RUI: Photon Impact Ionization of Fullerene and Endofullerene Molecules: Cross Sections, Resonances, and Time-Delays

Northwest Missouri State University, Maryville MO

Investigators

Abstract

The encapsulation of an atom or a cluster of atoms, or even a smaller fullerene (buckyball made of carbon atoms) inside a larger fullerene cage offers a unique molecular-level laboratory in which to examine the behavior of the guest system in sub-nanometer to nanometer (1 billionth of a meter) size confinements. Studies of these so called endofullerenes can not only lead to intriguing effects at the atomic scale but also can probe processes within the nanometric space that can be accessed by the current technology. In fact, the endofullerenes hold the promise of exciting applications in areas including quantum computations, superconductivity, biomedical fields, drug delivery research, magnetic resonance imaging, and organic photovoltaic devices. Further, the discovery of endofullerenes in extraterrestrial environments indicates their astrophysical relevance. Hence, understanding the influence of the confining fullerene cage on the behavior of the confined species, and vice versa, are matters of great scientific interest. For atoms confined in a fullerene, recent studies have predicted huge enhancements and alterations in the atom's response to radiation. However, it is not known how the process will evolve if instead a cluster of metal atoms or a smaller fullerene is confined. By examining couplings between such captive-captor pairs, researchers will be able to uncover fundamental effects, thereby substantially adding to the current knowledge. With capabilities of precision measurements being available, such findings shall motivate experiments involving cluster-doped endofullerenes. Furthermore, advancements in technology for generating extremely short attosecond (1 billion billionth of a second) laser pulses enable study of the light-matter interaction time with unprecedented precision. Results from this program produced the best agreement so far with the argon atom's time-delay measurements. Encouraged by this result, attosecond response studies of endofullerenes will be initiated. The outcome may bridge the gap between atto- and nano-sciences to establish a new domain of research in 'atto-nano-science'. Finally, another planned research area will focus on processes wherein a light-driven stimulation is caused at one location inside the compound which subsequently de-stimulates to transfer energy off-site to cause a dramatic response in a new location. The current program will access processes in endofullerenes where such local stimulations may cause a global response. This is similar to an antenna-receiver pair at the molecular scale where the antenna couples to the incoming light and transfers energy globally to enhance the efficiency of the ultimate output by enabling the antenna to also contribute to the process in sync with the receiver. The effect and related knowledge may have significant utilization in nanoscale antenna technology. This project involves the theoretical study of the response of neutral and ionic endofullerenes to an external photon. Photoelectron cross sections, angular distributions, Wigner-Smith time delays, and intercoulombic decay (ICD) resonances for both pure and hybrid levels of the compound will be calculated. This will help to understand better: (i) The many-body interactions that determine the absorption, temporal and resonant-decay properties at low plasmonic energies; and (ii) The diffraction-type oscillations due to multipath interferences between electron waves from various sites of the compound. Several areas will be studied. First, for atoms confined in C60, recent studies predicted huge enhancements in the atomic photoionization over the C60 plasmon resonance energy region. However, it is not entirely known how this coupling will evolve if instead a metal cluster or a smaller fullerene is confined, since these systems can excite their own plasmons. It is expected that by examining couplings between the plasmon-active captive-captor pair novel effects will be discovered, thereby substantially adding to the current knowledge. With recent capabilities of precision measurements such findings shall motivate experiments involving cluster-doped or onion-type endofullerenes. Second, for a confined atom the photo-liberation of atomic inner-electrons involves reflection off the fullerene shell. For the atom-fullerene hybrid-levels emissions from both the atomic and the fullerene sites occur. The quantum multipath interference between these modes of emissions carries a wealth of information on the geometry of the compound. Replacing the inner atom by a cluster or a fullerene will further compound this interference effect, producing far richer structures in photoionization cross section that can be diagnosed with our recently established Fourier photospectroscopy methods, thereby, significantly advancing scientific knowledge. Next the intercoulombic decay (ICD) process is a naturally abundant nonradiative relaxation pathway of a vacancy in a cluster and a topic of intense contemporary interest. The precursor excitation to form this vacancy can be accomplished by promoting an inner shell electron to an excited state by the photon or charged particle impact. Endofullerenes, being rotational analogues of asymmetric dimers of two concentric and unequal systems, can induce novel ICD processes. Research results in this topic can, therefore, generate significant experimental impetus, besides discovering fundamental effects. Finally, advancements in technology for generating attosecond laser pulses enable study of the light-matter interaction with unprecedented precision by pump-probe experiments. Attosecond photoemission studies of endofullerenes have been initiated. The outcome may bridge the gap between atto- and nano-sciences to establish a new domain of research in 'atto-nano-science'.