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Description
The melting of metallic nanoparticles under optical excitation has been one of the puzzling processes, wherein photoexcited electrons deposit their energy into the lattice via electron-phonon coupling on sub-picosecond time scales and consequent phonon dynamics takes over. While previous attempts [1,2] on gold nanoparticles have elucidated the electronic dynamics and the shape transformations, a complete dynamical trajectory of the system remains unresolved. This demands an extensive statistical analysis over large datasets.
We address this challenge by exploiting the single-particle-imaging technique at the European XFEL in a time-resolved pump-probe (pp) experiment on gold nanorods. More than 1M diffraction frames with precise pp-delay encoding were collected with a hit rate approaching 50%. With a plasmon resonant optical excitation (770nm) and pump fluences (3-8 mJ/cm2) much lower than the previous experiments [2,3], we suppress explosive fragmentation while preserving high data quality.
Within the first 100ps, a radial coherent phonon mode (period ~10ps) is observed which attributes to an acoustic phonon mode of the gold lattice, which subsequently decoheres and anharmonically couples to other vibrational modes within 60ps. Post melting, we observe an oscillatory rod-reshaping between ellipsoids and spheres, attributed to the elastic properties of the material. Hence, we attempt to comprehend for the first time a full dynamical trajectory in the first nanosecond after light absorption without ensemble averaging.
References:
1. Rubio et al., Acc. Chem. Res. 49 (2016)
2. Park et al., Nat Commun. 16 (2025)
3. Plech et al., ACS Nano 18 (2024)
| Scientific Topics | Solid State Physics |
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