Comparison of positron and electron binding to molecules

Positrons can attach to molecules via Feshbach resonances in which a vibrational mode absorbs the excess energy. Using a high-resolution positron beam, this process has been used to measure positron-molecule binding energies for many chemical species.\footnote{G. F. Gribakin, et al., Rev. Mod. Phys. {\bf 82}, 2557 (2010).}$^,$\footnote{J. R. Danielson, et al., Phys. Rev. A, in press (2012).} In particular, recent measurements have focused on molecules with large permanent dipole moments (i.e., $\mu > 2.5$ D), including aldehydes, ketones, and nitriles. Positron binding to these molecules is compared to the analogous weakly bound electron-molecule (negative-ion) states, commonly called ``dipole-bound'' states.\footnote{N.~I.~Hammer, et al., J. Chem. Phys. {\bf 119}, 3650 (2003).} Positron binding energies are found to be one to two orders of magnitude larger than those of the negative ions due to two effects: the orientation of the molecular dipole moment allows the positron to approach it more closely; and for positrons, lepton correlations (e.g., via dipole polarizability) contribute more strongly. Comparisons to available calculations will be presented, as will comparisons to binding to molecules with $\mu\sim 0$ (e.g., polarizability bound states).