Onset of charge separation in clusters: size-selective infrared spectroscopy of uncharged solute-solvent clusters

物质在溶剂中的溶解是大家所熟悉的。当固体溶解在溶剂中时，它会在分子尺度上分解成简单的组成部分，如小离子或小分子，它们与溶剂分子密切混合。虽然我们对这一过程了解很多，但要获得分子尺度上发生的事情的详细知识并不容易。解决这个问题的一种方法是使用“簇方法”，即对气相中的溶质和溶剂原子/分子的小簇进行详细研究，例如通过光谱学。如果簇大小不是太大，支持理论研究是可行的，然后可以将理论和光谱数据结合起来解释以提取详细信息，例如溶质周围溶剂分子的排列。如果不同大小的团簇的光谱可以被区分和分配，那么人们就有可能探索团簇性质如何随溶剂分子数量的变化而变化。这使得从一个小的溶质-溶剂集群到集群大小的演变成为可能，其中溶质-溶剂行为可能开始类似于散装溶液。团簇理论已经建立，近年来已开始应用于离子溶剂团簇的详细光谱研究。带电团簇特别适用于这种方法，因为不同大小的团簇很容易通过质谱法分离。通过将这种质量选择性与新的可调谐红外激光技术相结合，几个研究小组在过去几年中利用质量选择性红外光谱技术提取了关于离子溶剂团簇的重要和令人兴奋的新信息。这里提出的工作采用了一种不同的方法。我们的目的是应用尺寸选择性红外光谱对不带电的溶质-溶剂团簇。针对两种体系：(1)金属原子溶质与常见溶剂分子（如水和氨）结合；(2)水和氨中的碱卤化物分子（MX）。在这两种情况下，在散装溶液中都有电荷分离的趋势。对于高正电性的金属，如碱，金属原子可以释放一个电子到溶剂中形成溶剂化电子。这种现象在团簇中发生的程度取决于金属的性质、溶剂的性质和溶剂分子的数目。众所周知，碱性卤化物MX在散装水溶液中溶解形成溶剂分离离子M+和X-，但在小簇中这种离子解离可能是不完全的。我们想回答以下基本问题：在MX盐分子完全解离成M+和X-离子之前，需要多少溶剂分子？我们的目的是利用红外光谱来提取这些信息。这项工作的一个挑战是在不带电团簇的光谱中实现质量选择性。我们选择的方法是通过观察红外激光激发对源自紫外激光光电离的质量选择离子信号的影响来记录作用光谱。将有三种相关的行动技术可用于解决这个问题，它们都需要基本相同的设备。我们在这里提出的技术并不局限于金属溶剂和碱卤化物溶剂团簇。其他溶质-溶剂体系，如碱-溶剂、酸-溶剂和其他盐-溶剂体系，都有可能通过这种方法观察到。因此，这个独特的项目也将成为研究中性团簇中溶质-溶剂相互作用及其与溶剂化机制的联系的长期方案的垫脚石。

The dissolution of substances in solvents is familiar to everyone. When a solid dissolves in a solvent it breaks down at the molecular scale into simple building blocks, such as small ions or molecules, which mix intimately with the solvent molecules. While much is known about this process, it is not easy to gain detailed knowledge of what is happening at the molecular scale. One way of tackling this is to use the 'cluster approach', whereby a small cluster of solute and solvent atoms/molecules in the gas phase is subjected to detailed study, e.g. by spectroscopy. If the cluster sizes are not too large, a supporting theoretical study is feasible and the combination of theory and spectroscopic data can then be interpreted to extract detailed information, such as the arrangement of solvent molecules around the solute. If the spectra from different sized clusters can be distinguished and assigned, then one potentially has the means to explore how the cluster properties change with the number of solvent molecules. This makes it possible to follow the evolution from a small solute-solvent cluster through to cluster sizes where the solute-solvent behaviour may begin to resemble the bulk solution.The cluster philosophy is well-established and in recent years it has begun to be applied to the detailed spectroscopic study of ion-solvent clusters. Charged clusters are especially amenable to this approach because different sized clusters are easily separated by mass spectrometry. By combining this mass selectivity with new tuneable infrared laser technology, important and exciting new information on ion-solvent clusters has been extracted by several research groups in the past few years using mass-selective infrared spectroscopy.The work proposed here takes a different approach. Our aim is to apply size-selective infrared spectroscopy to uncharged solute-solvent clusters. Two systems are targeted: (1) metal atom solutes combined with common solvent molecules such as water and ammonia; (2) alkali halide molecules (MX) in water and ammonia. In both cases there is a tendency for charge separation to occur in bulk solutions. For highly electropositive metals, such as the alkalis, the metal atom can release an electron into the solvent to form a solvated electron. The extent to which this occurs in clusters will depend on the identity of the metal, the identity of the solvent, and the number of solvent molecules. It is well-known that the alkali halides, MX, dissolve in bulk aqueous solutions to form the solvent-separated ions, M+ and X-, but in small clusters this ionic dissociation may be incomplete. We want to answer the following basic question: how many solvent molecules are required before the MX salt molecule fully dissociates into M+ and X- ions? Our aim is to employ infrared spectroscopy to extract this information.A challenge for this work is to achieve mass-selectivity in the spectroscopy of uncharged clusters. Our chosen approach is to record action spectra by observing the impact of the IR laser excitation on the mass-selected ion signals originating from UV laser photoionization. Three related action techniques will be available to tackle this problem, all requiring essentially the same equipment.The techniques we propose here are not limited to metal-solvent and alkali halide-solvent clusters. Other solute-solvent systems, such as base-solvent, acid-solvent and indeed other salt-solvent systems are potentially observable by this means. Consequently, this unique project will also serve as a stepping-stone for a longer term programme investigating solute-solvent interactions in neutral clusters and their link to solvation mechanisms.