Time resolved imaging of nanoscale dynamics of gas phase metal particles
Final Report Abstract
The project has successfully tackled the ambitious goal to study the dynamics of highly excited metal nanoparticles in gas phase by time-resolved X-ray diffraction. Several methods and techniques had to be developed or improved to bring this project to success. Several important findings have been made. Structures of large gas phase silver clusters and helium droplets could be characterized, with rather unexpected outcome. But most importantly the dynamics of superheated silver nanoparticles have been observed and understood in detail. Depending on the degree of heating such particles just melt, expand like soap bubbles or explode. The latter two processes are cavitation effects, caused by a decompression wave resulting from the strong thermally induced pressure in the particle. Which of the processes occur depends on whether and where in the phase diagram the spinodal, the boundary for the onset of homogeneous boiling, is crossed. These findings open a completely new approach to study the properties of highly excited matter. A measurement of the diffraction patterns of plasmonic nanoparticles with high time resolution and for different excitation strength will yield precise values for the time of the onset of void formation, which can be used to determine the velocity of sound in the superheated material, or the expansion velocity of the droplet, which yields detailed information about the heat induced pressure in the particle. This will also allow tackling the question of how strongly the heated electronic system contributes to this pressure. By comparing the results to simulations it will be possible to reconstruct the location of the spinodal in the phase diagram, a very sensitive test for the equation of state of the material. Using two pump pulses instead of one will furthermore permit to excite already liquid, spherical particles with temperatures far beyond the boiling point, yielding information about temperature dependent absorption properties as well as the time scales of electron-electron and electron-phonon interaction. Such measurements require stable, intense particle beams ideally with uniform sizes and structures. Here the use of chemically synthesized particles will be highly advantageous. For a beamtime at the EU-XFEL our collaborator Christina Graf (Darmstadt) has produced silver nanocubes, which were very successfully tested in a X-ray scattering experiment in the SQS endstation. A next set of time-resolved X-ray diffraction experiments is planned for the second half of 2023.
Publications
- The 3D-architecture of individual free silver nanoparticles captured by X-ray scattering. Nature Communications 6, 6187 (2015)
I. Barke et. al.
(See online at https://doi.org/10.1038/ncomms7187) - Time-resolved x-ray imaging of a laser-induced nanoplasma and its neutral residuals. N. J. Phys. 18, 043017 (2016)
L. Flückiger et al.
(See online at https://doi.org/10.1088/1367-2630/18/4/043017) - Deep neural networks for classifying complex features in diffraction images. Phys. Rev. E 99, 063309 (2019)
Julian Zimmermann et al.
(See online at https://doi.org/10.1103/physreve.99.063309)
