Hot electrons in metal nanoparticles thermalize with the lattice via electron−phonon coupling. Size dependency is controversially discussed in literature. Here we investigate poly-and monocrystalline gold nanoparticles via transient absorption spectroscopy. As reported earlier, electron−phonon coupling in polycrystalline particles is not size-dependent. However, we clearly observe a size-dependent electron−phonon coupling in monocrystalline particles. Larger monocrystalline particles show slower electron−phonon coupling due to the decreasing effect of electron-surface scattering, with electron−phonon coupling constants approaching the values reported for bulk gold. In polycrystalline particles, size dependencies are outweighed by effective electron scattering at grain boundaries. Linear absorption spectra indicate that plasmon damping is also enhanced in polycrystalline particles by grain boundaries.

The excitation of dark plasmons, i.e. coupled plasmon modes with a vanishing net dipole, is expected to favor Landau damping over radiative damping. The dark plasmon excitation might therefore lead to an increased absorption of energy within gold nanoparticles, resulting in a strong generation of hot electrons compared to the generation via bright plasmons. We performed transient-absorption spectroscopy on gold nanoparticle films to assess the initial electronic temperature before thermalization. We observe a significant increase in the electron-phonon coupling time if dark plasmon modes are excited in these films. The results indicate an efficient energy absorption within the nanoparticles due to the suppressed radiative decay of dark plasmon modes.

Dynamics of optically excited plasmonic nanoparticles are presently understood as a series of scattering events involving the initiation of nanoparticle breathing oscillations. According to established models, these are caused by statistical heat transfer from thermalized electrons to the lattice. An additional contribution by hot-electron pressure accounts for phase mismatches between theory and experimental observations. However, direct experimental studies resolving the breathing-oscillation excitation are still missing. We used optical transient-absorption spectroscopy and time-resolved singleparticle X-ray diffractive imaging to access the electron system and lattice. The time-resolved single-particle imaging data provided structural information directly on the onset of the breathing oscillation and confirmed the need for an additional excitation mechanism for thermal expansion. We developed a new model that reproduces all of our experimental observations. We identified optically induced electron density gradients as the initial driving source.

We provide a microscopic approach to describe the onset of radial oscillation of a silver nanoparticle. Using the Heisenberg equation of motion framework, we find that the coupled ultrafast dynamics of coherently excited electron occupation and the coherent phonon amplitude initiate periodic size oscillations of the nanoparticle. Compared to the established interpretation of experiments, our results show a more direct coupling mechanism between the field intensity and coherent phonons. This interaction triggers a size oscillation via an optically induced electron density gradient occurring directly with the optical excitation. This source is more efficient than the incoherent heating process currently discussed in the literature and well-describes the early onset of the oscillations in recent experiments.