Small Silver Clusters

As mentioned in Sect. 5.1, in the neutral silver trimer there is no excited electronic state which could be reached with a wavelength of 400 nm, and therefore the ionization is due to a nonresonant two-photon absorption pro- 

the real-time spectra I [At) of the various cations extracted from the trap were monitored. For all cluster sizes (Agn=3,5, ,9) the fragment ions showed the same dependence on the delay time as the parent ions. Thus, it can be assumed that the fragmentation takes place after the ionization, and that the clusters survive the detachment intact. Regarding the dynamics of the neutral cluster, it is therefore sufficient to concentrate on the unbroken 

In this book an overview has been given of the amazing opportunities provided by femtosecond real-time spectroscopy applied to small molecules and clusters. Fascinating phenomena such as control of molecular dynamics, selective state preparation, observation of vibrational wave packets on ground state PESs, ultrafast IVR, and photodissociation with unexpected and sometimes exceptional features have been introduced. 

A summary of the investigations described in the preceding chapters is given in Sect. 6.1. In Sect. 6.2 we look forward to a few of the next future challenges and new goals of this modern and highly vital branch of natural sciences. 

The main purpose of this book was to introduce the exciting field of femtosecond real-time spectroscopy applied to small model molecules and clusters. To carry out investigations in this field first of all an experimental setup was assembled with the following characteristics  

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Considerable effort has been expended on Ag atoms and small, silver clusters. Bates and Gruen (10) studied the spectra of sputtered silver atoms (a metal target was bombarded with a beam of 2-keV, argon ions produced with a sputter ion-gun) isolated in D, Ne, and N2. They found that an inverse relationship between Zett of the metal atom and the polarizability of rare-gas matrices (as determined from examination of... 

Ozin and Huber 112) synthesized and characterized very small silver particles, Ag n = 2-5) by conventional deposition methods, as well as by a novel technique that they have termed "cryophotoaggrega-tion. This study will be discussed in detail in Section III. Of interest here is a study of silver atoms and small, silver clusters entrapped in ice and high-molecular-weight paraffin (n-C22H46, n-C32Hg8) matrices 146) (see Figs. 7 and 8, and Tables IV and V). Besides the intriguing, multiple-site (solvation) occupancy of atomic silver in ice matrices, and their thermal and photochemical interconvertibility, their extremely... 

In 2007, Dickson et al. found that it is possible to stain fixed cells with fluorescent silver clusters instead of silver nanoparticles by tuning the staining conditions [57]. The new approach consists of staining fixed cells with a low concentrated silver nitrate solution 20-100 mM, within 20 h at ambient conditions, and reducing the silver by photoactivation, with the result of small silver clusters that present a broad emission band between 500 and 700 nm (Fig. 8a-d). The discovery that fluorescent silver clusters can be generated by photoactivation of cells fed with silver salt, opens up new paths for the application of silver clusters in biological systems. 

Interestingly, non-metallic silver clusters, depending on their sizes, may act either as electron donors or as electron acceptors. Using sulfonatopropyl-viologen, SPV (E° for SPV/SPV- = — 0.41 V/NHE), pulse radiolysis established that small silver clusters (n < 4) acted as electron donors (Le. E° for (Ag /Agn < E° for SPV/SPV - ) while, conversely, large silver clusters (n 2 4) were electron acceptors (i.e. E° for Ag/ /Agn > E° for SPV/SPV- ) [511]. Size-dependent electrochemical potentials of silver aggregates have been elucidated (Fig. 82) [506]. 

The electronic properties of small silver clusters chemisorbed on AgBr have been calculated by Baetzold (66) using MO theory. This problem deals with catalysis since, as Hamilton and Urbach (67) have described, the silver centers are catalysts for the chemical reduction of AgBr grains. By using various experimental techniques, they indicate that a minimum size cluster of 4 Ag atoms is required for the catalysis. This suggests that some properties of 4 bonded silver atoms are different from atomic and perhaps like bulk properties, which could account for the catalysis. 

Mostafavi et al. [92] generate very small silver aggregates by a pulse radiolysis method and follow their aggregation and growth in the presence of an electron donor, the radical anion of sulfonatopropyl viologen (SPV) generated in the same pulse. They use this method to study the size dependence of the electrochemical potential of small silver clusters and conclude that a critical silver aggregate is Ag5+/Ag5, which has E° = —410 mV (NHE). 

Fedrigo, S., Harbich, W., Buttet, J. Collective dipole oscillations in small silver clusters embedded in rare-gas matrices. Phys. Rev. B 47, 10706-10715 (1993)... 

Figure 19. Three photographic characteristic curves for a monodispersed AgBr cubic emulsion (0.5 m cube edges) bearing a low level of surface sulfur sensitization. In all cases the film was given a 10 3 s flash exposure through a photographic step tablet with a step interval of 0.15 density units. The film was then immersed for 15 min in a redox buffer of ferrous/ferric EDTA with a potential of —105.4 mv relative to bulk Ag/AgBr at the same Br concentration. It is concluded that the small silver clusters of the subdevelopable image are unexpectedly stable (56C).

HENGLEIN, A., Remarks on the Electrochemical Potential of Small Silver Clusters in Aqueous Solution , Ber. Bunsenges. Phys. Chem. 1990, 94, 600-603. 

Ultrafast Structural Relaxation in Small Silver Clusters... 

Since the observation of giant resonances in the photofragmentation of free Ag clusters indicating collective electronic excitations small silver clusters have attracted considerable experimental and theoretical attention. Additional interest in these small metal clusters was aroused by the unexpected observation of a very sharp absorption line of Ags clusters inside helium droplets.Because of the large capture cross sections of helium droplets (Sec. 2.2) sufficient numbers of metal atoms such as silver can be picked up by passing the droplets through an oven heated to 800°C with a vapor pressure of only 10 mbar. 

Bonacic-Koutecky and co-workers studied a model system consisting of a tryptophan-glycin (TrpGly) dipeptide and a small silver cluster (Ags). 

Original methods of synthesis of highly dispersed silver catalysts (based upon the application of strong reducing properties of electrons solvated in soldium-ammonia solutions, the adsorption-contact method of drying and a weak solubility in nitric acid of the Si02-supported small silver clusters) allowed us to synthesize Si02-supported silver particles of sizes less than 6 nm. It makes possible some unusual catalytic and other physico-chemical properties of these particles to be discovered. 

The size dependency of the electrochemical oxidation of metal nanoparticles can be partially understood in the light of several different considerations. " " First, data deriving from work on small silver clusters, Agn, formed in aqueous solution via pulse radiolysis coupled with other information such as mass spectroscopic equilibrium data and Gibbs energies of sublimation of silver has led to the estimation of the standard electrode potential of the redox couple... 

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