Studies of copper valence states with Cu L3 x-ray-absorption spectroscopy.

Abstract

We have used x-ray-absorption spectroscopy to study a series of compounds in which Cu assumes a formal valence between 0 and 3. We find that the shape, the threshold energy, and the intensity of the Cu ${L}_{3}$ absorption edge is strongly influenced by the chemical state of the Cu atoms. We use the Cu 2p x-ray-absorption spectra of a large number of Cu compounds, including sulfides, oxides, La-Sr-Cu-O compounds, a phthalocyanine complex, and various minerals to show that the presence of a strong 2p-3d excitonic transition is a fingerprint of the Cu(${d}^{9}$) contribution to the ground state. A simple ionic picture is generally inadequate to describe these compounds.