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Regulating the femtosecond excited-state lifetime of a single molecule

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Science  07 Sep 2018:
Vol. 361, Issue 6406, pp. 1012-1016
DOI: 10.1126/science.aat9688

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Selectively exciting desorption

Reactions of molecules adsorbed on surfaces can be induced by injecting electrons from the tip of a scanning tunneling microscope. Rusimova et al. show that for the tip-induced desorption of toluene molecules from a silicon surface, two activation channels exist: One is invariant, but the other depends on the height of the tip above the surface. When the tip is very close to the molecule, it can quench the excitation. The decreased lifetime, in turn, decreases the desorption probability.

Science, this issue p. 1012

Abstract

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.

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