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

通过将激光照射到一个分子上，人们可以探测到分子对辐射的反应和分子的连续弛豫过程。这是了解材料基本性质的有力科学方法，有利于基础科学和应用技术的进步。在目前的研究项目中，要研究的分子是巴克敏斯特富勒烯和更大的富勒烯，包括在这些分子内的原子/团（内富勒烯）和聚合富勒烯。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有着令人兴奋的应用前景。因此，了解这些系统的物理/化学结构和反应特性是具有重大科学意义的问题。使用大规模，高性能的计算机模拟程序旨在实时调查这些分子过程。将进行调查以了解系统内的电子如何单独耦合分子的振动，集体相互作用以在内部移动/放松或退出系统，以及系统的结构，几何形状和内部能量分布在每种机制中所起的作用。作为这项研究的一个有趣组成部分，计划通过将富勒烯材料与电子的奇异反粒子正电子碰撞来测试其中的一些光谱特性。本科学生参与，以提高他们的教育经验和激励他们为物理/科学事业。该项目包括以下详细研究：1)光致电荷转移（CT）是有机光伏的关键过程。离子自由度和电子自由度的强耦合驱动了这些超快过程。基于非绝热分子动力学的框架适合提供准确、全面的潜在机制描述。利用高性能的计算方法，该小组计划模拟和研究选定的内富勒烯的弛豫过程，以激发实验。将考虑约束原子或约束富勒烯内的初始光激发，或从原子到富勒烯的“转移”激发。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，包括瞬态延迟事件，将被实时跟踪。这一研究方向开辟了超快计时技术的新分支领域。二)富勒烯和内富勒烯的阿秒光发射计算和研究。这条研究路线将与超快AMO的积极研究相吻合，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。库仑间衰变（ICD）是簇中空位的一种非辐射弛豫，是当代关注的一个话题。内富勒烯是同心体系的不对称二聚体，可诱导新型icd。计划中的簇内富勒烯ICDs研究可以预测基本效应和驱动实验。V)正电子（Ps）在正电子撞击物质后形成。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知识将引领Ps光谱学的新方向，催生实验。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.