MULTIPLE IONIZATION OF LEAD BY ELECTRON-IMPACT

A pulsed crossed-beam method incorporating time-of-flight spectroscopy , previously developed in this laboratory, has been used to obtain rel ative cross sections sigma(n) for n = 1- to 6-fold ionization of groun d-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 b een normalized to the previously measured cross section rrl for electr on impact ionization of Ga at 200 eV. At our high-energy limit of 3000 eV, our measured cross sections n, decrease by much less than an orde r of magnitude with increasing n and sigma(2) and sigma(3) are also co nverging rapidly at high velocities, indicating large contributions fr om Auger processes. Observed near-threshold structure in the sigma(1) cross section curve is consistent with known autoionization transition s, and while there is also some indication of structure in cross secti on curves for multiple ionization, the large number of possible levels and experimental uncertainties preclude a detailed analysis. Recent t heoretical predictions of multiple ionization based on approximate sca ling procedures have been shown to be only partially successful in des cribing the present data.