Start the clock: a new direct method to study collisions of electronically excited molecules

了解大气、燃烧系统和技术等离子体的化学成分在现代世界中非常重要。在这些气相环境中，一个非常重要的过程是碰撞过程中分子之间的能量转移。分子所拥有的内部能量的总量很重要，但同样重要的是这种能量的形式。它可以是分子的平移、旋转或振动运动的形式，甚至可以是电子的排列形式。大量的能量可以储存在分子的电子结构的重排中，从而产生电子激发态。这种激发态出现在燃烧或等离子体等高能环境中，它们的衰变是典型观察到的光发射的原因。当使用光学方法在实验中探测分子时，通常也会产生激发态。电子能量可以重新辐射，但通常不会在分子与其他气相分子发生碰撞之前进行。这可能导致激发电子态内的平移、旋转或振动能量转移。值得注意的是，它还可能涉及一种反应，使激发的分子完全消失，或者一种猝灭碰撞，在这种碰撞中，电子能量损失给碰撞伙伴。尽管它们很重要，但令人惊讶的是，人们对电子激发的分子碰撞所涉及的基本力知之甚少。确定这种分子碰撞的详细动力学的最好方法是使用一种称为交叉分子束(CMB)散射的技术。在这种方法中，不同分子的特定光束在真空室中碰撞，并以某种方式观察到它们的传播方向和内部能量的细节。我们将把以前用于电子基态分子的这项技术扩展到电子激发分子的碰撞，特别是NO，这是大气和燃烧中的一种重要分子。我们将建造一台最先进的CMB散射装置，通过使用不同波长的激光脉冲的组合来制备处于激发态的分子。在这些分子与目标分子相撞后，我们将使用一种名为速度图离子成像的基于激光的检测技术来探测散射分子，这种技术能够准确地测量它们的速度。产生被激发的NO的激光脉冲定义了实验的零时间，因此开始了时钟的滴答。这将使我们比通常在CMB实验中更好地定义实验，从而对散射动力学具有更高的灵敏度。我们将利用这种敏感性来研究NO与与燃烧和大气有关的简单分子，即氮气、氧气和一氧化碳的碰撞。这些都是已知的，显示出非常不同的猝灭和反应行为与激发的NO，但很少或根本不知道这些相互作用涉及的力。我们的实验结果将用来加深我们对什么是指导电子激发分子中的能量转移的理解，并将有助于推动更好的理论模型和计算的发展，以及为从事NO燃烧和大气探测工作的科学家提供直接相关的信息。

Understanding the chemistry of the atmosphere, combustion systems and technological plasmas is of great importance in the modern world. A very significant process in these gas-phase environments is the transfer of energy between molecules during collisions. The total amount of internal energy that a molecule possesses is important, but equally if not more important is what form that energy takes. It may be in the form of translational, rotational or vibrational motion of the molecule, or even in the arrangement of the electrons. A large amount of energy may be stored in a rearrangement of the electronic structure of a molecule, producing an electronically excited state. Such excited states occur in energetic environments such as combustion or plasmas, and their decay is what is responsible for the typically observed light emission. The excited states are often also created when molecules are probed in experiments using optical methods. The electronic energy may be re-radiated, but generally not before the molecule has undergone collisions with other gas-phase molecules. This may result in translational, rotational or vibrational energy transfer within the excited electronic state. Significantly, it may also involve either a reaction that removes the excited molecule altogether, or a quenching collision in which the electronic energy is lost to the collision partner.Despite their importance, surprisingly little is known about the fundamental forces involved in collisions of electronically excited molecules. The best way to determine the detailed dynamics of the collisions of such molecules is using a technique called crossed molecular beam (CMB) scattering. In this method, defined beams of different molecules are collided in a vacuum chamber, and the details of their directions of travel and internal energies are observed in some fashion. We will extend this technique previously used for electronic ground state molecules to the collisions of electronically excited molecules, specifically NO, which is an important molecule in the atmosphere and combustion. We will build a new state-of-the-art CMB scattering apparatus, and by using a combination of laser pulses of different wavelengths will prepare NO molecules in their excited state. After these have collided with the target molecules we will probe the scattered molecules with a laser-based detection technique called velocity map ion-imaging, which is capable of accurately measuring their velocities. The laser pulse that prepares the excited NO defines a zero-time for the experiment, and so 'starts the clock' ticking. This will give us much better definition of the experiment than is usual in CMB experiments, and hence higher sensitivity to the scattering dynamics. We will use this sensitivity to study the collisions of NO with simple molecules relevant to combustion and the atmosphere, namely N2, O2 and CO. These are known to show very different quenching and reactive behaviours with excited NO, but little or nothing is known about the forces involved in these interactions. The results from our experiments will be used to deepen our understanding of what is important is directing energy transfer in electronically excited molecules, and will help to drive the development of better theoretical models and calculations, as well as providing information of direct relevance to scientist working in the combustion and atmospheric probing of NO.