AMOC in Positron and Positronium Chemistry

Hermann Stoll, Petra Castellaz and Andreas Siegle Max-Planck-Institut fur Metallforschung, D-70569 Stuttgart, Germany 

The two quantities which can be observed when an individual positron annihilates in condensed matter are the positron age r, which is the time interval between implantation and annihilation of the positron, and the momentum p of the annihilating positron-electron pair. Time-resolved information on the evolution of positron states is obtained by correlated measurements of the individual positron lifetime (= positron age) and the momentum of the annihilating positron-electron pair (Age-Momentum Correlation, AMOC). AMOC measurements are an extremely powerful tool for the study of reactions involving positrons. It not only provides the information obtainable from the two constituent measurements but allows us to follow directly, in the time domain, changes in the e+e momentum distribution of a positron state (cf. Sect. 1). 

In the field of positronium chemistry AMOC combines the positronlifetime analysis which is especially sensitive to detect long-lived ortho-positronium (o-Ps) with the Doppler-broadening-measurement technique which is particularly suitable for the observation of the para-positronium (p-Ps) state with its characteristic narrow momentum distribution. In addition, 

AMOC allows time-dependent observations of the occupations and transitions of different positron states tagged by their characteristic Doppler broadening. Chemical reactions of positronium have been studied by beam-based AMOC as well as bound states between positrons (e+) and halide ions (cf. Sect. 2). 

For all positronium forming solids and liquids investigated so far epithermal positronium and its slowing-down could be observed, indicating that no special sites are required for positronium formation (cf. Sect. 3). Evidence for the existence of two kinds of o-Ps in liquid rare gases and for irreversible transitions between them is given in Sect. 4. 

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