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Regarding Confinement Resonances Print
Wednesday, 27 July 2011 00:00

When an atom is encapsulated inside a hollow spherical carbon buckyball, the complex is called an “endofullerene.” Theoretically, if the atom is an unreactive noble gas like xenon, it should be centered within the cage. If one or more of the atom’s electrons are boosted out of the cage by an x-ray photon, the electron waves may be transmitted through or reflected off the carbon cage, giving rise to an interference effect similar to waves in a water tank. These so-called “confinement resonances” were predicted theoretically a decade ago but have never been observed. In the first experimental test of this theory, members of an international team led by Ronald Phaneuf, University of Nevada, and working at ALS Beamline 10.0.1 produced and isolated xenon endofullerenes and observed confinement resonances.

Using Carbon Cages

Atoms and molecules encaged within carbon fullerenes have potentially important applications in medical diagnosis and treatment, astrophysics, and energy research. Endohedral fullerenes (EHs) are effective tools for isolating caged species, preventing chemical interactions with their environment. Extremely reactive or even radioactive species could be inserted into tissue using an EH, preventing chemical bonding between the tissue and encaged species. This could serve as a delivery system for gases or other molecules, transporting them to a desired location within the body before a reaction begins.

EHs have been found in meteor and asteroid impact craters on the earth. Measurements of the isotopic abundances of caged atoms found therein indicate that these molecules were created elsewhere in the universe, not upon impact with the Earth. Infrared signatures of fullerene molecules have also been recently observed in interstellar space. Fullerenes, even EHs, can evidently be created by natural processes in the universe and may perhaps play a role in the origin of life as we know it.

In energy research, EHs are being explored as a convenient means to safely store large quantities of hydrogen, as temporary storage to move unreactive hydrogen to the right place. The merged-beams technique discussed here, using synchrotron radiation, is a powerful probe of the internal structure and dynamics of these exotic molecules, even when only small numbers of them are available for study.

Illustration of the endofullerene Xe@C60.

Previous photoionization measurements taken at beamline 10.0.1 of the endohedral fullerene molecular ion Ce@C82+ (a Ce atom inside an 82-carbon atom cage) revealed distinct signatures of photoexcitation of the Ce 4d inner subshell, but no evidence of oscillatory structure in the cross sections attributable to confinement resonances. Their absence is likely explained by the fact that the Ce atom is triply ionized and located significantly off­-center inside the C82 cage, masking any signatures of multipath interference.

A measurement of photoionization of Xe@C60 in the energy range of Xe 4d ionization is therefore optimal for revealing evidence of confinement resonances and for exploring other multielectron phenomena associated with an atom in a cage. Such an experiment has not previously been performed because yields of synthesized noble-gas endohedral fullerenes have been too small, precluding their purification and isolation in quantities sufficient for experiments with free endohedral molecules.

 

Setup used to synthesize Xe@C60

Preparation of samples containing Xe@C60 was done by David Kilcoyne (ALS) over several months prior to taking measurements. A 50–200 eV beam of Xe+ from an ion sputter gun was directed onto the surface of a rotating metal cylinder onto which 99.95% pure C60 was continually being deposited in vacuum by evaporation from a small oven. The accumulated powder (few tens of mg) was scraped from the surface and placed into a small oven for evaporation into a low-power discharge in an electron cyclotron resonance (ECR) ion source.

Only one in 5,000 C60 molecules in the accumulated samples contained a Xe atom. This was sufficient to produce a mass-analyzed Xe@C60+ ion beam current of less than a picoampere. The team at ALS Beamline 10.0.1 pushed the sensitivity of the merged-beams technique using synchrotron undulator radiation to its practical limits. On average, only a few tens of Xe@C60+ ions were present in the interaction region, corresponding to a target ion density of the order of one ion per cubic centimeter.

 

Excess cross section for double photoionization accompanied by release of C2 for Xe@C60+ relative to the same process for empty C60+. Error bars are statistical, representing 1 standard  deviation from the mean. Plotted for comparison are RPAE theoretical cross sections for 4d photoionization of free Xe (solid black curve), Xe in a C60 cage of zero thickness (dashed blue curve), and Xe in a cage of finite thickness (dotted red curve). A time-dependent local-density approximation (TDLDA) calculation for Xe@C60 is shown by the green dash-dotted curve. The theoretical curves have each been divided by 10 to match the measurement.

The resulting experimental data indicate a strong, statistically significant enhancement of the photoionization cross section due to the presence of Xe inside the C60+ cage. They also suggest the oscillatory structure of amplitude and period comparable to the predictions, although shifted in photon energy by several eV. It should be noted that in the calculations the cage is initially uncharged, whereas it was singly ionized in the measurement. The energy shift of the oscillatory structure relative to the model-potential calculations likely results from differences in their characterizations of the fullerene shell.

 


 

Research conducted by A. Müller and S. Schippers (IAMP, Justus-Liebig University, Giessen); C. Cisneros (Instituto de Ciencias Físicas); G. Alna’Washi (The Hashemite Univ.); N.B. Aryal, K.K. Baral, D.A. Esteves, C.M. Thomas, and R.A. Phaneuf (Univ. of Nevada); and A.L.D. Kilcoyne and A. Aguilar (ALS).

Research funding: U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES); the German Research Foundation; and the National Council on Science and Technology (Mexico). Operation of the ALS is supported by DOE BES.

Publication about this research: A.L.D. Kilcoyne, A. Aguilar, A. Müller, S. Schippers, C. Cisneros, G. Alna’Washi, N.B. Aryal, K.K. Baral, D.A. Esteves, C.M. Thomas, and R.A. Phaneuf, “Confinement resonances in photoionization of Xe@C+60,” Phys. Rev. Lett. 105, 213001 (2010).

ALS Science Highlight #233

 

ALSNews Vol. 322