the birth of an attosecond light pulse
Generation of single attosecond flashes of extreme ultraviolet light is a striking manifestation of steering electrons on atomic length and time scales with the electric field of light. Here is
how it happens.
Electron detachment
The form of the electric field wave of a pulse of red laser light lasting several femtoseconds is shaped such that its most intense waving is at the center of the pulse as depicted by the red line. This light wave is beamed into an ensemble of atoms the electron cloud of one of which is depicted in green. The force this light field exerts on an atomic electron changes direction (up or down) with a period of T0 ≈ 2500 attoseconds. If this force overcomes the atomic binding force, the electron can be pulled away from the nucleus. The intensity of the few-cycle red wave is adjusted so that this happens at the wave peak preceding the pulse center. Our attosecond chronoscope, the attoscope, is released at this instant, t0 = 0 (Fig.1).
Breaking away
Shortly after t0 the electron (represented by a "cloud" because its position is not precisely known) is rapidly removed from the vicinity of the nucleus. The electric field of the light wave (red line) points downwards, accelerating the electron upwards (owing to its negative charge) following its liberation. At t1 ≈ T0/4, the electron's speed is boosted to some ten thousand kilometers per second (Fig.2).
Turning back
However, it cannot break away from the nucleus much farther than a few nanometers in spite of this breakneck speed. This is because at t1 the light force is reversed and starts braking the electron. It comes to a standstill when the attoscope turns to t2 (Fig.3).
Coming to rest and giving birth to a photon
The standstill lasts only a few attoseconds, because the light force of reversed direction shortly thereafter hurls the electron back towards the atomic nucleus. This results in a recollision of the electron with its parent atom roughly another quarter period later, at t3 ≈ ≈T0. Those electrons that leave their atoms 100 - 200 attoseconds after t0 return to the nucleus with the highest speed. Upon being captured by the atomic binding force, the electron must release all the energy previously acquired from the light field, which it does by emitting high-frequency, extreme ultraviolet (XUV) photon. The duration of the emission is confined to a fraction of a femtosecond (Fig.4).
An attosecond pulse is born
The attosecond burst emitted from a single atom is immeasurably weak. However, the light field waving through a macroscopic volume makes this story happen in millions of atoms and makes it happen in perfect synchronism. As a result, the faint atomic xuv emissions add to build up an intense xuv pulse. As the atoms radiate unisono, the xuv pulse is delivered in a highly-collimated laser-like beam.
The world's first attosecond pulse source (Nature 414, 509 (2001)) emits 250-attosecond pulses - one per one millisecond - of extreme ultraviolet light at a wavelength of 13 nanometers (Nature 427, 817 (2004); New Scientist, 6 November 2004, p. 33) and affords promise of being extended to wavelengths of less than 1 nanometer (Nature 433, 596 (2005)). Control of strong light fields (Nature 421, 611 (2003)) was crucial in its reproducible, controlled creation.
further reading
Press release, June 2008: New record in ultrafast metrology. Physicists at Max-Planck Institute of Quantum Optics and the Ludwig-Maximilians-University Munich are the first to produce light pulses lasting only 80 attoseconds.
To observe the motion of electrons in atoms one has to be fast. The speed needed has once again been achieved by a team of physicists of the “Munich-Centre for Advanced Photonics” (MAP). In cooperation with their colleagues at the Advanced Light Source in Berkely (USA) researchers of the team of Professor Ferenc Krausz at Max-Planck Institute of Quantum Optics (MPQ) in Garching and Ludwig-Maximilians-University of Munich (LMU) and Prof. Ulf Kleineberg at LMU have produced the first light pulses lasting just approx. 80 attoseconds with ultrashort laser flashes. An attosecond is a billionth of a billionth of a second. This is the first time that scientists have advanced metrology into the temporal range below 100 attoseconds. This affords access to real-time observation of the fastest electron motions inside atoms, molecules and solids. Insight into electron processes can lead to the development of new light sources, exploration of the microscopic origin of serious illnesses, or gradual advancement of electronic data processing towards the ultimate limits of electronics (Science, 6/20/2008 
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Guiness Certificate for the shortest light flashes worldwide.
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Fig. 1. Electron detachment.
Fig. 2. Breaking away.
Fig. 3. Turning back.
Fig. 4. Coming to rest and giving birth to a photon.
Fig. 5. Laser pulses are focused on a neon gas streaming out of a thin tube. The intense (ionizing) laser field induces electron oscillations in the neon atoms, which emit attosecond pulses of extreme ultraviolet light. (© thn)