In an unprecedented achievement, physicists have managed to directly observe electrons moving about the outer orbit of an atom. It's all thanks to some nifty quantum trickery and a machine that measures time in quintillionths of a second.

The actual process used by the scientists, called attosecond absorption spectroscopy, is about as fiendishly complicated as its name, so let's take this slowly. They started by taking some atoms of krypton, one of the nobles gases. They then ionized the atoms using a near-infrared laser pulse. This pulse operated in cycles of a few femtoseconds each. A femtosecond is 10^-15 second, or a quadrillionth of a second. This ionization pulse caused anywhere from one to three of the eight electrons in the krypton's outermost shell to leave the atom, leaving an empty space in this furthest valence.


Next, it was time for the attosecond pulse. An attosecond is a thousandth of a femtosecond, which is also 10^-18 second (not to mention a quintillionth of a second, just so all our bases are covered). They sent an extreme-ultraviolet attosecond pulse on the same path as that of the earlier, femtosecond pulse. And this is where the physicists were able to directly observe electrons at work in the wake of ionization.

The attosecond pulse excited one or more of the electrons in the next energy orbital beneath the outermost shell, causing them to jump to the outer orbital and fill the gap created by the departed electrons. At that point, the electron starts "flopping" between the two orbits, creating complementary interference patterns that essentially merge into one, thanks to the quantum concept of coherence.

It's that short-lived electron coherence that the attosecond pulses are able to measure, giving the physicists a direct measurement of the changing levels of coherence between the electron's two quantum states. The graph below shows how the coherence dipped and peaked over the tiny fractions of a second. Each black dot represents a direct moment of electron observation.

This is one of the first direct applications of attosecond pulses, but according to Berkeley researcher Stephen Leone, this is just the tip of the iceberg for what the technology can do:

"his revealed details of a type of electronic motion – coherent superposition – that can control properties in many systems. The method developed by our team for exploring coherent dynamics has never before been available to researchers. It's truly general and can be applied to attosecond electronic dynamics problems in the physics and chemistry of liquids, solids, biological systems, everything.

[via UC Berkeley]