February 25, 2004
With the most modern microscopes scientists can observe atoms at rest. If, however, the atoms are in motion, very short light pulses are needed to reconstruct the motion from a series of snapshots. Whereas an exposure time of less than a thousandth of a second is sufficient for sharp imaging of a tennis-ball in flight, the light pulses have to be shortened by a billionth, to just a few femtoseconds, in order to record the fastest atomic motions in molecules. Inside the electron shell of excited atoms electrons fly a thousand times faster. They change from one energy state to another typically within 10 to a few 1000 attoseconds and in the process cause atoms originally bound in a molecule to fly apart or emit ultraviolet radiation or X-rays. These processes are of fundamental significance for controlling chemical reactions and synthesising new materials. They could even be applied for designing a versatile X-ray laser.
Firstly, the microscopic process to be investigated - and simultaneously the stopwatch - has to be initiated. In order to set the interior of an atom in motion, energy has to be imparted to it. The start of the process must be defined more exactly than the duration of the process, and so energy input has to happen in a flash - within a fraction of a femtosecond. If atoms at rest are to be set in motion, the input energy has to overcome the binding of the electrons to the atomic nucleus. For this purpose the Vienna-Munich-Bielefeld research team uses X-ray flashes lasting 250 attoseconds, the shortest pulses in the world at present. The attosecond flash excites the electron shell of the irradiated atoms. Many of the electrons set in motion thereby acquire such a high energy that they are released from their atomic binding and become independent. The duration and evolution of this electron emission afford direct information on the evolution of excitation and relaxation processes inside the interior of the atom.
The research team has now developed the first method for measuring these processes. This involved reverting to a well-known concept, streak imaging. Untill recently this method was used exclusively for measuring the duration of short light flashes. In its lifetime the light flash ejects electrons from a metal plate which are then accelerated with a static electric field to a fluorescent screen (see "streak camera" illustration). Before they hit the screen, they are deflected aside with another field increasing linearly with time. The temporally varying deflection "streaks" the point of impact of the electrons on the screen. The spatial width of this "streak image" is directly proportional to the duration of the electron emission, i.e. the duration of the light flash. The faster the deflecting field varies, the shorter are the pulses that can be recorded. The most modern streak cameras attain a resolution in the region of 100 femtoseconds.
The novelty of the new method is that deflection of the electrons is done with a light field - varying millions of times faster - that imparts its effect in space and time immediately on release of the electrons. For this purpose the attosecond X-ray flash irradiating the atoms is supplemented with an intense laser light pulse comprising just a few oscillations. In the "light-field-controlled streak camera" illustration (Fig. 3) the red and the blue curves represent, respectively, the evolution of the electric field of the laser light and the intensity of the attosecond x-ray flash. The latter excites the atoms, which then emits electrons. The electrons ejected in the direction of the electric field of the laser light pulse simultaneously beamed in then are detected. They undergo - depending on the time of their emission within half the oscillation period of the laser light - a change in velocity: in the case illustrated the electrons emitted first are decelerated, while those released on termination of the X-ray flash are accelerated.
In this manner the successively emitted electrons are detected separately again, but this time not spatially on a screen but alongside one another on the energy scale. The width and shape of the measured energy distribution of the electrons reflect the duration and evolution of the electron emission, just as their spatial distribution in conventional streak imaging. In this case, however, "deflection" occurs within half a light period, which opens the way to measurement in the attosecond region.
In order to implement this concept of a light-field-controlled streak camera successfully in practice, two conditions have to be satisfied: i) the oscillations of the "deflecting" light field have to be precisely controlled and ii) the exciting X-ray flash has to be synchronised to these oscillations with attosecond precision. The required intense laser pulses consisting of just a few oscillations and having a controlled waveform were demonstrated by the research team a year ago for the first time in the world. The controlled light waveforms opened the way to generating single attosecond X-ray pulses reproducibly from laser pulse to laser pulse by controlled excitation of gigantic oscillations of the electron shell in atoms. The excited atoms emit - like tiny antennas - an X-ray flash perfectly synchronised to the exciting laser field.
In the latest experiment the attosecond flash and the light wave generating it are beamed into a group of atoms to excite them and "deflect" (i.e. accelerate or decelerate) the emitted electrons, respecitvely, thus meeting both of the conditions stated above. The evolution of the emission of the electrons directly ejected by the X-ray flash, so called photoelectrons reflects the evolution of the exciting X-ray flash, while the evolution of the emission of so-called secondary or Auger electrons affords direct information on the relaxation processes occurring in the electronic shell in the wake of the flash-like excitation. By measuring the emission time of the photoelectrons by the new streak method the German-Austrian research team was able to determine an X-ray flash time of 250 attoseconds. These are the shortest pulses ever reported and also the shortest timespan measured hitherto. When the method is applied to secondary electrons, the new device allows direct measurement of the evolution of processes inside the electron shell of atoms with a resolution of approx. 100 attoseconds.
Observation of the motion of electrons deep inside atoms and molecules is thus now within reach. For the first time there is now a chance of presenting experimental answers to questions of effective, collective X-ray emission from atoms (X-ray laser) or of the creation and destruction of chemical bonds (control of chemical reactions).