Positron binding to molecules

While there is theoretical evidence that positrons can bind to atoms,\footnote{Mitroy, {\it et. al.}, {J. Phys. B} \textbf{35}, R81 (2002).} calculations for molecules are much less precise.\footnote{Strasburger, {J. Chem. Phys.} \textbf{114}, 615 (2001).} Unfortunately, there have been no measurements of positron-atom binding, due primarily to the difficulty in forming positron-atom bound states in two-body collisions. In contrast, positrons attach to molecules via Feshbach resonances (VFR) in which a vibrational mode absorbs the excess energy. Using a high-resolution positron beam, this VFR process has been studied to measure binding energies for more than 40 molecules. New measurements will be described in two areas: positron binding to relatively simple molecules, for which theoretical calculations appear to be possible;\footnote{Danielson, {\it et. al.}, {Phys.~Rev.~Lett.}, \textbf{104}, 233201 (2010).} and positron binding to molecules with large permanent dipole moments, which can be compared to analogous, weakly bound electron-molecule (negative-ion) states. Binding energies range from 75 meV for CS$_2$ (no dipole moment) to 180 meV for acetonitrile (CH$_3$CN). Other species studied include aldehydes and ketones, which have permanent dipole moments in the range 2.5 - 3.0 debye. The measured binding energies are surprisingly large (by a factor of 10 to 100) compared to those for the analogous negative ions,\footnote{Hammer, {\it et. al.}, J. Chem. Phys. 119, 3650 (2003).} and these differences will be discussed. New theoretical calculations for positron-molecule binding are in progress, and a recent result for acetonitrile will be discussed.\footnote{Tachikawa, {\it et. al.}, Phys. Chem. Chem. Phys. {\bf 13}, 2701 (2011).} This ability to compare theory and experiment represents a significant step in attempts to understand positron binding to matter.