Pump-probe reflectivity study of ultrafast dynamics of strongly correlated 5f electrons in UO₂

5f electrons in the Mott insulator UO₂ produce intriguing electronic states and dynamics, such as strong correlation and f-f excitations. We have performed femtosecond pump-probe reflectivity measurements on a single crystal UO₂ at temperatures of 5-300 K to study the ultrafast dynamics of photoexcited 5f electrons. Laser pulses at 400 nm pump 5f electrons across the Mott gap, while those at 800 nm probe the pump-induced change of reflectivity. We find temperature-dependent excitation and relaxation processes and long-lived acoustic phonons, and extract picosecond risetimes and microsecond relaxation times at low temperatures. The observed slow relaxation is ascribed to the decay of Hubbard excitons formed by U³⁺-U ⁵⁺ pairs.