Direct observation of electron dynamics in the attosecond domain

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(1-6). 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 characterized(7), and used to directly measure light waves(8) and electronic relaxation in free atoms(2-4). 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(9-19), 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.