Momentum-transfer dependence of the K absorption edge of lithium

The lithium K absorption edge has been studied as a function of momentum transfer in a high-resolution inelastic-electron-scattering experiment and found to exhibit no significant change with momentum transfer up to 1.2 -1. This result places severe restrictions on theories which interpret the shape of the threshold in terms of the final-state electron-hole interaction, and compatibility with this experiment could only be obtained in such theories by the utilization of a range (ξ<0.01 eV) or an s-wave phase shift (δ0<0.5), values which differ from the presently accepted calculations. Our results are consistent with an edge shape determined by a simple step function in the one-electron transition probability convoluted with broadening due to phonon or lifetime effects.