On the Raman Spectrum of Plutonium dioxide: vibrational and crystal electric field modes

The Raman spectrum of plutonium dioxide is studied both experimentally and theoretically. Particular attention has been devoted to the identification of high-energy modes at $2120$ cm${}^{-1}$ and $2625$ cm${}^{-1}$, the attribution of which has so far been controversial. The temperature dependence of both modes suggests an electronic origin. Crystal Field (CF) calculations reported in this work shows that these two modes can be respectively assigned to the ${\mathrm{\Gamma }}_1$ $\to$ ${\mathrm{\Gamma }}_5$ and ${\mathrm{\Gamma }}_1$ $\to$ ${\mathrm{\Gamma }}_3$ CF transitions within the ${}^5{I}_4$ manifold. These two modes, together with the only vibrational line foreseen by the group theory for the Fm$-3$m PuO${}_2$ symmetry the T${}_{2g}$ PuO stretching mode observed at $478$ cm${}^{-1}$ can thus be used as Raman fingerprint of fcc plutonium dioxide.