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May 13, 2005
Making the Buckyballs Ring

Ronald Phaneuf, Foundation Professor of Physics at the University of Nevada, Reno, is fascinated by plasma, the state of matter in which unbound electrons, atomic and molecular ions (lacking one or more electrons and therefore positively charged), and photons (particles of light) may mix freely with electrically neutral atoms and molecules.

"More than 99.99 percent of the known matter in the universe is highly ionized plasma, and most of what we know about the distant universe comes to us by way of photons and energetic particles produced in hot plasmas, such as the atmospheres of stars," Phaneuf says. "Yet there is very little experimental information on the electronic structure of ions that interact with photons in this state — mainly because it has been so technically difficult to do the experiments."

Beamline 10.0.1 at the Advanced Light Source is specially designed to study the interaction of ions and photons. Bright beams of ultraviolet or soft x-ray photons from the ALS enter lower left. A beam of ions is created in the ECR ion source, far right. The tightly focused beams collide in the interaction region, and ionization products are collected and measured by the detector.

Enter beamline 10.0.1 at the Advanced Light Source (ALS), which is under the direction of David Kilcoyne of the ALS Scientific Support Group. The beamline is equipped with a specialized research facility that collides opposing beams of photons and ions, an experiment designed by Phaneuf's University of Nevada research group.

Here a versatile ECR (for electron-cyclotron-resonance) ion source, developed by Phaneuf's long-standing collaborators Alfred Müller and Stefan Schippers at the Justus-Liebig-University of Giessen, Germany, sends a beam of singly or multiply charged ions to interact with an extraordinarily bright beam of photons. The energy of the photon beam, produced by the beamline's associated undulator magnet, can be tuned from ultraviolet to soft x-rays with very high spectral resolution.

Working with collaborators from the U.S. and Germany, as well as from Brazil, Great Britain, Denmark, and Hungary, Phaneuf and his group have conducted numerous electronic-structure studies of photoexcited ions over the years, including carbon, nitrogen, oxygen, fluorine, neon, scandium, titanium, manganese, nickel, and other atomic and molecular species. Their latest results, however, by tackling the electronic structure of photoexcited carbon-60 (so-called buckminsterfullerenes, or buckyballs), break new ground.

Fred Schlachter, one of the group of collaborators on the ALS staff, puts it this way: "For a molecule made from a single element, C-60 is a very large. It marks the transition from atoms to solids." In atoms and small molecules, the behavior of electrons is accounted individually; in bulk materials, a sea of innumerable electrons behaves en masse, yielding a very different description of electronic structure.

Buckyballs perch on the cusp between these states, as evidenced by the discovery in the early 1990s that, when subject to excitation energy of about 22 electron volts (22 eV), the four valence electrons belonging to each of the 60 carbon atoms in a buckyball, 240 in all, act collectively, resulting in a "giant resonance." Such collective motion, when constrained to a so-called normal mode of oscillation, is called a plasmon.

The collective motion first identified in C-60 was a surface plasmon, a back-and-forth normal-mode oscillation of the whole cloud of valence electrons, relative to the effectively rigid cage of carbon cores. (Carbon atoms occupy the vertices of the hexagons and pentagons that trace out a buckyball's characteristic soccer-ball geometry.)

Surface plasmons can produce obvious effects. A February 22, 2005 article by Kenneth Chang in the New York Times notes that the deep red color in medieval stained-glass windows is actually produced by nanoparticles of gold: "Electrons at the surface of the nanoparticles slosh back and forth in unison, absorbing blue and yellow light. But longer-wavelength red light reflects off the particles." The Times calls the stained-glass artisans "the first nanotechnologists."

Stained-glass window makers in the Middle Ages achieved deep reds by dissolving gold in the molten glass. Surface plasmons on the nanosized particles of metal cause them to reflect red light selectively.

Phaneuf's group, headed up by then-postdoctoral fellow Shane Scully, now at Queen's University, Belfast, and including graduate students Mohammad Gharaibeh, now at the Jordan University of Science and Technology, and Erik Emmons, was inspired by electron-ion studies of C-60 in Germany to try similar experiments using the photon-ion facility at the ALS. David Kilcoyne says of these experiments, "The beauty is that this beam line and this end station, plus the international team Ron Phaneuf assembled, put us far ahead in the effort to gather detailed information about such phenomena."

Information gathering starts with a pinch of soot: soot containing fullerenes is evaporated in the ECR ion source. A fine beam of the resulting buckyball ions is accelerated from the ion source and turned 90 degrees to collide head-on with a similarly fine beam of ultraviolet photons. All but buckyballs of the desired charge state (+1 for most measurements, corresponding to a total of 239 valence electrons) are stripped out of the ion beam. For about a meter the overlapping ion and photon beams interact in the beamline.

The continuous beam of ions interacts with the photon beam as it is tuned through a range of values, from less than 20 eV to more than 70 eV. Photoexcited ions are deflected to a detector; there relevant measurements are made, notably the number of ions reaching the detector at different photon energies and their ion charge states. Because the charge state of the ions in the incident beam is known, the number of ions that have lost an additional electron through photoionization can be directly compared with the energy of the photons that collided with them.

The group's first experiment with C-60 resulted in a clearer-than-ever picture of the giant resonance at 22 eV — evident as a sharp peak in a graph showing the number of photoionized buckyballs arriving at the detector as a function of the energy of the photon beam. But instead of falling off smoothly from this peak as photon energy was increased, there was a secondary rise or shoulder in the curve.

Shortly after this experiment, Ron Phaneuf visited his collaborators in Germany, where Alfred Müller presented the results at a workshop in Berlin. A group of theorists from the Max Planck Institute for Complex Systems in Dresden was in attendance; on the basis of theoretical calculations, Jan-Michael Rost, Himadri Chakraborty, and Mohamed Elamine Madjet had predicted a higher-energy resonance than the known 22 eV giant resonance in C-60, but because of a lack of experimental evidence they had not published their prediction.

"We had an experiment looking for an explanation and they had an explanation looking for an experiment," says Phaneuf. "It was a scientific marriage made in heaven."

When stimulated by photons at an energy of about 20 electron volts, a buckyball displays collective electron motion as a surface plasmon. But when stimulated by photons of about 40 eV, the result is a different mode of collective electron motion, a volume plasmon.

The second resonance in C-60, occurring at a photon energy of 38 eV, is called a volume plasmon — not a back-and-forth oscillation of the valence electron cloud but rather an in-and-out contortion, like squeezing a beach ball. Such collective motion would be impossible if C-60 were a solid sphere instead of a hollow charged shell; for this reason it is not been observed in metal clusters like gold nanoparticles, where surface plasmons are common.

When a 22-eV photon smacks into a singly charged buckyball having 239 outer electrons, often the whole spherical cloud surrounding the cage structure oscillates with enough energy to eject another electron: this is photoionization. The same thing happens when a 38-eV photon smacks into a charged buckyball, except that the electron cloud wobbles in and out, penetrating the cage — a phenomenon unique to charged buckminsterfullerenes. Like hitting a big bronze bell with a clapper, it's a way to make the buckyballs ring.

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