Energy-resolved positron annihilation rates for molecules

Abstract

The development of high resolution positron beams has enabled measurements of annihilation rates for molecules as a function of incident positron energy. Vibrational Feshbach resonances in these spectra provide evidence for the existence of positron-molecule bound states. In this paper we present further studies of this phenomenon. Evidence is presented for positronically excited bound states (i.e., in addition to the ground state) in ${\mathrm{C}}_{12}{\mathrm{H}}_{26}$ and ${\mathrm{C}}_{14}{\mathrm{H}}_{30}$. Measurements of the annihilation spectra of the halomethanes, $\mathrm{C}{\mathrm{H}}_{3}\mathrm{F}$, $\mathrm{C}{\mathrm{H}}_{3}\mathrm{Cl}$, and $\mathrm{C}{\mathrm{H}}_{3}\mathrm{Br}$, exhibit strong resonances that vary significantly with the substituted halogen. Annihilation spectra for linear alkanes and ring molecules are compared. Annihilation spectra and infrared absorption spectra are compared for a number of molecules. Finally, annihilation rate measurements are presented for a variety of molecules at energies $\ensuremath{\geqslant}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ (i.e., above the vibrational resonances). These provide a measure of the annihilation rates in the absence of vibrational resonances.