Positronium formation into excited states

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... 

Fig. 4.7. The differential cross sections for positronium formation into the ground state and the nPS = 2 excited states in positron-hydrogen collisions at...

Positronium formation into nPs = 2 excited states in positron H2 scattering was investigated in the first Born approximation by Ray, Ray and Saha (1980) and also by Biswas et al. (1991b). 

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. 

To date, it has been found that the positronium formation potential, see equation (1.14), is positive for nPs > 1, so that excited state positronium emission by a work function process is forbidden. Only positrons which reach a surface epithermally (which occurs predominantly at low impact energies where they do not penetrate far into the solid) may... 

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