Positron binding to molecules*

\def\appgteq{\mathrel{\vcenter{\hbox{$\buildrel{\textstyle >}\over{\sim}$}}}} Positron annihilation on molecules as a function of incident positron energy exhibit vibrational Feshbach resonances (VFR).\footnote{J.~A.~Young and C.~M.~Surko, {\it Phys.~Rev.~A} {\bf 77}, 052704 and {\bf 78}, 032702 (2008).} The energy differences between the vibrational modes and the VFR provide a measure of the positron-molecule binding energy, $\epsilon_b$. Measurements of $\epsilon_b$ for 30 molecules are now available from this procedure and from comparison with theoretical spectra in the case of very small binding energies.$^2$ The dependence of $\epsilon_b$ on molecular parameters can be expressed as $\epsilon_b = 12.4(\alpha + 1.6\mu + 2.4N_\pi - 5.6)$ [meV], where $\alpha$ is the molecular dipole polarizability, $\mu$ is the permanent dipole moment, and $N_\pi$ is the number of $\pi$ bonds in aromatic molecules.\footnote{J.~R.~Danielson, J.~A.~Young, and C.~M.~Surko, {\it J.~Phys.~B} {\bf 42}, 235203 (2009).} Insights into chemical trends from this analysis will be discussed. A key outstanding goal is to measure $\epsilon_b$ for molecules that are sufficiently small and simple that $\epsilon_b$ can be calculated theoretically. The empirical formula is used to identify candidate molecules. New measurements of $\epsilon_b$ for CS$_2$ (predicted $\epsilon_b \appgteq 40$ meV) will be discussed.