Attosecond surface science

University of Birmingham

Surface science apparatus connected to the attosecond beamline.

The Birmingham team has accomplished construction of an experimental set-up consisting of an in-UHV STM apparatus and surface analysis chamber equipped with Time-of-Flight energy and mass analyzer. The system has been successfully attached to the XUV atto-source beam line at Imperial College and is now fully operational. There are two main experimental areas which are currently being explored: (a) observation of extreme nonlinear phenomena on surfaces using bare graphite as a benchmark material (b) O+ desorption from TiO₂ in an IR/XUV pump-probe experiment. The set (a) of experiments is intended to assess the poorly understood mechanism(s) of multiphoton electron excitation in the perturbative regime where the laws of conventional optics are not applicable. The experiments in (b) are aimed at observing the dynamics of the initial steps of photodesorption processes on the attosecond time scale. The surface reaction leading to O+ emission has been chosen because of the extremely short lifetime (hundreds of attoseconds) of core holes involved in the process. This also provides an opportunity to test the basic technological capabilities of the optical set-up in the detection of processes on sub-femtosecond time scales.

At this time the set (a) of experiments has almost been completed. The results have suggested that at moderate laser intensities (~10¹¹ W/cm2) the band-to-band electron transition excited by ~10 fs IR pulses is governed by tunnelling enhanced by the combined electrical field of the ponderomotive force and surface plasmon generation. This interaction leads to emission of very fast electrons and breakdown of surface electrical neutrality accompanied by impulsive Coulomb explosion. Under these circumstances the strength of the electrical field induced at the surface can be gently tuned in such a way that intact graphene layers are emitted from the surface.