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Photoexcitation of a Volume Plasmon in Buckyballs Print

For molecules made from a single element, buckyballs (carbon-60) are very large. They mark 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 can behave 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 eV, the four valence electrons belonging to each of the 60 carbon atoms in a buckyball, 240 in all, act collectively, resulting in a "surface plasmon." This collective motion is a back-and-forth oscillation of the whole cloud of valence electrons, relative to the effectively rigid cage of carbon cores. Now, the latest results from a U.S.–German collaboration on the electronic structure of photoexcited buckyball ions show an additional resonance near 40 eV, characterized as a volume plasmon made possible by the special fullerene geometry.

Beyond the Surface Plasmon

Plasma is the state of matter in which electrons, ions, and photons mix freely with electrically neutral particles—an ionized gas, if you will. The "electron gas" in a metal can be considered one form of plasma. When such electrons oscillate back and forth to form a wave, the corresponding quasiparticle is called a "plasmon." Plasmons confined to the surface of a material 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." Despite fundamental and practical interest in the interaction between plasmas and radiation, there is very little experimental information available—mainly because experiments have been so technically difficult. At ALS Beamline 10.0.1, specially designed to study the interaction of ions and photons, Scully et al. have found that the geodesic-dome structure of buckyballs makes possible volume plasmons—which exhibit not the back-and-forth oscillation of a surface plasmon but rather an in-and-out contortion, like a beach ball being squeezed.

The researchers measured absolute photoionization cross sections for C-60 ions at the Ion-Photon Beamline at the ALS (Beamline 10.0.1). There, information gathering starts with a pinch of fullerene soot evaporated in an ion source. A fine beam of the resulting buckyball ions is accelerated from the ion source and turned 90 degrees to collide with a 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. The photon beam is tuned through a range of values, from 17 to 75 eV. Photoexcited ions are deflected to a detector; there the number of ions reaching the detector at different photon energies and their ion charge states are recorded.

Absolute cross-section measurement of single photoionization of C-60 (open circles). The thin solid line results from the fit to the measured data of a linear background (not shown) plus the two separately displayed Lorentzian curves representing the surface and volume plasmons.

The group's first experiment with buckyballs 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. The results were presented at a workshop in Berlin, where they were heard by theorists from the Max Planck Institute for Complex Systems, who had predicted such a higher-energy resonance but had not published their prediction because of a lack of experimental evidence.

Combining their experimental observations and theoretical calculations, the collaborators interpreted the second resonance, occurring at a photon energy of 38 eV, as a volume plasmon, corresponding to a radial compression of the electron density, as opposed to the back-and-forth motion of a surface plasmon. The excitation of a volume plasmon in a solid conducting sphere is dipole forbidden, leading to its suppression in photoabsorption by metal clusters. However, a volume plasmon is possible in C-60 because of its shell geometry. The researchers' analysis considered the induced surface charges of the inner and outer surfaces of the shell. These surface charges can oscillate in two modes: one where they oscillate together relative to the shell, and one where they oscillate out of phase, creating local compression of the electron density with respect to the C-60 shell. This latter mode corresponds in effect to a dipole-allowed volume plasmon excitation.

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

When a 22-eV photon smacks into a charged buckyball, often the electron cloud surrounding it oscillates with enough energy to eject an electron. 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.

Research conducted by S.W.J. Scully, E.D. Emmons, M.F. Gharaibeh, and R.A. Phaneuf (University of Nevada, Reno); A.L.D. Kilcoyne and A.S. Schlachter (ALS); S. Schippers and A. Müller (Justus-Liebig-Universität, Germany); and H.S. Chakraborty, M.E. Madjet, and J.M. Rost (Max Planck Institute for the Physics of Complex Systems, Germany).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES); Deutsche Forschungsgemeinschaft; and Alexander von Humboldt Foundation. Operation of the ALS is supported by BES.

Publication about this research: S.W.J. Scully, E.D. Emmons, M.F. Gharaibeh, R.A. Phaneuf, A.L.D. Kilcoyne, A.S. Schlachter, S. Schippers, A. Müller, H.S. Chakraborty, M.E. Madjet, and J.M. Rost, "Photoexcitation of a Volume Plasmon in C60 Ions," Phys. Rev. Lett. 94, 065503 (2005).