The encapsulation of atoms in fullerenes offers a unique natural laboratory to examine the behavior of an atom in confinement. Studies of these endohedral compounds can not only lead to intriguing effects at the atomic scale but also can probe subtleties of quantum effects in the nanometer region. Besides the single-wall confinement, the multi-walled confining shell of nested concentric single-walled fullerenes, bucky-onions,can make the coupling of the central atom with the shell rich in novel effects. Technologically also the endohedral fullerenes hold the promise of exciting applications: (a) Research is underway to use endohedrally doped fullerenes and bucky onions as seed materials in solid state quantum computations, in which quantum bits can be encoded in the electronic and nuclear spins of the encapsulated atoms. (b) Recent experiments with Ar@C60 find evidence of encaged atoms significantly improving the superconducting ability of materials. (c) A proposed biomedical application of endohedral materials is to shield radioactive tracers inside fullerene cages, and then inject the material into human blood to monitor blood flow. (d) The ability of fullerenes to sequester metal atoms inside has led to exploring their potential as contrast-enhancing agents for magnetic resonance imaging. The discovery of endo fullerenes with trapped noble gases in extraterrestrial environments indicates the astrophysical relevance of their studies. Therefore, understanding the influence of the confining cage on the spectroscopy of the atom inside, and vice versa, are matters of significant interest. A theoretical photoionization study of these compounds is proposed. Photoionization is a well known method to obtain a fairly undistorted account of the many-electron dynamics of the discrete and continuum states of atomic systems, since the coupling of the photon with the electrons is so weak and the photon disappears in the final channel. Employing this tool we will investigate the role of the collective motion of delocalized electrons in endohedrally doped single- and multi-walled fullerenes. Cross sections and asymmetry parameters of the ionization from both atomic and fullerene sub-shells will be calculated to understand the many-body interactions that determine the photoabsorption properties of these nanoparticles. Moreover, the diffraction of atomic photo-liberated electrons in crossing the confining wall will be studied in detail in the spirit of a recently discussed Fourier photo-spectroscopy approach.
Intellectual Merit: The encapsulation of an atom or an atomic cluster, or even a smaller fullerene (buckyball) in a fullerene cage offers a unique natural laboratory in which to examine the behavior of the guest system in confinement. Studies of these endo-fullerenes can not only lead to intriguing effects at the atomic scale but also can probe subtleties of physical processes in the nanometer (one billionth of a meter) dimension. Endo-fullerenes also hold the promise of exciting applications in areas including, quantum computations, superconductivity, biomedical fields, drug delivery research, magnetic resonance imaging, and organic photovoltaic devices. Hence, understanding the influence of the confining carbon cage on the confined species and vice versa, are matters of great scientific interest. A theoretical/computational study of the response of these compounds to the optical, ultra-violet and x-ray radiations has been performed in this research. The roles of (i) the collectivized plasmonic motion of electrons, like the fictional (Star Trek) Borg – the assimilated cybernetic organisms of a singular consciousness, (ii) the dopant-fullerene bonding and hybridization, and (iii) the oscillations due to interferences between electrons simultaneously dislodged from various sites of the compound have been investigated. Broader Impacts: (I) For single-walled endo-fullerenes, while there have been some studies with radiations having energies well beyond where the plasmons arising from the collective motion of the fullerene electrons appear, much about the influence of the plasmon behavior on atomic dynamics was unknown before our research. Studies undertaken in this research have revealed spectacular increase in the number of electrons knocked off (ionized) from the encaged atom by the incoming radiations when radiation energies match the energies of electrons in their plasmonic motion. With the capability of precision measurements, experiments are expected to be performed with the current technology to test this prediction. (II) Previous theoretical methods, due to their inherent limitations, were oblivious to the mixing between the electron cloud of captor fullerenes and captive atom. Our research has been able to explore these processes to detect novel atom-fullerene hybrid states and their rich ionization properties. (III): For a confined atom the liberation of atomic electrons involves scattering off the edges of the confining wall, as well as collateral ejection from the wall itself. The quantum interference signal of all these electron waves carries a wealth of information on the geometry of the confining shell. While specific features in the electron flux from atoms in C60 due to this effect have been predicted, no detailed scrutiny of the result with an aim to extract geometric information of the confinement has made until our research. Our findings on this aspect have significantly impacted the field and are likely to stimulate experiments.
