Direct observation of electron dynamics in the attosecond domain

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Dynamical processes are commonly investigated using laser pump–probe experiments, with a pump pulse exciting the system of interest and a second probe pulse tracking its temporal evolution as a function of the delay between the pulses1,2,3,4,5,6. Because the time resolution attainable in such experiments depends on the temporal definition of the laser pulses, pulse compression to 200 attoseconds (1 as = 10-18 s) is a promising recent development. These ultrafast pulses have been fully characterized7, and used to directly measure light waves8 and electronic relaxation in free atoms2,3,4. But attosecond pulses can only be realized in the extreme ultraviolet and X-ray regime; in contrast, the optical laser pulses typically used for experiments on complex systems last several femtoseconds (1 fs = 10-15 s)1,5,6. Here we monitor the dynamics of ultrafast electron transfer—a process important in photo- and electrochemistry and used in solid-state solar cells, molecular electronics and single-electron devices—on attosecond timescales using core-hole spectroscopy. We push the method, which uses the lifetime of a core electron hole as an internal reference clock for following dynamic processes9,10,11,12,13,14,15,16,17,18,19, into the attosecond regime by focusing on short-lived holes with initial and final states in the same electronic shell. This allows us to show that electron transfer from an adsorbed sulphur atom to a ruthenium surface proceeds in about 320 as.

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Figure 1: Core-hole clock spectroscopy—schematic overview.
Figure 2: Core-hole clock spectroscopy—the spectroscopic signatures.
Figure 3: Quantitative charge transfer analysis of sulphur L1L2/3M1/2/3 Coster–Kronig autoionization spectra of c(4 × 2)S/Ru(0001) as a function of photon energy.
Figure 4: Theoretical charge transfer time for S in f.c.c. and h.c.p. hollow sites computed as S 3 p resonance lifetime.


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We acknowledge support by the staff of MAX-lab, Lund, Sweden, in particular J. N. Andersen and the ARI program. This work was supported by the Deutsche Forschungsgemeinschaft under Schwerpunktprogramm 1093 “Dynamik von Elektronentransferprozessen an Grenzflächen”, the Basque Departamento de Educación, the University of the Basque Country, the Spanish MEC, European Network of Excellence NANOQUANTA, and Max-Planck Awards for Scientific Cooperation to P.M.E. and D.M.

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Correspondence to W. Wurth.

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