Reionisation in low-energy He-Pb scattering

Abstract

The scattering of He off Pb under conditions at which the quasiresonant process is not observed has revealed two interesting features. Over a limited range of energies at particular scattering angles two distinct peaks in the energy spectra are observed which will hereafter be referred to as the high- and low-energy peaks respectively. The high-energy peak is observed only below a critical incident energy which depends on scattering angle, while the low-energy peak is only observed at energies above a threshold energy which also depends on scattering angle. There is an overlap region of incident energies at which the ion yield is initially dominated by the high-energy peak and gradually with increasing energy this peak disappears and the low-energy peak dominates. An intriguing feature of these two peaks is the energy width which for the high-energy peak is much less than that expected for scattering off a real surface and is equal to the analyser resolution. The low-energy peak has a much larger energy width than would be expected for the combined effects of analyser resolution, thermal broadening and scattering effects. The greater energy width of the low-energy peak and the energy loss associated with it suggests a reionisation process which has a particular threshold involving a distance of closest approach.