First Ever Realtime Lightshow of Krypton Atom Electrons

Forthefirstt An international team of scientists led by groups from the Max Planck Institute of Quantum Optics (MPQ) and from the U.S. Department of Energy's Lawrence Berkeley National Laboratory and the University of California at Berkeley has used ultrashort flashes of laser light to directly observe the movement of an atom's outer electrons for the first time.

A classical diagram of a krypton atom above shows its 36 electrons arranged in shells. Researchers have measured oscillations of quantum states (foreground) in the outer orbitals of an ionized krypton atom, oscillations that drive electron motion.

Through a process called attosecond absorption spectroscopy, researchers were able to time the oscillations between simultaneously produced quantum states of valence electrons with great precision. These oscillations drive electron motion.

"With a simple system of krypton atoms, we demonstrated, for the first time, that we can measure transient absorption dynamics with attosecond pulses," says Stephen Leone of Berkeley Lab's Chemical Sciences Division, who is also a professor of chemistry and physics at UC Berkeley. "This revealed details of a type of electronic motion – coherent superposition – that can control properties in many systems."

Leone says an example of the importance of coherent dynamics is its crucial role in photosynthesis, citing recent work by the Graham Fleming group at Berkeley. "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."

Valence electrons control how atoms bond with other atoms to form molecules or crystal structures, and how these bonds break and reform during chemical reactions. Changes in molecular structures occur on the scale of many femtoseconds and have often been observed with femtosecond spectroscopy, in which both Leone and Krausz are pioneers. 

The experimenters separately generated extreme-ultraviolet attosecond pulses (using the technique called "high harmonic generation") and sent the beam of attosecond probe pulses through the krypton gas on the same path as the near-infrared pump pulses.

By varying the time delay between the pump pulse and the probe pulse, the researchers found that subsequent states of increasing ionization were being produced at regular intervals, which turned out to be approximately equal to the time for a half cycle of the pump pulse. (The pulse is only a few cycles long; the time from crest to crest is a full cycle, and from crest to trough is a half cycle.)

"The femtosecond pulse produces a strong electromagnetic field, and ionization takes place with every half cycle of the pulse," Leone says. "Therefore little bursts of ions are coming out every half cycle.".

"When the bursts of ions are made quickly enough, with just a few cycles of the ionization pulse, we observe a high degree of coherence," Leone says. "Theoretically, however, with longer ionization pulses the production of the ions gets out of phase with the period of the electron wave-packet motion, as our work showed."

The physical demonstration of attosecond transient absorption by the combined efforts of the Leone and Krausz groups and their colleagues will, in Leone's words, "allow us to unravel processes within and among atoms, molecules, and crystals on the electronic timescale" – processes that previously could only be hinted at with studies on the comparatively languorous femtosecond timescale.

Casey Kazan via by Lawrence Berkeley National Laboratory

Image credit: Courtesy Lawrence Berkeley National Laboratory.

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