  Ultrafast electron delocalization induced by a fs laser pulse is a well-known
process and is the initial step for important applications such as
fragmentation of molecules or laser ablation in solids. It is well understood
that an intense fs laser pulse can remove several electrons from an atom within
its pulse duration. [1] However, the speed of electron localization out of an
electron gas, the capture of an electron by ion, is unknown. Here, we
demonstrate that electronic localization out of the conduction band can occur
within only a few hundred femtoseconds. This ultrafast electron localization
into 4f states has been directly quantified by transient x-ray absorption
spectroscopy following photo-excitation of a Eu based correlated metal with a
fs laser pulse. Our x-ray experiments show that the driving force for this
process is either an ultrafast reduction of the energy of the 4f states, a
change of their bandwidth or an increase of the hybridization between the 4f
and the 3d states. The observed ultrafast electron localization process raises
further basic questions for our understanding of electron correlations and
their coupling to the lattice.
