Resonant core level studies of molecules and clusters:: Electronic structure and femtosecond dynamics

Core level spectroscopy offers probing of atomic matter localized both in time and space. The temporal localization is due to the short lifetime of core holes, which opens a window from the energy domain to the low femtosecond time domain. The thus enabled studies of dynamic phenomena are exemplified by ultrafast dissociation of molecules. It is shown how information may be obtained about a characteristic dissociation time scale and the fragment velocities. The latter are also shown to be photon-energy-dependent. The spatial localization is due to the atomic character of core levels even in polyatomic systems. Clusters have a high and size-dependent fraction of atoms at the surface, and this leads to size-dependent properties. The spatially local character of core level spectroscopy is shown to be very valuable for separating contributions from surface and bulk atoms in free clusters.