Abstract
Size selected water clusters are generated by photoionizing sodium doped clusters close to the ionization threshold. This procedure is free of fragmentation. Upon infrared excitation, size- and isomer-specific OH-stretch spectra are obtained over a large range of cluster sizes. In one application of this method the infrared spectra of single water cluster sizes are investigated. A comparison with calculations, based on structures optimized by genetic algorithms, has been made to tentatively derive cluster structures which reproduce the experimental spectra. We identified a single all-surface structure for n = 25 and mixtures with one or two interior molecules for n = 24 and 32. In another application the sizes are determined at which the crystallization sets in. Surprisingly, this process strongly depends on the cluster temperature. The crystallization starts at sizes below n = 200 at higher temperatures and the onset is shifted to sizes above n = 400 at lower temperatures.
Highlights
The most general one is the deflection of clusters in a scattering experiment with an atomic beam
The doping with Na atoms and the subsequent fragmentation free photoionization proved to be an effective method for generating size selected water clusters
We present here the application in two different fields, the determination of the cluster structure and the onset of crystallinity
Summary
The most general one is the deflection of clusters in a scattering experiment with an atomic beam. IR excitation of the clusters before photoionization leads to an enhancement signal, which is used to generate the desired spectrum.[25] The resulting spectra, obtained in the first application of this method, were those of Na(H2O)n and for smaller clusters they were clearly dominated by features of the solvated electron.[25,26] In the meantime we found out that for many experimental conditions the IR spectra are even for smaller clusters dominated by a class of isomers, in which the Na is attached to the intact water or methanol cluster.[27,28,29] For larger clusters the influence of the single Na atom anyway disappears and essentially the spectra of the pure water clusters are recorded This approach is the basis for obtaining size resolved IR spectra over a broad range of water cluster sizes, which will be presented in this perspective article. The possibilities of revealing water cluster structures, opened by the available facilities for a combined theoretical–experimental effort, are briefly presented
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