Comparison of Positron- and Electron-Molecule Bound States

Positrons can 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 used to measure positron-molecule binding energies for many chemical species.\footnote{Danielson, et. al., {\it J. Phys. B}, {\bf 42}, 235203 (2009).}$^,$\footnote{Danielson, et. al., {\it Phys. Rev. Lett.}, {\bf 104}, 233201 (2010).} New measurements will be discussed of positron binding to relatively simple molecules and molecules with large permanent dipole moments ($\mu$). For example, the binding energy is 75 meV for CS$_2$ ($\mu = 0$) and 180 meV for acetonitrile (CH$_3$CN, $\mu = 3.9$ debye). Other species studied include aldehydes, ketones, and nitriles, which have $\mu$ in the range $2.5 - 4.0$ debye. These data will be compared to analogous, weakly bound electron-molecule (negative-ion) states.\footnote{Hammer, et. al., {\it J. Chem. Phys.} {\bf 119}, 3650 (2003).} The positron binding energies are surprisingly large (i.e., by a factor of 10 to 100) compared to those for the analogous negative ions, and origins of these differences will be discussed.