Elastic electron-alkali atom scattering near the first inelastic threshold

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Abstract
Differential elastic cross sections have been measured as a function of angle and energy for the e--Na and e--K systems in a crossed-beam experiment. The angular range was 20 degrees to 145 degrees and the energies were chosen to span the region containing the first excited state for each atom which are at 2.1 eV and 1.61 eV respectively. At threshold a cusp is observed in the energy dependence of the cross section. At some angles the cusp is much more pronounced than at others, and though it is usually a peak in the cross section, it can also be a dip, as for example with K at 145 degrees . The threshold feature is superposed on a broad D-resonance which also occurs in the threshold region. The angular distributions for e--K have been remeasured at 1.2 eV and 2.2 eV.