Key insights into the behaviour of materials can be gained by observing their structure during phase transitions or when they undergo lattice distortion. Laser pulses on the femtosecond time scale can be used to induce disorder in a "pump-probe" experiment with the subsequent transients being probed stroboscopically using femtosecond pulses of visible light 1 ,

There is a fundamental interest in studying photoinduced dynamics in nanoparticles and nanostructures as it provides insight into their mechanical and thermal properties out of equilibrium and during phase transitions. Nanoparticles can display significantly different properties from the bulk, which is due to the interplay between their size, morphology, crystallinity, defect concentration, and surface properties. Particularly interesting scenarios arise when nanoparticles undergo phase transitions, such as melting induced by an optical laser. Current theoretical evidence suggests that nanoparticles can undergo reversible nonhomogenous melting with the formation of a core-shell structure consisting of a liquid outer layer. To date, studies from ensembles of nanoparticles have tentatively suggested that such mechanisms are present. Here we demonstrate imaging transient melting and softening of the acoustic phonon modes of an individual gold nanocrystal, using an X-ray free electron laser. The results demonstrate that the transient melting is reversible and nonhomogenous, consistent with a core-shell model of melting. The results have implications for understanding transient processes in nanoparticles and determining their elastic properties as they undergo phase transitions.X-ray laser | coherent diffraction | phase transition | ultrafast imaging | pump-probe N anoparticles also display interesting properties due to their size and morphology; for example, they can exhibit anomalously low thermal conductivity due to the phonon mean free path approaching the size of the particle. This has significant implications for developing novel thermoelectric devices (1). Further interest in transient processes in nanoparticles comes from their role in photoacoustic imaging (2) and photothermal cancer therapy (3) and their potential in high-harmonic generation (4). Therefore, it is critical from both a fundamental and a practical point of view to understand the photon, electron, and lattice interactions of nanoparticles irradiated with a short pulse (<100 fs) laser up to the point of melting.Molecular dynamics (MD) simulations have provided the most detailed models of the nanoparticles' response to laser irradiation at low intensity, with reversible nonhomogenous surface premelting predicted (5, 6). The simulations indicated that isolated regions on the nanoparticle surface begin to melt before forming a continuous layer. Additionally, these simulations suggest that before the formation of a nonhomogenous liquid outer layer surrounding a solid inner core, preferential facet premelting can occur with liquid-like atoms appearing first in the (100) and (110) crystallographic directions. It was also observed that during the formation of the liquid outer layer, liquid regions extended inward toward the solid core. Simulations on larger nanorods (7) have suggested more exotic behavior during laser-induced premelting, such as internal structural changes from face-centered cubic to hexagonal close-packed structures along with the for...

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