Plasmons in amorphous CaAℓ alloys determined from photoemission and electron energy loss spectroscopy

Abstract

We have used electron spectroscopy to study amorphous CaAℓ alloys across the entire composition range. Synchrotron radiation photoemission was used to examine the region of the Aℓ 2p core level, with the energy of plasmon loss features giving the electron density around the Aℓ atoms, while energy loss spectroscopy excited by low energy electrons probed the average electron density in the alloys. The plasmon energy is found to vary continuously with composition. For alloys with low Ca concentrations (below ∼20 at%) there is reasonable agreement with expectations for free-electron-like plasmons with Ca and Aℓ having their elemental valencies. At higher Ca concentrations the Aℓ plasmon energy is larger than expected. Although this would be consistent with an increased electron density in the vicinity of the Aℓ atom, the plasmon energies are affected because part of the oscillator strength of the electron gas is associated with one-electron excitations around the Ca atoms. We argue that there is little charge transfer in the alloys, a result which agrees with our studies of the Auger parameter.