1. Institut für Experimentalphysik, Universität Hamburg, Luruper Chaussee 149, D-22761 Hamburg, Germany
2. Physik Department E20, Technische Universität München, D-85747 Garching, Germany
3. Centro Mixto CSIC-UPV/EHU "Unidad de Física de Materiales", Donostia International Physics Center (DIPC), and Departamento de Física de Materiales, Universidad del País Vasco, Apdo. 1072, 20080 Donostia-San Sebastián, Spain

Correspondence to: W. Wurth¹ Correspondence and requests for materials should be addressed to W.W. (Email: wilfried.wurth@desy.de).

Abstract

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 pulses^{1, 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 atoms^{2, 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 processes^{9, 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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