Beyond the Surface Plasmon
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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).
ALSNews
Vol. 256, August 31, 2005 |