QLC: EAGER: Quantum control of energy transfer pathways and chemical reactions

化学中的挑战之一是利用光从化学反应中产生特定的产品。如果能够实现这一目标，将推进广泛的技术，从能量转换(例如，光到电或合成燃料)到化学传感，再到化学工艺效率的普遍提高。在这项由化学结构、动力学和机制-化学系计划支持的项目中，密歇根州立大学的Marcos Dantus和Benjamin Levine教授正在使用实验和理论建模相结合的方法来设计能够导致特定化学反应的激光脉冲。光脉冲的持续时间通常是几飞秒(一飞秒是一万亿分之一秒)，并且可以被设计成包含所需的光波长范围(一系列颜色)，或者甚至在脉冲持续时间内改变波长。根据其形状的不同，光脉冲以不同的方式影响分子内电子的运动。由于电子在分子的原子之间形成键，所以有可能控制键如何断裂和重新形成。换句话说，激光脉冲的形状可以控制化学反应的结果。参与这个项目的研究生和本科生学习光-物质相互作用，并与从不同角度考虑这些现象的小组(光谱学家、理论家和合成化学家)合作。研究人员定期将高中生纳入他们的研究努力，并与旨在增加从事研究生学习和研究事业的未被充分代表的学生数量的项目密切合作。该项目实施了一种新的战略，以实现对凝聚阶段大有机分子的能量流动和反应性的一致控制。认识到不同的电子激发态经历不同的化学反应，成形激光脉冲被用来(A)填充具有所需反应性的电子态，以及(B)最大限度地减少自发跃迁出所需电子态的可能性(例如，内部转换)。在追求(B)的过程中，使用了从半经典的量子控制策略(沿着特定的反应坐标驱动振动波包)到没有经典类似物的量子控制策略。例如，当势能面被光场覆盖时，可以利用电子态相交附近的拓扑效应来影响反应结果和强耦合。在这种情况下，自然能量流被改变，分子系统S与驱动场的相干性得以增强。先进的量子动力学模拟能够确定初始波包结构和反应结果之间的因果关系，从而为后续实验提供信息。通过较高激发态的荧光产额来跟踪内部转换的成功控制。随后，类似的策略被用来驱动一系列染料的解离反应，这些染料只有在激发到更高的激发态时才会释放高效的荧光团。这项实验和理论相结合的工作阐明了最大限度地提高驱动凝聚相化学反应所需的光子能量比例的策略。该奖项反映了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。

One of the challenges in chemistry is to produce specific products from chemical reactions using light. If this objective can be achieved, a wide range of technologies would be advanced, from energy conversion (e.g., light to electricity or synthetic fuel) to chemical sensing, to general improvement of chemical process efficiency. In this project supported by the Chemical Structure, Dynamics and Mechanisms-A Program of the Division of Chemistry, Professors Marcos Dantus and Benjamin Levine of Michigan State University are using a combination of experiment and theoretical modeling to design laser light pulses that can result in specific chemical reactions. The light pulses are typically a few femtoseconds in duration (a femtosecond is one-quadrillionth of a second), and can be designed ("shaped") to contain a desired range of light wavelengths (a range of colors), or even change wavelength over the pulse duration. Depending on their shape, the light pulses affect the motions of electrons inside the molecules in different ways. Since electrons form the bonds between the atoms of a molecule, it is possible to control how the bonds break and re-form. In other words, the shape of the laser light pulses can control the outcome of chemical reactions. The graduate and undergraduate students involved in this project learn about light-matter interactions and collaborate with groups that consider these phenomena from different perspectives (spectroscopists theorists, and synthetic chemists). The researchers regularly include high school students in their research efforts and work closely with programs aimed at increasing the number of underrepresented students who pursue graduate study and research careers.This project implements a novel strategy for achieving coherent control of the energy flow and reactivity of large organic molecules in the condensed phase. Recognizing that different electronic excited states undergo different chemical reactions, shaped laser pulses are being used to (a) populate electronic states with desirable reactivities, and (b) minimize the probability of spontaneous transition out of the desired electronic state (e.g. internal conversion). In pursuit of (b), quantum control strategies that range from semi-classical (driving the vibrational wave packet along a particular reaction coordinate) to quantum strategies with no classical analogue are being used.For example, topological effects near intersections between electronic states can be exploited to influence the reaction outcome and strong coupling, for example when potential energy surfaces are dressed by the light field. In such cases, the natural energy flow is altered and the molecular system?s coherence with the driving field can be enhanced. Advanced quantum dynamical simulations are enabling the determination of causal relationship between the structure of the initial wave packet and reaction outcomes, thus informing subsequent experiments. Successful control of internal conversion are tracked by the fluorescence yield from higher excited states. Subsequently, similar strategies are used to drive dissociative reactions in a series of dyes, which release a highly efficient fluorophore only when excited to a higher excited state. Together, this combined experimental and theoretical effort is elucidating strategies to maximize the fraction of photon energy needed to drive a condensed phase chemical reaction.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.