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Physical Review Letters

Bound states of positrons and large molecules

Clifford M. Surko, A. Passner, Marvin Leventhal, F.J. Wysocki

Published Oct 17, 1988• 110 citations
Publisher

Abstract

The lifetime of positrons contained in an electrostatic trap is studied when small quantities of alkanes (i.e., hydrocarbon molecules of the form ${\mathrm{C}}_{\mathrm{n}}$${\mathrm{H}}_{2\mathrm{n}+2}$) are added. As n is increased from 4 to 16, the annihilation rate per molecule is found to increase by a factor of 300 and then to level off. These data are interpreted as evidence for the existence of long-lived resonances and bound states of positrons and neutral molecules, and a model of the formation process is discussed.

DOI
10.1103/physrevlett.61.1831

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•Jan 1, 2007•0 citations
The lifetime of positrons contained in an electrostatic trap is studied when small quantities of alkanes (i.e., hydrocarbon molecules of the form {\mathrm{C}}_{\mathrm{n}}$${\mathrm{H}}_{2\mathrm{n}+2}) are added. As n is increased from 4 to 16, the annihilation rate per molecule is found to increase by a factor of 300 and then to level off. These data are interpreted as evidence for the existence of long-lived resonances and bound states of positrons and neutral molecules, and a model of the formation process is discussed.
Measurements of positron-on-molecule annihilation have established that positrons bind to a variety of molecules via vibrational Feshbach resonances. Data for deeply bound states in benzene and 1-chlorohexane and for positronically excited (i.e., second) bound states in alkanes are used to establish the dependence of annihilation rates on the binding energy and incident positron energy. With this dependence removed, annihilation rates for a broad class of molecules lie on a universal curve as a function of the number of molecular vibrational degrees of freedom. The implications of these results for theoretical models are discussed.
An overview is presented of recent studies of the interaction of low-energy positrons with atoms and molecules using a trap-based positron beam. The beam is tunable from ~100 meV to many electron volts, with a typical energy resolution of 25 meV (FWHM). A variety of scattering processes are investigated using the properties of positron orbits in a spatially varying magnetic field. Recent scattering measurements are reviewed, and new data for electronic excitation, ionization, and positronium formation in noble gases are presented. In a separate set of experiments, positron annihilation below the threshold for positronium formation is studied in molecules as a function of positron energy. The data show large Feshbach resonances in the range of energies of the molecular vibrations. Within the framework of this model, these results provide the first experimental evidence that positrons bind to hydrocarbon molecules.
The very large annihilation rates observed at room temperature in positron collisions with certain molecules have instigated the development of several models of positron annihilation based on the role of virtual processes, Feshbach resonances and bound states. One of the outcomes of these models has been the interest they have generated in investigations of the energy dependence of the annihilation cross-section in the vicinity of inelastic thresholds. The aim of this article is to provide a review of these recent studies and to highlight outstanding questions.
The angular correlation of the two-photon annihilation of positrons annihilating in gases at high pressures demonstrates the two modes of decay---(1) from a bound state, positronium, and (2) from an unbound state---in all gases tested that have low-lying energy states. The increase in the narrow component in argon, plus suitable thermalizing gases, due to the application of a dc magnetic field obeys the theoretical increase due to the mixing of the 11S1^{1}S11S and 13S1^{3}S13S m=0m=0m=0 states. The increase in positronium formation due to the application of a dc electric field is observed. Preliminary evidence for the existence of n=2n=2n=2 states of positronium is seen.
This is a personal account of the development of our buffer-gas positron trap and the new generation of cold beams that these traps enabled. Dick Drachman provided much appreciated advice to us from the time we started the project. The physics underlying trap operation is related to resonances (or apparent resonances) in positron-molecule interactions. Amusingly, experiments enabled by the trap allowed us to understand these processes. The positron-resonance "box score" to date is one resounding "yes," namely vibrational Feshbach resonances in positron annihilation on hydrocarbons; a "probably" for positron-impact electronic excitation of CO and NZ;an d a "maybe" for vibrational excitation of selected molecules. Two of these processes enabled the efficient operation of the trap, and one almost killed it in infancy. We conclude with a brief overview of further applications of the trapping technology discussed here, such as "massive" positron storage and beams with meV energy resolution.