Electronic structure of liquid lithium and sodium from electron spectroscopy

Abstract

The electronic structure of liquid alkali metals is of particular interest since these group of metals represent the electronically most simple metals known in the periodic system. This also means that there is the greatest chance of finding an electronic structure close to the prediction of the free electron model (FEM) for these metals, in particular for lithium. So far there exists no published valence band investigation of liquid alkali metal surfaces which proved to be atomically clean. In this study liquid Li and Na have been prepared revealing excellent stability against recontamination as checked both by UPS and XPS. The valence band spectra of lithium are in agreement with a parabolic band whose width however proves to be significantly reduced compared to the FEM. In contrast the experimental band width of sodium exhibits a minor reduction from the FEM prediction whereas the valence band shape is rather triangular. Only minor changes of the relative s to p band state photoionization cross sections as a function of the excitation energy have been observed within the range of hv = 11.8 − 48.4 eV in contrast to the results on polyvalent metals. The use of HeII radiation enabled a variety of observations. For example in the case of sodium, besides the different valence band spectra with their typical plasmon structures, core states and Auger transitions have also been examined. The splitting of the 2 p states can be observed by the high resolution UPS. The energetic position of the L 2,3 VV Auger transition is consistent with a negligible two hole final state energy.