Garching, Germany and Berkeley, CA--A team of scientists led by groups from the Max Planck Institute of Quantum Optics (MPQ), the U.S. Department of Energy's Lawrence Berkeley National Laboratory (LBNL), and the University of California at Berkeley (UCB) has used attosecond absorption spectroscopy to directly observe the movement of an atom's outer electrons for the first time.1
The technique allowed the researchers to time the oscillations between simultaneously produced quantum states of valence electrons with great precision. These oscillations drive electron motion.
The team began by first ionizing krypton atoms, removing one or more outer valence electrons with pulses of near-IR laser light that were typically a few femtoseconds in duration. Then, with far-shorter pulses of extreme-UV (EUV) light on the 100 attosecond timescale, they were able to precisely measure the effects on the valence electron orbitals.
"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."
Fine points of valence-electron motion
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.
Zhi-Heng Loh of Leone's group at Berkeley Lab and UC Berkeley worked with Eleftherios Goulielmakis of Ference Krausz's group at MPQ to perform the experiments at MPQ. The EUV attosecond pulses were created using high-harmonic generation.
By varying the time delay between the femtosecond pump pulse and the attosecond 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 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." Although expected from theory, these isolated bursts were not resolved in the experiment. The attosecond pulses, however, could precisely measure the production of the ionization, because ionization leaves gaps, or "holes"—unfilled orbitals that the ultrashort pulses can probe.
The attosecond pulses do so by exciting electrons from lower energy orbitals to fill the gap in krypton's outermost orbital—a direct result of the absorption of the transient attosecond pulses by the atoms. After the "long" femtosecond pump pulse liberates an electron from the outermost 4p orbital, the short probe pulse boosts an electron from an inner orbital (3d), leaving behind a hole in that orbital while sensing the dynamics of the outermost orbital.
In singly charged krypton ions, two electronic states are formed. A wave-packet of electronic motion is observed between these two states, indicating that the ionization process forms the two states in what is known as quantum coherence.
Indispensable attosecond pulses
"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."
So after just a few cycles of the pump pulse, the coherence is washed out. Thus, says Leone, "Without very short, attosecond-scale probe pulses, we could not have measured the degree of coherence that resulted from ionization."
1. Eleftherios Goulielmakis et al., Nature, Vol. 466, p. 739, 05 August 2010, doi:10.1038/nature09212