Excited states of positronium

The formation of excited states of positronium, usually termed Ps, through reaction 4.2, can make significant contributions to erPs. This has already been discussed briefly in the theoretical section 4.2 and in subsection 4.4.3, where we saw that excited state positronium is thought to dominate alkali metals. Here we describe the only experiment to date which has directly detected excited state positronium formation in gases, namely that of Laricchia et al. (1985). 

Charlton, M. (1990). Antihydrogen production in collisions of antiprotons with excited states of positronium. Phys. Lett. A 143 143-146. 

Day, D.J. (1993). An electrostatic positron beam and its use in an experimental investigation of the first excited state of positronium. Ph.D. thesis, University of London. 

Ho, Y.K. (1984). Doubly excited states of positronium negative ions. Phys. 

Many other attempts to observe this new excited state of positronium have been made. Indications of correlated, equal energy, two-photon decay with a summed energy of 1062 keV have been found by DANZMANN et al. [22] in the same reaction as produced the electron-positron pairs. [15, 18, 19] The authors believe that this line, at a new, fourth energy, may belong to the same neutral system which produced the correlated electron-positron pairs. [22]... 

Ziock, K.P., Howell, R.H., Magnotta, F., Failor, R.A. and Jones, K.M. (1990b). First observation of resonant excitation of high-n states of positronium. Phys. Rev. Lett. 64 2366-2369. 

Excitons are the simplest manifestations of many-body elementary excitations in crystalline solids. These are the bound states of an electron-hole system held by a Coulomb attraction - not as strong as in a hydrogen atom but more like in positronium. The ground and excited states of an exciton are properly represented only in a two-particle band picture (Fig. 1) [1]. The process of creation, stabilization and recombination of excitons could well be conveniently investigated through ambient optical absorption and photoluminescence in I-VII compounds. 

The formation potential is usually negative for nPs = 1, and therefore positronium emission is allowed. However, with the possible exception of a diamond surface (Brandes, Mills and Zuckerman, 1992), it is positive for nPs > 2, which therefore precludes the emission of excited state positronium following positron thermalization in the material. 

Most other calculations of positronium formation in positron-helium scattering have employed much simpler methods of approximation, but results have usually been obtained over energy ranges extending well beyond the Ore gap. It must therefore be borne in mind that the experimental results include contributions from positronium formation into excited states as well as into the ground state. The Born approximation, used first by Massey and Moussa (1961) and subsequently by Mandal, Ghosh... 

Walters (1996) and Kernoghan et al. (1996) both employed 11 states (five atomic and six positronium). A common feature of all the calculations is that, although they differ in the details of the magnitudes, they do show that the formation of positronium into excited states is a very important process for these targets and is responsible for the peak-shaped cross sections measured by Zhou et al. (1994b). A detailed study by McAlinden, Kernoghan and Walters (1996) shows how the breakdown into the various positronium states occurs. These authors obtained cross sections which, at least for positronium formed in the 2s and 2p states, were a factor 2-3 smaller than those calculated by Hewitt, Noble and Bransden (1993) using the seven-state approximation. 

Fig. 4.24. Apparatus of Laricchia et al. (1985) for the detection of excited state positronium formed in positron-gas collisions.

Similar coupled-state methods, both with and without the inclusion of positronium terms, have been applied to the excitation of other alkali atoms. The results of McAlinden, Kernoghan and Walters (1994, 1997) and Hewitt, Noble and Bransden (1994) for the dominant resonant excitation cross sections for sodium, rubidium and caesium all exhibit a similar energy dependence to that for lithium. Also, the neglect of positronium terms in the expansion, as in the work of McEachran, Horbatsch and Stauffer (1991), again has the effect of increasing the low energy excitation cross sections over those obtained when such terms are included. 

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