The universal response of a sudden created core hole, predicted to occur on an attosecond (10(-18)  s) time scale, lacks an experimental demonstration. With a two-dimensional coincidence spectrometer, we demonstrate an extensive energy sharing between the Ag 4p photoelectron and the N2,3VV Auger electron exceeding 10 eV. This energy width provides access to the time scale of the emission process. This is the fingerprint of the dynamic fluctuation process 4p(-1)⇌4d(-2)4f. The shakeup induced interband transitions from the Ag(100) surface are also identified by comparing the coincidence spectrum with the M4,5VV Auger transitions.

Electron pair emission spectroscopy (e,2e) is a tool well suited to probe the electron correlations. The probability of the electron pair emission depends in a crucial way on the localization properties of the electron wave functions describing the initial state of the system. One expects an enhanced coincidence signal from the localized electron states in oxides compared to that of itinerant states in metals. Our comparative (e,2e) study of the Ag(001) and NiO/Ag(001) system confirms this observation. We demonstrate that the intensity of the pair emission increases by an order of magnitude after the deposition of 15 monolayers of NiO onto the Ag(001) substrate.

The neutralization of a single He^{2+} ion near a Ir surface leads to the emission of an electron pair. Via coincidence spectroscopy we give evidence that a sizable amount of these electron pairs originate from a correlated single step neutralization of the ion involving a total of four electrons from the metal. These correlated electron pairs cannot be explained in the common picture of two consecutive and independent neutralization steps. We infer a characteristic time scale for the correlated electron dynamics in the metal of 40-400  as.