This paper presents an experimental study designed to address the long-standing question regarding the origin of very large positron annihilation rates observed for many molecules. We report a study of the annihilation, resolved as a function of positron energy (⌬Eϳ25 meV, full width at half maximum͒ for positron energies from 50 meV to several eV. Annihilation measurements are presented for a range of hydrocarbon molecules, including a detailed study of alkanes, C n H 2nϩ2 , for nϭ1-9 and 12. Data for other molecules are also presented: C 2 H 2 , C 2 H 4 ; CD 4 ; isopentane; partially fluorinated and fluorinated methane (CH x F 4Ϫx); 1-fluorohexane (C 6 H 13 F) and 1-fluorononane (C 9 H 19 F). A key feature of the results is very large enhancements in the annihilation rates at positron energies corresponding to the excitation of molecular vibrations in larger alkane molecules. These enhancements are believed to be responsible for the large annihilation rates observed for Maxwellian distributions of positrons in molecular gases. In alkane molecules larger than ethane (C 2 H 6), the position of these peaks is shifted downward by an amount ϳ20 meV per carbon. The results presented here are generally consistent with a physical picture recently considered in detail by Gribakin ͓Phys. Rev. A 61, 022720 ͑2000͔͒. In this model, the incoming positron excites a vibrational Feshbach resonance and is temporarily trapped on the molecule, greatly enhancing the probability of annihilation. The applicability of this model and the resulting enhancement in annihilation rate relies on the existence of positron-molecule bound states. In accord with this reasoning, the experimental results presented here provide the most direct evidence to date that positrons bind to neutral molecules. The shift in the position of the resonances is interpreted as a measure of the binding energy of the positron to the molecule. Other features of the results are also discussed, including large, qualitative changes in the annihilation spectra observed when hydrocarbon molecules are fluorinated.

The development of buffer-gas trapping and magnetized beam formation has provided positron beams with significantly improved energy resolution (⌬Eр0.025 eV) compared to those available previously. Analysis techniques have been developed to take advantage of the fact that the beam is in a magnetic field of ϳ0.1 T. This has enabled scattering experiments at lower energies and with significantly improved state selectivity for excitation experiments than had been possible previously. A detailed description of these techniques is presented. Data are presented for a variety of cross sections for scattering of positrons from atomic and molecular targets.

We have developed a versatile, pulsed source of cold (ΔE=0.018 eV), low-energy positrons (E≈0–9 eV). Multiple pulses of 105 positrons, each 10 μs in duration, are extracted from a thermalized, room temperature positron plasma stored in a Penning trap. The frequency, duration, and amplitude of the pulses can be varied over a wide range.

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Absolute cross sections for the vibrational excitation of CO, CO2, and H2 by positron impact are presented for incident positron energies from 0.5 eV to several electron volts. The measurements use a novel technique that exploits the adiabatic motion of a positron beam in a strong magnetic field. This work is the first systematic experimental study of vibrational excitation by positron impact, and extends to energies where positron measurements have traditionally been difficult. The measured cross sections are compared with available theoretical calculations.