No abstract

Positron annihilation on a wide variety of atoms and molecules is studied. Room-temperature positrons confined in a Penning trap are allowed to interact with molecules in the form of low-pressure gases so that the interaction is restricted to binary encounters between a positron and a molecule. Data are presented for the ␥-ray spectra resulting from positrons annihilating in such interactions. The Doppler broadening of these spectra is a measure of the momentum distribution of the annihilating electron-positron pairs. Consequently, these spectra provide information about the electron and positron wave functions. Systematic studies of annihilation line shapes are discussed for noble gases, a variety of inorganic molecules, alkanes, alkenes, aromatics, and perfluorinated and partially fluorinated hydrocarbons. In the case of molecules, the measurements are used to determine the probability of positrons annihilating at specific locations in the molecule. For example, in the case of partially fluorinated hydrocarbons, we have been able to determine the relative probability of annihilation on the fluorine atoms and on the C-H bonds. Insights that these studies provide in understanding the interaction of low-energy positrons with atoms and molecules are discussed. ͓S1050-2947͑97͒05405-X͔

Measurements are presented for the annihilation rates of thermalized positrons in a variety of substances, including noble gases, simple inorganic molecules, hydrocarbons, substituted hydrocarbons, and aromatics. The measurements were performed using trapped clouds of room-temperature positrons, into which substances under test were introduced as low-pressure gases, so that only two-body interactions were involved. These data are compared with other values in the literature, and a compilation of annihilation rates is presented. The measurements illustrate the importance of both chemical composition and the vibrational modes of excitation of the molecules in determining the annihilation rates. The anomalously high annihilation rates observed for large molecules provide evidence for the existence of long-lived resonances. The nature of these resonances is not yet understood, and the data presented are expected to provide useful constraints for the development of theoretical models.

Measurements are presented of the Doppler broadening of the 511-keV g-ray line from thermalized positrons annihilating with Ar, Kr, and Xe. The experiments are conducted at low pressures using positrons stored in a Penning-Malmberg trap, which ensures that only two-body interactions are involved. The contributions from annihilation on inner-shell electrons are studied quantitatively in an isolated atomic system for the first time. The spectra are compared with theoretical calculations using a simple Hartree-Fock model to identify the inner-shell contributions.[S0031-9007(97)03508-4]

Positron annihilation on molecules is known to depend sensitively on molecular structure. For example, in the case of hydrocarbon molecules, modest changes in molecular size produce orders of magnitude changes in the observed annihilation rates. Although this process has been studied for more than three decades, many open questions remain. Experimental studies are described which are designed to test specific features of the annihilation process. Two possible mechanisms of the annihilation are considered theoretically: direct annihilation of the positron with one of the molecular electrons, including possible enhancement of this process when low-lying virtual or bound positron-molecule states are present, and resonant annihilation through positron capture into vibrationally excited states of the positronmolecule complex. The dependence of annihilation rates, λ, on positron temperature, T p , is studied for the first time for molecules, and at low values of T p the dependence follows a power law, λ ∝ T −ξ , with ξ ≈ 0.5. These data are used to test the predictions of direct numerical calculations and theories of the virtual-level enhancement. Partially fluorinated hydrocarbons are studied in order to understand the rapid changes in annihilation rate produced in hydrocarbons as a result of fluorine substitution. These data are compared with the behavior expected due to direct annihilation when there is virtual or bound level enhancement. Measurements of positron annihilation on deuterated hydrocarbons are described which test the dependence of the annihilation on the nature of the molecular vibrations. The relationship of the presently available experimental data for annihilation in molecules to current theories of the annihilation process is discussed.