The theories regarding the formation of positronium in molecular substances are discussed. A modified theory based upon the “spur” theory is described. Theoretical derivation and experimental results suggest that the fraction of the long lifetime component due too-positronium annihilation has a complex origin. Initially, the electron of the positron-electron pair is likely to be separated from the positron by medium molecules even when the total kinetic energy of the pair is less than the potential energy between them. The formation of positronium depends on the mobilities of the positron and electron, the slowing down process and the potential energy between them. Positronium is formed in less than 10 ps. Then the positronium produced may react with the radicals created in the “spur” due to the positron. Only the fraction ofo-positronium able to escape the “spur” will have a long lifetime. Therefore, positronium formation is highly related to the fast reactions in the “spur” during the first 100 ps.

The formation of positronium in molecular substances

The decrease of positronium (Ps) formation in condensed matter caused by the presence of an external electric field is discussed in terms of the spur reaction model of Ps formation. The rather few experimental results available are shown to be in good agreement with the predictions of the model. Many new results are predicted by use of the available radiation-chemistry results. The work is also a reply to Brandt, who estimated that the spur process of Ps formation is insignificant in hydrocarbons. It is shown that Brandt's estimate disagrees with well established results of radiation chemistry.

Effect of an external electric field on the positronium formation in the positron spur

Influence of spur processes on positronium formation in some mixtures of organic liquids

Calculations of the binding energies and annihilation rates of bound atomic and molecular systems which contain a positron are reviewed. Emphasis is placed on methods of calculation and the quality of the numerical results. In this article we limit our attention to positrons interacting with atoms, diatomic molecules, and their ions.

Bound states of positrons with atoms and molecules: Theory

The progress of research into the behavior of the positronium in molecular substances in the last several years is reviewed. The Ps atom can be treated as a solute in a liquid. The repulsive exchange force is the dominant force which reacts against the van der Waals attractive force. One of the most useful general relationships found is the one between the pick-off quenching rate and the surface tension of the liquid. The pick-off quenching rate is found, in general, to be an additive property of the functional groups of a molecule. The quenching of theo-Ps atom by an active quenching agent in an inert solvent has been found to be primarily diffusion controlled. Some deviations from the simple Arrhenius equation have been found. For gases, more theoretical work on the collision problems of Ps with atoms or molecules is needed. For amorphous molecular solids, it is likely that the property of pick-off quenching ofo-Ps is similar to that in liquids. However, in crystalline molecular solids, the situation is quite different. A few definite cases have been found where a Ps atom does not form or exist. Further work in this area should be very interesting. The foregoing leads us to believe that positronium or positron annihilation can be useful in the study of fundamental properties of molecular liquids and solids.

Positronium in molecular substances

A new conceptual basis is outlined for chemical reactions between positronium and ionic or neutral species in liquids. It is first shown that the positronium formation fraction in pure liquids depends not only on the dissociation energy of the medium (Ore-gap theory) but also on the threshold, IA–IP, where IA and IP are the ionisation potentials of the liquid and positronium. The energy regions in which positronium reactions may occur are defined in terms of ionisation potentials and bond strengths of the constituents of the liquid and of positronium. Special features of positronium reactions are described, and general conditions and examples are given of oxidation of Ps by electron transfer, compound formation, and double decomposition, and of reduction. ‘Anomalous’ features of earlier experimental data and recent experiments are satisfactorily explained in terms of the new concepts. These concepts are then applied to describe the conditions for reactions in mixtures. The effects are additive except when complexes are formed.A simple mathematical analysis is given to predict the effects on the experimental position annihilation lifetime data which depend on ortho–para conversion, pick-off, and compound formation. Experimental data are used, when available, to check the predictions.

J. H. Green

Papers

Systematic positronium chemistry