|Photoexcitation of a Volume Plasmon in Buckyballs|
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.
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.
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.
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).