Photoacoustic spectroscopy of actinide oxides

Abstract

Photoacoustic spectroscopy has been used to measure directly the absorption spectra of powdered actinide oxides (thorium, uranium, neptunium, plutonium, americium and curium). The photoacoustic spectra in the UV, visible and near-IR region were measured with a single beam instrument supplemented by a small glove box which contained the sample cell and the detector. In contrast with the photoacoustic spectra of the lanthanides, which show sharp, narrow absorption bands, the absorption spectra of the actinide oxides reveal quite different spectroscopic properties. All the spectra are characterized by intense and broad absorption bands, especially in the visible and UV ranges. The narrow, weak absorption bands, which are attributed to electron transitions within the 5f shells, cannot be observed in uranium, americium and curium dioxides. In neptunium and plutonium dioxide as well as in americium sesquioxide they are observable, but they are overlapped by broader absorption bands. Most of the 5f-electron levels are significantly hybridized and it seems that covalent metal-oxygen ligand mixing is an important factor. The intense, broad absorption bands in the UV and visible regions may be explained by electron transfer processes between 5f-electron states and oxygen molecular orbitals.