Enhancement of positron binding energy in molecules containing π bonds

Abstract

Observation of vibrational Feshbach resonances in the annihilation spectra of positrons on molecules provides a direct measurement of the positron-molecule binding energy ${\ensuremath{\varepsilon}}_{B}$. Annihilation measurements are presented for ring hydrocarbons with different numbers of $\ensuremath{\pi}$ bonds, for which it is observed that the presence of $\ensuremath{\pi}$ bonds generally increases the positron binding energies. These molecules were chosen because other global molecular parameters (e.g., polarizability, dipole moment, and geometry) are approximately constant, so the observed differences in ${\ensuremath{\varepsilon}}_{B}$ can be related to changes in the nature of the bonds. The molecular ionization potential ${E}_{\mathrm{i}}$ is an exception: for these molecules, the inclusion of $\ensuremath{\pi}$ bonds tends to decrease ${E}_{\mathrm{i}}$, and the number of $\ensuremath{\pi}$ bonds also exhibits a correlation with ${\ensuremath{\varepsilon}}_{B}$. Comparison with other molecules with $\ensuremath{\pi}$ bonds indicates that the changes in ${\ensuremath{\varepsilon}}_{B}$ are better correlated with the changing electronic structure of the bonds rather than with a direct dependence of ${\ensuremath{\varepsilon}}_{B}$ on ${E}_{\mathrm{i}}$. The relationship between the dependence of ${\ensuremath{\varepsilon}}_{B}$ on the number of $\ensuremath{\pi}$ bonds and electron-positron correlation effects (such as virtual positronium formation) is discussed.