Solute clusters being trapped

Neumark and co-workers [56] pointed out the similarity of the cluster results to the transient behavior in aqueous 1 solution, which has been studied via ultrafast pump-probe measurements [50]. Bradforth and co-workers [50] observed IR (800 nm) transient absorption after UV (255 nm) excitation with 50 fs time resolution. In 1 solution, a promptly arising transient disappears within 50 fs, and absorption due to solvated electron rises with a 200 fs time constant. For longer time-scales, the trapped electron shows a biexponential decay with time constants of 8 and 60 ps, which is due to recombination with the nearby iodine atom. The close resemblance of time-scales for the rise of the solvated electron and isomerization in I (water) ( = 5 and 6) implies that the electron trapping pathway in solution can be modeled as a rearrangement of the solvent hydrogen-bond network in gas-phase clusters. 

Ousters adsorbed on the outside surfaces of zeolites can often be easily extracted with neutral solvents or with salt solutions which remove the cluster ions by cation metathesis (ion exchange). Comparison of the infrared spectra of the extracted spedes and those of known spedes helps identify the encaged spedes. When treatment of a sample with such solutions fails to remove sorbed dusters, they are inferred to be trapped within the cages. The inference is supported when the same clusters adsorbed on the surface of a large pored material such as an amorphous metal oxide are removed by extraction. 

As demonstrated in Section 3.1.1, the Au SR clusters formed in reaction (1) correspond to trapped intermediates of the growing clusters and are thus not always thermodynamically stable. The stabilities of the Au SG clusters (1-9) are acutely dependent on the core sizes. The Aui8(SG)i4 Au25(SG)i8, and Au39(SG)24 clusters were found to be stable when allowed to stand in aqueous solution while other Au SG clusters were degraded into smaller clusters (Figure 7a) [16]. 

In particular, the application of multi-exponential decay kinetics anticipated from models that assume distinct photophysical species within polymer chains may be inappropriate in some cases. The possibility of non-exponential fluorescence decay behaviour arising from energy migration and trapping (11) should also be considered. Additional studies of the mobilities of fluorescent probes incorporated in PMA using time-resolved fluorescence anisotropy measurements provide further support for a "connected cluster" model to describe the conformation of this polyelectrolyte in aqueous solution at low pH. 

This review will emphasize SERS in the context of electrochemical systems. The liberty has been taken of including in this category work done on colloids suspended in (mostly aqueous) solutions. Colloids, anyway, have many common features with systems in electrochemistry. Thus SERS at the solid-electrolyte interface is the main question of interest here. Of course, one cannot ignore the work on other systems, nor does one want to. Therefore we will also discuss the other systems, such as various films in ultrahigh vacuum, in air or in tunnel junctions, on specially prepared lithographic structures, on metal clusters trapped in a noble-gas matrix, or on an oxide in catalytic systems, though they will not be at the main focus of this review. 

The reticulated structures are made up of clusters. If all the clusters have a finite size, the system is soluble and the solution is called sol. On the other hand, if the structure contains a cluster of infinite size, the system is a gel which is not soluble but which may swell in a solvent. The same reaction may lead either to sols or to gels according to the final branching rate. The sol-gel transition may be considered as a percolation transition. Note that an infinite cluster can be made either by chemical binding or (partially) by topological trapping [see Fig. 1.8]. From a mechanical point of view, a sol is viscous, a gel is elastic. Thus a piece of vulcanized rubber can be considered as a gel. 

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