Attosecond surface science
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
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.