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 , 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 characterized 7 , and used to directly measure light waves 8 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.