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Getting a Kick Out of Light

In the usual photoionization process, the absorption and emission of photons between the light field and the photoexcited electron packet is an arbitrary process that broadens the energy distribution of the electrons. However, there is no net exchange of energy between the two, and the drift velocity of the electron packet is unchanged. Kienberger et al. (p. 1144; see the Perspective by Lewenstein) used ultrafast xray pulses only several hundred attoseconds (1 attosecond is 10−18 second) in duration to generate a packet of electrons. They could then control where the packet interacted relative to the phase of much more intense (but slower and longer wavelength) laser light field. The confinement of the packet to a particular region of the light field not only allows the acceleration and deceleration of the electron packets, but also provides a powerful probe for use in attosecond metrology.