Ultrafast observations of heat transfer in palladium nanocrystals

Laser pump-probe x-ray experiments have been used to study the dynamics of processes that occur on ultrafast timescales, such as chemical reactions, phase transitions, and electronic and vibrational excitations. Here, we use ultrafast laser pulses to heat octahedral palladium nanocrystals at the European XFEL to observe heat transfer at picosecond timescales. We observe the evolution of 111 Bragg peaks as a function of delay time at various laser fluences. Above a critical laser fluence threshold, the Bragg peak disintegrates into two smaller peaks around 30 ps. We attribute the broken-up Bragg peak to be representative of inhomogeneous strain within the crystal, caused by a short skin depth of electromagnetic penetration. As the delay between the pump and probe increases, there is sufficient time for heat to dissipate throughout the entire nanocrystal, thereby alleviating the inhomogeneous strain. The Bragg peak then returns to a single peak, though shifted in detector position due to an average lattice change. This study opens the possibility of observing ultrafast heat transfer in nanoparticles, which have been extensively used in catalysis and sensors.