The absorption spectra of thirteen sintered trivalent lanthanide oxides (La to Lu) and six actinide dioxides (Th to Cm) have been measured by photoacoustic spectroscopy. Though both series of elements have in common successive filling of the 4f and 5f shells, respectively, the absorption spectra of the two classes of oxides are very different. While absorption spectra of the thirteen trivalent lanthanide oxides show narrow weak absorption bands and lines in the uv, visible, and near-IR wavelength ranges due to internal 4f<sup><i>n</i></sup> → 4f<sup><i>n</i></sup> transitions between the numerous energy levels of the Xe 4f<sup><i>n</i></sup> configuration, the absorption spectra of the actinide oxides display very intense and broad absorption bands extending from the ultraviolet to the visible and sometimes to the infrared. In the absorption spectra of the lanthanide oxides of higher valencies, such as Tb<sub>4</sub>O<sub>7</sub> or Pr<sub>6</sub>O<sub>11,</sub> the sharp absorption lines cannot be detected, as the much stronger 4f<sup><i>n</i></sup> → 4f<sup><i>n</i>−1</sup>5d transitions overlap the weak 4f<sup><i>n</i></sup> → 4f<sup><i>n</i></sup> transitions. The conclusion is that the 5f<sup><i>n</i></sup> electrons in the actinide oxides are not as well localized as the 4f<sup><i>n</i></sup> electrons of the lanthanide sesquioxides. The 5f electrons probably participate in bonding, e.g., in electron transfer processes from a molecular oxygen orbital to a partly filled or empty 5f electron state of the oxidizing central atom which gives rise to intensive broad absorption bands in the ultraviolet and visible.

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