BaDElPh Highlights

The effective attenuation length (EAL) of low-energy electrons in oxides (CoO, MgO) and in a rare earth film (Yb) is investigated by photoemission spectroscopy experiments (5<hv<28 eV). In all cases the EAL is found to increase when lowering the electron energy, but the experimental values are smaller than expected from the predictive formula, i.e. the so-called “universal curve”. In particular, for the Yb case the experimental EAL is found to be a factor of four smaller than those predicted from the “universal curve”, while for the small-gap (E_g^CO= 2.5 eV) insulator CoO a factor of six is found. For the large gap (E_g^MgO= 7.77 eV) insulator MgO, instead, a steep increase in the EAL for energies smaller than the insulator optical gap of MgO is found. The comparison of the experimental results with calculated optical properties available in the literature suggests that, for energies lower than 20 eV, the relevant scattering mechanisms are described by the imaginary part of the dielectric function, accounting in particular for the steep increase of the EAL for energies smaller than the insulator band gap.
We point out that the bulk sensitivity in low-energy photoemission may be lower than the expectation and it may strongly depend on the investigated material, with the further warning that theoretical efforts have to be undertaken for a clear interpretation of the photon energy dependence of the LEPES spectra.