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

将一个原子或一簇原子，甚至一个更小的富勒烯（由碳原子组成的巴基球）封装在一个更大的富勒烯笼中，提供了一个独特的分子水平实验室，可以在亚纳米到纳米（10亿分之一米）的尺寸范围内检查客体系统的行为。对这些所谓的内富勒烯的研究不仅可以在原子尺度上产生有趣的效应，而且可以在纳米空间内探测到当前技术可以进入的过程。事实上，内富勒烯在量子计算、超导、生物医学领域、药物输送研究、磁共振成像和有机光伏器件等领域有着令人兴奋的应用前景。此外，在地外环境中发现的内富勒烯表明了它们在天体物理学上的相关性。因此，了解限制富勒烯笼对限制物种行为的影响，反之亦然，是具有重大科学意义的问题。对于被限制在富勒烯中的原子，最近的研究预测，原子对辐射的反应会有巨大的增强和改变。然而，如果一簇金属原子或更小的富勒烯被限制，则不知道该过程将如何发展。通过检查这些捕获者-捕获者对之间的耦合，研究人员将能够发现基本的影响，从而大大增加了现有的知识。有了精确测量的能力，这些发现将激发涉及团簇掺杂内富勒烯的实验。此外，产生极短阿秒（十亿分之一秒）激光脉冲的技术的进步使光-物质相互作用时间的研究具有前所未有的精度。该程序的结果与氩原子的时间延迟测量结果最吻合。受此结果的鼓舞，内皮富勒烯的亚秒反应研究将启动。这一结果可能会弥合atto-nano-science之间的差距，从而建立一个新的“atto-nano-science”研究领域。最后，另一个计划的研究领域将集中于在化合物内的一个位置引起光驱动刺激的过程，该过程随后释放刺激以转移能量，从而在新位置引起戏剧性的反应。目前的项目将进入内富勒烯的过程，在那里这种局部刺激可能引起全局反应。这类似于分子尺度上的天线-接收器对，天线与入射光耦合，并在全球范围内传输能量，通过使天线也与接收器同步促进过程，从而提高最终输出的效率。该效应及其相关知识在纳米天线技术中具有重要的应用价值。本项目涉及中性和离子型内富勒烯对外部光子响应的理论研究。将计算该化合物纯能级和杂化能级的光电子截面、角分布、Wigner-Smith时间延迟和库仑衰变（ICD）共振。这将有助于更好地理解：(i)在低等离子体能量下决定吸收、时间和共振衰减特性的多体相互作用；（ii）由于来自化合物不同位置的电子波之间的多径干扰而产生的衍射型振荡。将研究几个领域。首先，对于被限制在C60中的原子，最近的研究预测在C60等离子体共振能量区原子的光离会有巨大的增强。然而，如果金属团簇或更小的富勒烯被限制，这种耦合将如何演变还不完全清楚，因为这些系统可以激发它们自己的等离子体激元。预计通过检查等离子体-活跃的捕获者-捕获者对之间的耦合，将发现新的效应，从而大大增加了现有的知识。随着最近精确测量的能力，这些发现将激发涉及簇掺杂或洋葱型内富勒烯的实验。其次，对于一个受限原子，原子内部电子的光释放涉及到富勒烯壳层的反射。对于原子-富勒烯杂化能级，原子和富勒烯两个位置都有辐射。这些发射模式之间的量子多径干涉携带了化合物几何形状的丰富信息。用团簇或富勒烯取代内部原子将进一步复合这种干涉效应，在光电离截面上产生更丰富的结构，可以用我们最近建立的傅立叶光谱学方法进行诊断，从而显著推进科学知识。其次，库仑衰变（ICD）过程是团簇中空位的自然丰富的非辐射弛豫途径，也是当代强烈关注的话题。形成该空位的前体激发可以通过光子或带电粒子撞击将内壳层电子促进到激发态来实现。内富勒烯是两个同心和不对称体系的不对称二聚体的旋转类似物，可以诱导新的ICD过程。因此，本课题的研究成果除了发现基本效应外，还能产生重大的实验推动作用。最后，产生阿秒激光脉冲的技术的进步使得通过泵浦探针实验以前所未有的精度研究光-物质相互作用。内富勒烯的亚秒光发射研究已经开始。这一结果可能会弥合atto-nano-science之间的差距，从而建立一个新的“atto-nano-science”研究领域。

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'.