Dynamics of Molecules in Extreme Rotational States Made with an Optical Centrifuge

大多数分子在室温下是稳定的，但在更高的温度下变得更具反应性。温度升高会使分子在空间中飞行得更快（换句话说，它们的“平移动能”增加），这反过来会导致更大的化学反应性，因为分子之间的碰撞会产生更大的能量。 随着温度的升高，分子也会振动得更多（它们的键会以更多的能量拉伸和弯曲）;更高的振动能量也会增加化学反应性。 在过去的许多研究中，平移能和振动能是如何影响化学反应的。 然而，还有另一种分子运动没有得到很好的研究：分子也会旋转，如果它们以非常高的速度旋转，它们的键可能会拉伸甚至断裂。本研究由化学系化学结构动力学与机理（CSDM-A）项目资助。马里兰州大学的穆林利用精密的激光技术来制备和研究具有大量旋转能量的分子。穆林教授和她的学生利用他们所谓的光学离心机创造出处于高能旋转状态的分子。这种技术使用强光脉冲来捕获分子，并使它们以非常高的能量旋转。这种方法使他们能够研究分子旋转的速度如何影响原子在化学键中的紧密程度，以及如何通过与其他分子的碰撞来分享能量。这项研究具有更广泛的潜在社会效益的影响，因为它增加了对高能环境中分子的理解，如在自然界中发现的分子（即，在天体物理学和行星大气中）以及在高温工业和地质应用中。该项目还培训学生设计、建造和使用先进的实验仪器和计算机建模。此外，该研究通过互动直播视频促进与公众的对话。该项目正在研究CO2，N2 O，CO和H2 CO分子在极端旋转状态下的行为（例如，J 200）。光学离心机的功能是在空间和时间上组合两个相反的啁啾光脉冲，每个脉冲具有相反的圆偏振，从钛：蓝宝石激光系统与两级放大。光学离心机将分子单向旋转到具有定向角动量的高旋转状态。使用时间分辨IR偏振光谱测量离心分子的产生和碰撞动力学。个别旋转状态的多普勒谱线给出了关于离心分子在碰撞时的平移能量分布和空间取向的信息。光学离心机中不同物质的相对捕获和加速效率确定了高效离心的分子特性。这些高能态的光谱研究正在测试低能分子模型是否适用于高转动能。实验观察解释使用当前模型的结构和碰撞动力学的低能分子，以确定旋转激发分子的键和碰撞力。研究结果可能会影响对具有非局部热力学平衡的环境中分子行为的认识，例如天体物理学，行星和恒星大气，等离子体，燃烧以及高温工业和地质应用。该研究是在控制和询问分子运动的前沿训练研究生进行复杂的基于激光的研究。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Most molecules are stable at room temperature, but become more reactive at higher temperatures. Increasing the temperature makes molecules fly through space faster (in other words their "translational kinetic energy" increases), and this in turn can lead to greater chemical reactivity because the collisions between molecules occur with greater energy. As temperature increases, molecules also vibrate more (their bonds stretch and bend with more energy); higher vibrational energies also can increase chemical reactivity. Many studies in the past have asked how translational and vibrational energies affect chemical reactions. However, there is another kind of molecular motion that is not so well-studied: molecules also rotate, and if they do so at very high rates, their bonds can stretch or even break. In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Amy S. Mullin of the University of Maryland uses sophisticated laser techniques to prepare and study molecules with large amounts of rotational energy. Professor Mullin and her students create molecules in high-energy rotational states using what they call an optical centrifuge. This technique uses intense pulses of light to trap molecules and cause them to spin with very high energy. This approach enables them to study how fast molecular spinning affects how closely atoms are held together in a chemical bond and how energy is shared by collisions with other molecules. The research has broader impacts of potential societal benefits from an increased understanding of molecules in high-energy environments such as found in nature (i.e., in astrophysics and planetary atmospheres) and in high temperature industrial and geological applications. The project is also training students in the design, construction and use of advanced experimental instruments and computer-based modeling. In addition, the research promotes dialogue with the general public through interactive live-streaming videos.This project is investigating the behavior of CO2, N2O, CO and H2CO molecules in extreme rotational states (e.g., J 200). The optical centrifuge functions by spatially and temporally combining two oppositely chirped pulses of light, each with opposite circular polarization, from a Ti:sapphire laser system with two stage amplification. The optical centrifuge spins the molecules unidirectionally into high rotational states with oriented angular momenta. The creation and collision dynamics of the centrifuged molecules are measured using time-resolved IR polarization spectroscopy. Doppler line profiles for individual rotational states give information about the translational energy distribution and spatial orientation of the centrifuged molecules as they undergo collisions. Relative capture and acceleration efficiencies for different species in the optical centrifuge identify the molecular properties of efficient centrifugation. These spectroscopic studies of high-energy states are testing whether low-energy molecular models are valid for high rotational energies. The experimental observations are interpreted using to current models for structure and collision dynamics of low energy molecules to identify the bonding and collision forces of rotationally excited molecules. Research results may impact knowledge about molecular behavior in environments that have non-local thermodynamic equilibrium such as in astrophysics, planetary and stellar atmospheres, plasmas, combustion, and high temperature industrial and geological applications. The research is training research students in sophisticated laser-based research at the forefront of controlling and interrogating molecular motion.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.