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都熟悉的。当固体溶解在溶剂中时，它在分子水平上分解成简单的积木，如小离子或分子，它们与溶剂分子紧密混合。虽然对这一过程有很多了解，但要详细了解分子水平上正在发生的事情并不容易。解决这一问题的一种方法是使用“团簇方法”，即对气相中的一小群溶质和溶剂原子/分子进行详细研究，例如通过光谱分析。如果团簇的大小不是太大，那么支持理论研究是可行的，然后可以解释理论和光谱数据的组合以提取详细的信息，例如溶质周围的溶剂分子的排列。如果来自不同大小的团簇的光谱能够被区分和分配，那么人们就有可能探索团簇的性质如何随着溶剂分子的数量而变化。这使得可以跟踪从一个小的溶质-溶剂团簇到溶质-溶剂行为可能开始类似于整体溶液的团簇的大小的演化。团簇哲学是公认的，近年来它已经开始被应用于离子-溶剂团簇的详细光谱研究。带电星团特别适合这种方法，因为不同大小的星团很容易被质谱学分开。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，在过去的几年里，几个研究小组利用质量选择红外光谱提取了关于离子溶剂团簇的重要和令人兴奋的新信息。我们的目标是将尺寸选择红外光谱应用于不带电荷的溶质-溶剂团簇。目标是两个系统：(1)金属原子溶质与水和氨等常见溶剂分子结合；(2)水和氨中的碱卤化物分子(MX)。在这两种情况下，都有在整体溶液中发生电荷分离的趋势。对于高电正性的金属，如碱，金属原子可以向溶剂中释放一个电子，形成溶剂化电子。这在团簇中发生的程度将取决于金属的同一性、溶剂的同一性和溶剂分子的数量。众所周知，碱卤化物MX溶解在大体积水溶液中，形成溶剂分离的离子M+和X-，但在小团簇中，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，需要多少个溶剂分子？我们的目标是利用红外光谱来提取这一信息。这项工作的一个挑战是在未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紫外激光光致电离产生的质量选择离子信号的影响来记录作用光谱。有三种相关的行动技术可用于解决这一问题，所有这些技术都需要基本上相同的设备。我们在这里提出的技术并不局限于金属-溶剂和碱卤化物-溶剂簇合物。其他溶质-溶剂体系，如碱-溶剂、酸-溶剂，甚至其他盐-溶剂体系，都有可能通过这种方法观察到。因此，这一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.