Role of inelastic escape channels in Feshbach-resonant positron annihilation

Energy-resolved positron-molecule annihilation spectra show that positrons can bind to molecules via the vibrational Feshbach resonance (VFR) mechanism [1,2]. Here we take a closer look at VFR in large molecules, paralleling the talk by J. A. Young (this meeting). After normalizing out a weak dependence on binding energy, the magnitudes of C-H stretch mode resonances in large hydrocarbons obey a power-law scaling with the number of vibrational degrees of freedom [3]. The only known outliers are partially fluoronated alkanes. New results are presented for these molecules showing that this behavior can be explained by a strong inelastic channel. For ordinary alkanes, we show that annihilation data taken using 300~K thermal positrons is quantitatively consistent with data taken at higher positron energies. The role played by intramolecular vibrational relaxation (IVR) will also be discussed. \\ ~[1] L. D. Barnes, J. A. Young, and C. M. Surko, \textit{Phys. Rev. A} {\bf 74}, 012706 (2006). \\ ~[2] G. F. Gribakin and C. M. R. Lee, \textit{Phys. Rev. Lett.} {\bf 97}, 193201 (2006). \\ ~[3] J. A. Young and C. M. Surko, \textit{Phys. Rev. Lett.} {\bf 99}, 133201 (2007).