The key to controlling reactions of molecules induced with the current of a scanning tunneling microscope (STM) tip is the ultrashort intermediate excited ionic state. The initial condition of the excited state is set by the energy and position of the injected current; thereafter, its dynamics determines the reaction outcome. We show that a STM can directly and controllably influence the excited-state dynamics. For the STM-induced desorption of toluene molecules from the Si(111)-7x7 surface, as the tip approaches the molecule, the probability of manipulation drops by two orders of magnitude. A two-channel quenching of the excited state is proposed, consisting of an invariant surface channel and a tip height-dependent channel. We conclude that picometer tip proximity regulates the lifetime of the excited state from 10 femtoseconds to less than 0.1 femtoseconds.
The tip of a scanning tunnelling microscope can inject hot electrons into a surface with atomic precision. Their subsequent dynamics and eventual decay can result in atomic manipulation of an adsorbed molecule, or in light emission from the surface. Here, we combine the results of these two near identical experimental techniques for the system of toluene molecules chemisorbed on the Si(111)−7×7 surface at room temperature. The radial dependence of molecular desorption away from the tip injection site conforms to a two-step ballistic-diffusive transport of the injected hot electrons across the surface, with a threshold bias voltage of +2.0 V. We find the same threshold voltage of +2.0 V for light emission from the bare Si(111)−7×7 surface. Comparing these results with previous published spectra we propose that both the manipulation (here, desorption or diffusion) and the light emission follow the same hot electron dynamics, only differing in the outcome of the final relaxation step which may result in either molecular displacement, or photon emission.
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