Resonant Raman scattering in UO2 revisited

Abstract

Due to ${\mathrm{UO}}_{2}$ being a strong electron-phonon interaction system, resonant Raman scattering is particularly helpful in exploring the relationship between its optical properties and electronic structure. Fr\"ohlich interaction induced resonant 2LO(\ensuremath{\Gamma}) phonon Raman scattering exhibits excitation energy $({E}_{i}=1.58--3.06\phantom{\rule{0.16em}{0ex}}\mathrm{eV})$, temperature (77--873 K), and pressure (\ensuremath{\le}29 GPa) dependent weighted contributions of two resonances. The first (incoming) resonance occurs when the incident photon energy equals the energy of an optical transition positioned at \ensuremath{\delta} \ensuremath{\sim} 0.19 above the \ensuremath{\sim}2.1 eV band gap. The second (outgoing) resonance occurs when the scattered-photon energy is equal to that of the optical transition. The energy of this transition is proposedly related to the Brillouin zone \ensuremath{\Gamma} point electronic density of states, positioned to be slightly above the band gap, by recent density functional theory calculations. The interplay between the simultaneous temperature dependent effects of tuning the band gap and altering the lifetime broadening of the excited electronic state is elucidated and demonstrated to strongly depend on the excitation energy and its position with respect to the band gap. The core difference between pressure and temperature dependent Raman response is mostly attributed to the ``resonant-suppressing'' role played by excited-state lifetime shortening, which becomes increasingly dominant at increased temperatures.