Initiating and imaging the coherent surface dynamics of charge carriers in real space

The tip of a scanning tunnelling microscope is an atomic-scale source of electrons and holes. As the injected charge spreads out it can induce adsorbed molecules to react. By comparing large-scale `before' and 'after' images of an adsorbate covered surface, the spatial extent of the nonlocal manipulation is revealed. Here we measure the nonlocal manipulation of toluene molecules on the Si(111)-7x7 surface at room temperature. Both the range and probability of nonlocal manipulation have a voltage dependence. A region within 5 to 15 nm of the injection site shows a marked reduction in manipulation. We propose that this region marks the extent of the initial coherent (i.e., ballistic) time-dependent evolution of the injected charge carrier. Using scanning tunnelling spectroscopy, we develop a model of this time-dependent expansion of the initially localized hole wavepacket within a particular surface state and deduce a quantum coherence (ballistic) lifetime of ∼ 10 fs.