Look at a hydrogen atom made of a proton and an electron. Now imagine a photon impinges upon it, and the electron may be kicked out. The probability for such an event can be easily evaluated with quantum mechanics. And, according to quantum mechanics, the physics here is simple: the moment the photon disappears, the elctron is emitted. However, this is not so for a multi-electron atom. For such atoms, how an electron is thrust out by a photon is still being investigated, due to electron interactions. In this case, a senseful scenario might be like this: the photon is aborbed by the electron cloud of this atom, meanwhile the electron cloud changes to a higher energy state, and later on, this cloud relaxes to a lower energy state in the accompany of electron emission. Thus, there is a delay between the photon vanishing and electron escape. This delay is of the order of a thousandth of femtosecond, difficult to be spotted. A recent study [1] just attacks this barrier.
[1] M. Schultze et al., Science 328, 1658 (2010).
A perspective and a useful list of references on this work is found here.
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