlorohexane, benzene, and naphthelene. When a relatively weak dependence of resonant annihilation rate on incident positron energy and binding energy is removed, the resulting rates for hydrocarbons are shown to obey a universal scaling as a function of the number of vibrational degrees of freedom. Previously measured annihilation rates for alkanes using thermal positrons at 300 K are shown to be in quantitative agreement with measurements at higher energies. Results are presented for partially fluorinated hydrocarbons, providing evidence that the unusual suppression of resonant annihilation observed in these molecules occurs via a vibrational deexcitation process. The role of molecular temperature and inelastic escape channels in VFR-mediated positron annihilation in molecules is discussed. The implications of these results for theories of the annihilation process are also discussed.