Timing photoemission—Final state matters

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Science  01 Jul 2016:
Vol. 353, Issue 6294, pp. 28-29
DOI: 10.1126/science.aag1090

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The photoemission of electrons from atoms, molecules, and condensed matter provides the experimental basis of our understanding of electronic structure. During the process of photoemission, a sufficiently large quantum of electromagnetic radiation (a photon) is absorbed by matter and converted into an electronic excitation, promoting a bound electron into a final state above the vacuum energy Evac. In photoemission spectroscopy, the kinetic energy and momentum of electrons in such final states are analyzed after their propagation to a distant detector. To determine the electronic structure of the sample, the “sudden approximation” has to be fulfilled, whereby the photoelectron leaves the sample fast enough, without further interaction with the remaining electronic structure. On page 62 of this issue, Tao et al. (1) provide unprecedented insight into final-state dynamics by measuring the time a photoelectron takes to leave a solid material for characteristically different final states. By comparing an electron excited to a final state of a nickel solid Ψ Nif with one excited to a state of vacuum Ψ vacf, they establish that a photoelectron resides in the final state for 200 attoseconds (as) (2 × 10−16 s) before it leaves the nickel (see the figure). Such time scales would still allow for the electron to interact with its surroundings and, thus, are relevant for the validity of the sudden approximation.