240 Electrons Set in Motion

A soccerball-shaped carbon-60 molecule, possessing a mobile team of up to about 240 valence electrons holding the structure together, is sort of halfway between being a molecule and a solid. To explore how all those electrons can move as an ensemble, a team of scientists working at the Advanced Light Source synchrotron radiation lab in Berkeley, turned the C-60 molecules into a beam (by first ionizing them) and then shot ultraviolet photons at them. When a photon absorbed, the energy can be converted into a collective movement of the electrons referred to as a plasmon.

Previously a 20-electron-volt “surface plasmon” was observed: the absorption of the UV energy resulted in a systematic oscillation of the ensemble of electrons visualized as a thin sphere of electric charge. Now a new experiment has found evidence of a second resonance at an energy of 40 eV. This second type of collective excitation is considered a “volume plasmon” since the shape of the collective electron ensemble is thought to be oscillating with respect to the center of the molecule (see figure at http://www.aip.org/png/2005/230.htm).

The collaboration consists of physicists from the University of Nevada, Reno (Ronald Phaneuf, 775-784-6818, phaneuf@unr.edu), Lawrence Berkeley National Lab, Justus-Liebig-University (Giessen, Germany), and the Max Planck Institute (Dresden). (Scully et al., Physical Review Letters, 18 February 2005)