Abstract
A pulsed crossed-beam method incorporating time-of-flight spectroscopy, previously developed in this laboratory, has been used to obtain relative cross sections for n = 1- to 6-fold ionization of ground-state Pb atoms by electrons at energies ranging from near threshold to 3000 eV. These results greatly extend the energy range of previous measurements which, in any case, exhibit serious discrepancies. Rather than rely on any previous measurements on Pb, the present data have been normalized to the previously measured cross section for electron impact ionization of Ga at 200 eV. At our high-energy limit of 3000 eV, our measured cross sections decrease by much less than an order of magnitude with increasing n and and are also converging rapidly at high velocities, indicating large contributions from Auger processes. Observed near-threshold structure in the cross section curve is consistent with known autoionization transitions, and while there is also some indication of structure in cross section curves for multiple ionization, the large number of possible levels and experimental uncertainties preclude a detailed analysis. Recent theoretical predictions of multiple ionization based on approximate scaling procedures have been shown to be only partially successful in describing the present data.
Published Version
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