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Time-resolved single-molecule reactions

Final Activity Report Summary - TSR (Time-resolved single-molecule reactions)

The vivid progress in the ultrafast optics in the past decade facilitated the generation of femtosecond laser, and attosecond Vacuum-Ultraviolet (VUV) pulses. The duration of these pulses can be made shorter than the intrinsic time scale of the atomic motion, principally allowing the observation of fast dynamic processes on surfaces. On the other hand, the continuous development in Scanning Tunnelling Microscopy (STM) can deliver nowadays real-space information on the surface morphology, as well as the electronic structure of surfaces with atomic resolution. Thus, the overall objective of the project was to study the electronic properties of surfaces by taking advantage of both the unique spatial resolution of a state-of-the-art STM microscope as well as the excellent temporal resolution provided by ultrashort light pulses.

In particular, electron and ion emission studies on the Highly Oriented Pyrolitic Graphite (HOPG) surface have been performed. The study of the velocity of the emitted ions revealed that Coulomb explosion (CE) is the major generation mechanism for positive ions at laser intensities close to the ablation threshold. We proved not only the existence of CE, but also the possibility of the emission of intact graphene sheets during laser irradiation, that is consistent with earlier theoretical predictions.

Single-shot pump-probe experiments shedded light into the structural dynamics of the detachment of highly excited graphene layers from graphite in femtosecond laser ablation. The measurements revealed strong quenching and revival of Coulomb Explosion (CE) as a function of delay time in the range of 100-200fs and suggested oscillatory motion between the topmost surface layers which regulates the optical properties of the system. We have experimentally demonstrated that CE imaging is applicable to track the lattice dynamics of solid surfaces. We have shown that a quasi-periodic displacement of the layers is driven by the competition between laser-induced repulsive and intrinsic attractive forces, that provide a novel indicator for tracking the surface dynamics.

The work has also included ex-situ 'Atomic force microscopy' (AFM) studies of the laser-treated HOPG surface, with nanometre height, and micrometer lateral resolution, respectively. The studies indicated nanoscopic removal of intact monolayers at intensities close to the damage threshold. During the laser illumination, the ToF spectrum of the emitted positive ions at different laser fluences has been recorded. In the soft-ablation regime we have found evidence of CE (similarly to the multi-shot measurements), based on the measured velocity distributions and using the momentum-scaling criterion. As CE is apparently the dominant mechanism for ion emission and the removal of the material leaves nanoscale structures on the HOPG surface, we concluded that the charge is localised in the topmost surface layers.

The STM development has been performed in collaboration with the Max Planck Institute for Quantum Optics in Garching, Germany. This work was focused on the exploration of the plasmonic behaviour of metal nanostructures with a laser-assisted STM microscope. Statistics on the near-field microscopy of plasmons generated on different nanostructures revealed that the direct plasmon signal shows a narrow Gaussian or Poisson-like distribution, whereas the thermal signal of the plasmon oscillation corresponds to a Boltzmann-type distribution as expected for a signal of thermal origin. The width of the direct plasmon signal corresponds in most cases to the width expected for an ideal Poissonian or is narrower. If the signal fluctuations resulted uniquely from intensity variations in time, this would indicate reduced shot noise.

A final result to mention, albeit tentative, is the discovery of the beating of the high-energy electrons upon photoexcitaion. In other words, electrons in the kinetic energy range of a few tens of eV are emitted with a different periodicity as the low-energy thermal electrons as a function of pump-probe delay time. The evaluation of the results is on the way.