Positron Binding to Molecules: Interplay between permanent dipole moments and polarizability

Energy resolved studies of positron-molecule collisions exhibit vibrational Feshbach resonances in annihilation, thus providing evidence that positrons can bind to these pecies.\footnote{G. F. Gribakin, et al., Rev. Mod. Phys. {\bf 82}, 2557 (2010).} The downshifts of the observed resonances provides a measure of the positron-molecule binding energies which range from 1 to 300 meV. Presented here are annihilation spectra and binding energies for a wide range of chemical species, including aldehydes, ketones, formates, acetates, nitriles, alcohols, and halogenated compounds.\footnote{J. R. Danielson, et al., Phys. Rev. A, in press (2012).}$^,$\footnote{A. C. L. Jones, et al., New J. Phys., in press (2012).} Within a group, the measured binding energies often show an approximate linear correlation with molecular dipole polarizability. However, other effects, including the permanent dipole moment ($\mu$) and molecular geometry, play significant roles as well. For example, for compounds with $\mu \geq 2$ D, it appears that localization of the positron wave function leads to enhanced binding and an increased dependence upon both $\mu$ and electron-positron correlations.\footnote{Danielson, op. cit.} The relationship of these results to theoretical calculations is discussed.