Helium Droplet Spectroscopy of Atmospherically Significant Reaction Intermediates

在这个由化学部门化学结构动力学和机制（CSDM-A）和环境化学科学（ECS）项目资助的项目中，乔治亚大学的Gary Douberly教授正在使用液氦液滴分离和表征化学反应的中间体。反应中间体很难研究，因为它们在反应形成更稳定的分子之前只存在很短的时间。杜伯利教授和他的学生们克服了这个长期存在的实验难题，他们将单个反应物依次添加到液氦液滴中。在氦液滴的超冷环境中，大约零下452华氏度，反应中间体被“冻结”，从而有时间用激光光谱来观察它们的分子结构。这个项目的目标是在大气中发挥关键作用的两种基本化学反应，因此影响了我们对与空气污染和称为气溶胶的小液滴形成有关的化学反应的复杂网络的理解。该项目针对的第一个化学反应是异戊二烯的氧化，异戊二烯是一种小分子，占植物排放到大气中的有机化合物的一半，因此对我们了解对流层（地球大气的最低部分）非常重要。异戊二烯被羟基自由基氧化，羟基自由基被添加到氦滴中，红外光谱用于探测反应分支成各种中间体。然后中间体与氧反应产生相关的羟基过氧化自由基，这也是用红外技术研究的。该项目的第二个目标是确定观测质子从硫酸到水的自发转移所需的最小水分子数量，这一过程对大气气溶胶粒子的成核至关重要。这个过程是通过将水分子依次加入到含有单个硫酸分子的氦滴中来研究的。然后使用物种选择光谱来确定这些小的酸-水簇的结构，并确定酸中质子损失的开始。类似的测量探测氨（和其他胺）对减少自发质子转移所需的临界簇大小的影响。除了提供对基本大气过程的重要见解外，该项目还为学生提供了培训、指导和实验室研究与以本科生为中心的教育活动相结合的机会。

In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) and Environmental Chemical Sciences (ECS) programs of the Chemistry Division, Professor Gary Douberly of the University of Georgia is using liquid helium droplets to isolate and characterize the intermediates of chemical reactions. Reaction intermediates are difficult to study because they are only present for a short period of time before they react to form more stable molecules. Professor Douberly and his students overcome this long-standing experimental challenge by sequentially adding the individual reactants to liquid helium droplets. In the ultra-cold environment of the helium droplet, which is about 452 degrees Fahrenheit below zero, the reaction intermediates are "frozen out", allowing time for their molecular structures to be observed using laser spectroscopy. This project targets two fundamental chemical reactions that play key roles in the atmosphere, and is thus impacting our understanding of the complex web of chemical reactions associated with air pollution and the formation of small liquid droplets called aerosols.The first chemical reaction targeted by the project is the oxidation of isoprene, a small molecule that accounts for half of the organic compounds emitted into the atmosphere by plants, and is thus important to our understanding of the troposphere, the lowest portion of the Earth's atmosphere. Isoprene is oxidized by hydroxyl radicals, which are added to the helium droplets, and infrared spectroscopy is used to probe the branching of the reaction into the various intermediates. The intermediates are then reacted with oxygen to produce the associated hydroxyperoxy radicals, which are also studied using infrared techniques. The second aim of the project centers on identifying the minimum number of water molecules that are needed to observed the spontaneous transfer of a proton from sulfuric acid to water, a process that is central to the nucleation of atmospheric aerosol particles. This process is studied through the sequential addition of water molecules to helium droplets containing a single sulfuric acid molecule. Species selective spectroscopy is then used to determine the structures of these small acid-water clusters and identify the onset of proton loss from the acid. Analogous measurements probe the effect of ammonia (and other amines) on reducing the critical cluster size necessary for spontaneous proton transfer. In addition to providing important insight into fundamental atmospheric processes, the project provides opportunities for student training, mentoring, and the integration of laboratory research with undergraduate-centered educational activities.