RUI: Photoinduced Ultrafast Relaxation, Ionization, and Impact-Induced Positronium Formation of Fullerene Class of Molecules

通过将激光照射到分子上，人们可以探测分子对辐射的反应以及分子的连续弛豫过程。这是一种了解材料基本性质的强有力的科学方法，有利于基础科学和应用技术的进步。当前研究计划中要研究的分子是巴克明斯特富勒烯和较大的富勒烯，包括笼罩在这些分子(内富勒烯)中的原子/簇合物，以及聚合的富勒烯。这些材料在量子计算、超导、生物医学领域、药物输送研究、磁共振成像、分子器件、能量存储、转换和有机光伏等领域具有令人兴奋的应用前景。因此，了解这些体系的物理/化学结构和响应性质是非常有科学意义的事情。该程序使用大规模、高性能的计算机模拟，旨在实时研究这些分子过程。将进行研究，以了解系统中的电子如何单独耦合到分子的振动，共同相互作用以在系统内部移动/松弛或离开系统，以及系统的结构、几何形状和内部能量分布在每个机制中所起的作用。作为这项研究的一个有趣的组成部分，已经制定了计划，通过将富勒烯材料与正电子(电子的奇异反粒子)碰撞来测试其中一些光谱性质。本科生的参与是为了增强他们的教育经验，并激励他们投身物理/科学事业。该项目包括以下详细的研究：i)光致电荷转移(CT)是有机光伏中的一个关键过程。离子和电子自由度的强烈耦合推动了这些超快过程。基于非绝热分子动力学的框架适合于提供对潜在机制的准确、全面的描述。使用高性能计算方法，该小组计划模拟和研究选定的内富勒烯的松弛过程，以推动实验。我们将考虑限制原子内的初始光激发，或限制富勒烯，或从原子到富勒烯的“转移”激发。将实时跟踪在弛豫过程中导致电子在原子和富勒烯之间切换的重复中间CT，包括瞬时延迟事件。这一研究轨迹开启了超快计时器的一个新子领域。二)富勒烯和内富勒烯的阿秒光电子能谱计算和研究。这一研究路线将与超快AMO的活跃研究很好地吻合，在超快AMO中，新的阿秒激光脉冲使光与物质相互作用的精确研究成为可能。3)库仑间衰变(ICD)是原子团中空位的非辐射驰豫，是当代研究的热点。内富勒烯是同心体系的不对称二聚体，可以诱导新的ICD。计划中的簇内富勒烯ICD研究可以预测基本效应并推动实验。V)正电子作用于物质后形成的正电子(Ps)。从富勒烯/内富勒烯在等离子体共振能量下形成Ps的研究中学到的科学将为Ps光谱带来新的方向，产生实验。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

By shining laser light on a molecule, one can probe the molecule’s response to radiation and the molecule’s successive relaxation processes. This is a powerful scientific method to learn fundamental properties of materials and is therefore beneficial for the progress of basic science and applied technology. The molecules to be studied in the current research program are Buckminster fullerenes and larger fullerenes, including atoms/clusters caged inside these molecules (endofullerenes), and polymerized fullerenes. These materials hold the promise of exciting applications in areas including quantum computations, superconductivity, biomedical fields, drug delivery research, magnetic resonance imaging, molecular devices, energy storage, conversion and organic photovoltaics. Hence, understanding the physical/chemical structure and response properties of these systems are matters of great scientific interest. Using large scale, high performance computer simulations the program aims to investigate in real time these molecular processes. Investigations will be performed to learn how electrons inside the system individually couple to the molecule’s vibration, collectively interact with each other to move/relax internally or exit the system, and the role that structure, geometry, and internal energy distribution of the system plays in each mechanism. As an interesting component of the research, plans are in place to test some of these spectroscopic properties by colliding fullerene materials with positrons, the exotic antiparticles of electrons. Undergraduate students are involved to enhance their educational experience and motivate them for physics/science careers. The project includes the following detailed studies: I) Photoinduced charge transfer (CT) is a key process in organic photovoltaics. The strong coupling of ionic and electronic degrees of freedom drives these ultrafast processes. Frameworks based on nonadiabatic molecular dynamics are appropriate for providing accurate, comprehensive descriptions of the underlying mechanism. Using high-performance computational methodology, the group plans to simulate and study relaxation processes in selected endofullerenes to motivate experiments. Initial photoexcitations within the confined atom, or the confining fullerene, or from-atom-to-fullerene “transfer” excitations will be considered. Repeated intermediate CTs causing the electron to toggle between the atom and the fullerene, including transient delay events, during the relaxation will be followed in real time. This track of research opens a new subfield of ultrafast chronoscopy. II) Attosecond photoemission calculations and studies of fullerenes and endofullerenes. This line of research will dovetail well with active research in ultrafast AMO, where novel attosecond laser pulses have enabled precision studies of light-matter interactions. III) Inter-Coulombic decay (ICD) is a nonradiative relaxation of a vacancy in a cluster and a topic of contemporary interest. Endofullerenes, being asymmetric dimers of concentric systems, can induce novel ICDs. The planned studies on ICDs in cluster-endofullerenes can predict fundamental effects and drive experiments. V) Positronium (Ps) formation following the impact of positrons on matter. The science to be learned from the study of Ps formation from fullerenes/endofullerenes at plasmon resonance energies will usher new directions of Ps spectroscopy, spawning experiments.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.