Investigation of charging dynamics in atomic clusters on an attosecond timescale
Final Report Abstract
During this research fellowship, the interaction of ultrashort, few-cycle laser pulses with nanoparticles (so called atomic clusters) was studied. While in the past two decades, a large number of studies had been dedicated to investigate the interaction of clusters with laser pulses consisting of many optical cycles, the electron and ionization dynamics evolving during the first few cycles of a laser pulse are not well understood. In a first experiment, the electron dynamics in clusters induced by mid-infrared laser fields at a wavelength of 1.8 μm were investigated. I used two-cycle laser pulses, corresponding to a pulse duration of 12 femtoseconds. In comparison to previous experiments that were performed in the near-infrared region of the spectrum, a substantially more efficient acceleration of electrons was found using mid-infrared pulses. The emission of fast electrons with keV kinetic energies could be explained by the strong force that is induced by the mid-infrared laser field. The measurement of an angularly resolved electron spectrum showed that the electrons were preferentially emitted parallel to the laser polarization, demonstrating that fast electrons are driven out from the cluster by the laser within a few femtoseconds. We found that the electron emission showed two distinct features for kinetic energies below and above 500 eV. The origin of the lower-energy electrons was attributed to direct emission, while the higher energy electrons are a signature of rescattering, where electrons gain energy during a laser-driven interaction in the potential of individual ions or in the cluster potential. To the best of my knowledge, this result is the first direct observation of a rescattering plateau from a cluster target. The findings are expected to have implications for condensed systems in general. In the second experiment, I studied the question whether enhanced ionization processes can be observed in cluster targets for laser pulses as short as 3.5 femtoseconds. It is well known that the ionization of clusters is dramatically enhanced compared to an atomic target, when long (>30 femtoseconds) laser pulses are used. As a consequence, high ion charge states were observed from clusters. These results can be explained by an efficient avalanching, where electrons are heated by the laser field, allowing the electrons to ionize other atoms or ions during collisions. It is typically assumed that the heating of electrons requires some time and is therefore not efficient for laser pulses shorter than 10 femtoseconds. In my experiment, I compared ion charge spectra from a cluster and an atomic target using 1.5 cycle near-infrared laser pulses (corresponding to 3.5 femtoseconds). I found that charges up to Xe^4+ were created in the cluster target, whereas only charges up to Xe^2+ were generated in atoms, showing that ionization of clusters is enhanced even for these short pulses. Furthermore, we performed an ignition experiment, where seed electrons were generated with a first near-infrared laser pulse at an intensity above the threshold for multiphoton ionization. A second laser pulse at a lower intensity was used to heat the generated electrons. Indeed, we found that the second laser pulse substantially enhanced the degree of cluster ionization, while it did not change the ionization degree of atoms. Both laser pulses had a duration of 3.5 fs, showing once more that cluster ionization is enhanced on these short time scales. I attribute the enhanced ionization to avalanche processes. While it is likely that the pulse duration is too short for traditional avalanching, multiphoton avalanching may become important for such short pulses. The obtained results are not only important to understand strong-field ionization of nanoparticles, but also for other systems like solids and liquids. In particular, they could be relevant for practical applications such as laser material processing of dielectrics or nanosurgery.
Publications
- Ionization avalanching in clusters ignited by extreme-ultraviolet driven seed electrons, Phys. Rev. Lett. 116, 033001 (2016)
B. Schütte, M. Arbeiter, A. Mermillod-Blondin, M. J. J. Vrakking, A. Rouzée, and T. Fennel
(See online at https://doi.org/10.1103/PhysRevLett.116.033001)
