If some of the glamour of carbon-based nanostructures has passed to carbon nanotubes, C₆₀ clusters (buckminsterfullerene, or even more familiarly, buckyballs) retain considerable allure in many areas of science. Cations (C₆₀⁺) are of particular interest. In astrophysics, the infrared bands due to molecular vibrations in these cations have recently been implicated in the long-standing problem of diffuse interstellar infrared bands. In condensed matter, it appears that some of the properties of solids comprising clusters weakly bonded by van der Waals forces are connected to the electronic structure of the isolated cations. For example, superconductivity may be mediated by an electron–phonon interaction that is strengthened by the Jahn–Teller effect in the cations.

Buckyballs joined diamond and graphite in the 1980s as the third known form of pure carbon.

When Buckyballs Go Out of Round

Stability in nature is associated with the lowest possible energy. The arrangement of atoms in a molecule is a prime example; the atoms assume a configuration with the lowest energy, and then they stay put except for the slight quiver of molecular vibrations. Quantum mechanics, which governs motion on the atomic level, adds a subtlety. When the nominally lowest-energy configuration is highly symmetric like cubes or pyramids, interactions between the molecule's electrons and the atomic vibrations make it possible to lower the energy still further by a slight distortion to a less symmetric configuration.
All of this might seem to lean toward the arcane side, but it happens that such refinements can work important results at our everyday macroscopic level. At the ALS, Canton et al. have identified for the first time in positively charged ions of the soccerball-shaped carbon clusters (C₆₀) familiarly known as buckyballs a particular example of a distortion with the jargon name "dynamic Jahn–Teller effect." Buckyball ions are of interest to scientists partly because it appears that properties such as superconductivity (resistanceless flow of electricity) in solids comprising loosely bound collections of buckyballs are closely related to the structure of isolated ions.

The Jahn–Teller effect in buckyball cations is associated with the breakdown of the widely used Born–Oppenheimer approximation that allows theorists to calculate the electronic and vibrational states separately with any interaction between the two systems treatable as a small perturbation. In highly symmetric molecules or solids, where otherwise distinct electron states may be degenerate (have the same energy), the Born–Oppenheimer approximation is not necessarily valid because the electronic and vibrational systems can then interact strongly to form coupled "vibronic" states. In the Jahn–Teller effect, this interaction results in structural distortion that lowers the symmetry while enabling the atoms to assume a "relaxed" geometry. A splitting of the degenerate electronic states and a lower energy is another consequence.

At the ALS, the international collaboration investigated free C₆₀⁺ ions produced by ionizing with synchrotron radiation a beam of neutral particles from a heated oven. The group conducted valence photoelectron spectroscopy measurements at a photon energy of 50 eV in this crossed-beam configuration. They found that the first band in the photoionization spectrum, which is due to excitation from the highest occupied molecular orbitals (HOMO), consisted of three components. Curve fitting with three asymmetric Gaussian peaks and a simplified cation potential well enabled the group to reproduce the measured spectrum and determine the symmetry and energy of each component.

The structural distortion that accompanies the Jahn–Teller effect is reflected in the photoionization spectrum. The measured spectrum for buckyball cations exhibits three features (the two peaks and the shoulder marked by arrows). The differential spectrum (inset) obtained from the signal derivative clearly defines the positions of these features.	The photoionization spectrum when curve-fitted with three asymmetric Gaussian peaks and a simplified cation potential (inset) reproduces the measured spectrum and provides the symmetry and energy of each peak.

Interpreting these findings, the group concluded that the three components were due to vibronic states that tunnel between energetically equivalent potential wells in the distorted geometry, which has the overall symmetry D_3d. Tunneling is what makes this Jahn–Teller effect dynamic, as opposed to a static effect in which nuclear states are confined to one well. The observed peak with three bands with relative intensities appropriate to this symmetry appears to rule out an alternative static Jahn–Teller D_5d geometry suggested by previous experiments by other groups with optical and infrared spectroscopy of C₆₀⁺ ions trapped in glassy or rare-gas matrices. Earlier photoionization measurements also ruled out the possibility that the three peaks were due to individual vibrational states.

Research conducted by S.E. Canton (Western Michigan University and ALS), A.J. Yencha (State University of New York at Albany), E. Kukk (Oulu University, Finland), J.D. Bozek (ALS), M.C.A. Lopes (Universidade Federal de Juiz de Fora, Brazil), and G. Snell and N. Berrah (Western Michigan University).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES), and CNPq (Brazil). Operation of the ALS is supported by BES.

Publication about this research: S.E. Canton, A.J. Yencha, E. Kukk, J.D. Bozek, M.C.A. Lopes, G. Snell, and N. Berrah, "Experimental Evidence of a Dynamic Jahn–Teller Effect in C₆₀⁺" Phys. Rev. Lett. 89, 045502 (2002).

ALSNews Vol. 212, November 27, 2002