Energy-resolved positron annihilation rates for molecules

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 C₁₂H₂₆ and C₁₄H₃₀. Measurements of the annihilation spectra of the halomethanes, CH₃F, CH₃Cl, and CH₃Br, 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 0.50.3em0exeV (i.e., above the vibrational resonances). These provide a measure of the annihilation rates in the absence of vibrational resonances.