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., {RMP} {\bf 82}, 2557 (2010).} Recent measurements have focused on molecules with large permanent dipole moments (i.e., $\mu > 2.5$ D), including aldehydes, ketones, and nitriles.\footnote{J. R. Danielson, et al., {PRA} {\bf 85}, 022709 (2012).} Positron binding to these molecules is compared to the analogous weakly bound electron-molecule states, commonly referred to as ``dipole-bound'' negative anions.\footnote{N.~I.~Hammer, et al., {JCP} {\bf 119}, 3650 (2003).} Positron binding energies are found to be one to two orders of magnitude larger than those of the analogous negative ions due to two effects: the orientation of the molecular dipole allows the positron to approach it more closely; and for positrons, lepton correlations (e.g., via polarizability) contribute more strongly.\footnote{J. R. Danielson, et. al, {PRL} {\bf 109}, 113201 (2012)} Comparisons to available calculations will be presented, as will comparisons to binding to molecules with $\mu\sim 0$ (e.g., polarizability bound states).