2, Stanford Institute for Materials and Energy Sciences, Menlo Park, California, United States
To date, experimental studies of radiation damage are limited to post-mortem analysis of irradiated samples. Because of this, processes that occur on sub-nanosecond timescales have to be inferred from observations at time-infinity. Ultrafast lasers, which electronically excite materials, can be used to replicate electronic stopping damage in nuclear materials and perform time-resolved measurements. Here, we couple ultrafast laser irradiation with high pressure generated by a diamond anvil cell to study the behavior of compressed, electronically excited UO2. Excitation was found to produce a photoinduced polaron, as evidenced by a low frequency phonon mode. Frequency and lifetime of the phonon mode were measured as a function of pressure. A change in polaron behavior was observed at ~10 GPa—consistent with a previously observed electronic transition in UO2—showing a coupling between electronic structure and the polaron. Measurements were made at multiple probe wavelengths to help understand the dispersion relation of the phonon mode, and thus the nature of the polaron. This polaron has been a proposed energy carrier in radiation damage of UO2, though until now little of its behavior has been understood.