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Schumann, Frank O., Kirschner, J├╝rgen

On the timescales of correlated electron dynamics
Journal of Electron Spectroscopy and Related Phenomena 241, pp 146943/1-6 (2020)
Recent developments yielding intense and short light pulses in the atto-second regime makes it possible to address fundamental questions on the time evolution of the electron dynamics. We demonstrate in our studies that electron pair emission from surfaces holds the promise to unravel the time scale of correlated electron dynamics. This can be achieved without atto-second light sources. We will discuss two different approaches. First we studied the Auger decay following the emission of a core-electron due to photon absorption. With coincidence spectroscopy, we demonstrate an extensive energy sharing between the Ag 4p photoelectron and the NVV Auger electron exceeding 10 eV. This result is at odds with a sequential emission of first the photoelectron and then the Auger electron. This energy width of the sharing provides access to the time scale of the emission process. We convert this to a timescale of 60 as over which the fluctuations takes place. This value is fair agreement with the theoretical calculation of the timescale to fill an exchange-correlation hole. In a second study we utilized the neutralization of ions near a surface. This is known to be efficient process and leads to electron emission via Auger-type processes. Specifically, the neutralization of He2+ ions makes available the double ionization energy. We demonstrate that the neutralization of a single He2+ ion near an Ir(100) 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 4 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.