When Does Photoemission Begin?

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Science  25 Jun 2010:
Vol. 328, Issue 5986, pp. 1645-1646
DOI: 10.1126/science.1191842

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The process of photoemission was one of the effects that led to the formulation of quantum mechanics. If an atom or surface absorbs sufficient energy from incoming light, it can transfer that energy to an electron, which is then emitted. Theories of photoemission mainly focus on energetics—the temporal or dynamic aspects are ignored—but complex electron interactions occur that will create a slight delay between light absorption and electron emission. This time delay has been poorly understood for a fundamental reason: We cannot “see” an atom absorbing a photon. At best, we can follow subsequent emission events and use them to establish a “time zero” when the light was absorbed. A practical challenge has been that the time delay is extremely short, and only recently have direct experiments been feasible with the advent of lasers that emit pulses on the attosecond (as, 10−18 s) time scale. On page 1658 of this issue (1), Schultze and co-workers present measurements of time delays between different photoemission processes generated by the same ultrashort light pulse. This finding not only allows further studies of the timing of photoemission but also provides a new way to investigate electron interactions in atoms.