Atomically precise pieces of matter of nanometer dimensions composed of noble metals are new categories of materials with many unusual properties. Over 100 molecules of this kind with formulas such as Au25(SR)18, Au38(SR)24, and Au102(SR)44 as well as Ag25(SR)18, Ag29(S2R)12, and Ag44(SR)30 (often with a few counterions to compensate charges) are known now. They can be made reproducibly with robust synthetic protocols, resulting in colored solutions, yielding powders or diffractable crystals. They are distinctly different from nanoparticles in their spectroscopic properties such as optical absorption and emission, showing well-defined features, just like molecules. They show isotopically resolved molecular ion peaks in mass spectra and provide diverse information when examined through multiple instrumental methods. Most important of these properties is luminescence, often in the visible–near-infrared window, useful in biological applications. Luminescence in the visible region, especially by clusters prot...

Atomically Precise Clusters of Noble Metals: Emerging Link between Atoms and Nanoparticles

The field of cluster research can trace its origins back to the mid-nineteenth century when early studies of colloids, aerosols, and nucleation phenomena were reported. The field underwent a resurgence of interest several decades ago when well-defined clusters were observed in supersonic expansions that could be investigated using mass spectrometers. The advent of the laser provided a new dimension, enabling detailed spectroscopic observations through the probing of systems of varying size and degree of solvation. Modern interest derives from recognition that interrogating clusters provides a way of studying the energetics and dynamics of intermediate states of matter as cluster systems evolve from the gas toward the condensed state. Herein, we endeavor to highlight some of the significant advances which have been made during the past several decades that have led to a nearly explosive growth of interest in the field of cluster science. Finally, we conclude that the field will continue to expand through i...

Clusters: Structure, Energetics, and Dynamics of Intermediate States of Matter

The heterogeneous reaction of NO2 with water on the surface of laboratory systems has been known for decades to generate HONO, a major source of OH that drives the formation of ozone and other air pollutants in urban areas and possibly in snowpacks. Previous studies have shown that the reaction is first order in NO2 and in water vapor, and the formation of a complex between NO2 and water at the air–water interface has been hypothesized as being the key step in the mechanism. We report data from long path FTIR studies in borosilicate glass reaction chambers of the loss of gaseous NO2 and the formation of the products HONO, NO and N2O. Further FTIR studies were carried out to measure species generated on the surface during the reaction, including HNO3, N2O4 and NO2+. We propose a new reaction mechanism in which we hypothesize that the symmetric form of the NO2 dimer, N2O4, is taken up on the surface and isomerizes to the asymmetric form, ONONO2. The latter autoionizes to NO+NO3−, and it is this intermediate that reacts with water to generate HONO and surface-adsorbed HNO3. Nitric oxide is then generated by secondary reactions of HONO on the highly acidic surface. This new mechanism is discussed in the context of our experimental data and those of previous studies, as well as the chemistry of such intermediates as NO+ and NO2+ that is known to occur in solution. Implications for the formation of HONO both outdoors and indoors in real and simulated polluted atmospheres, as well as on airborne particles and in snowpacks, are discussed. A key aspect of this chemistry is that in the atmospheric boundary layer where human exposure occurs and many measurements of HONO and related atmospheric constituents such as ozone are made, a major substrate for this heterogeneous chemistry is the surface of buildings, roads, soils, vegetation and other materials. This area of reactions in thin films on surfaces (SURFACE = Surfaces, Urban and Remote: Films As a Chemical Environment) has received relatively little attention compared to reactions in the gas and liquid phases, but in fact may be quite important in the chemistry of the boundary layer in urban areas.

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The heterogeneous hydrolysis of NO2 in laboratory systems and in outdoor and indoor atmospheres: An integrated mechanism

The multiple pickup of up to 30 foreign particles by large (N≳103 atoms) liquid helium clusters and the formation of clusters of the foreign particles within the helium cluster has been observed in a molecular beam experiment for Ar, Kr, Xe, H2O, and SF6. Evaporation of helium atoms due to the pickup process has been measured quantitatively by two methods. The measured apparent cross sections for capture and coagulation of these particles are significantly smaller than the total scattering cross sections of the clusters. After electron impact ionization the size distribution of the foreign particle cluster ion fragments can be well described by a Poisson distribution. This is interpreted as indicating negligible fragmentation of the coagulated foreign particle clusters. The measured coagulation cross sections are smaller than the capture cross sections. This is explained through a model which involves only partial coagulation of the foreign particles. The observed processes allow the preparation of foreign particle clusters with well‐defined size distributions inside large helium clusters.

Successive capture and coagulation of atoms and molecules to small clusters in large liquid helium clusters

This paper presents results concerning size distribution and growth in the gas phase of ions in the series H+(H2O)n. The measurements span a range of cluster sizes up to n =28, a domain not previously investigated quantitatively. The molecular beam sampling technique developed for this study promises to be a powerful tool in the study of ion nucleation phenomenon. Noteworthy observations include the unusual stability of the cluster with n =21 and the absence of trapped molecules of carrier gas within a cluster.

Clustering of water on hydrated protons in a supersonic free jet expansion

Evidence for the existence of structures in gas-phase homomolecular clusters of water

Studies of gas-phase clusters: the solvation of HNO3 in microscopic aqueous clusters

Studies of the influence of source temperatures and pressures on the distributions of water, ammonia and sulfur dioxide clusters are reported. The experiments reveal that nearly identical cluster distributions occur in cases where the pressure of dimer is maintained constant according to a simple equation involving stagnation temperature and pressure. In similar experiments covering the same range of pressures and temperatures, widely differing cluster distributions are obtained under conditions where the dimer concentration is not fixed. Our results suggest that large clusters proceed largely from pre-existing dimers, and that very few new ones are created early enough in the expansion to effect cluster growth. The gas-phase heat of dimerization of sulfur dioxide is determined to be 4.3 ± 0.3 kcal/mole.

The use of similarity profiles in studying cluster formation in molecular beams: Evidence for the role of pre-existing dimers

A synchronous time‐of‐flight (TOF) technique is employed to make velocity distribution measurements in pulsed supersonic free‐jet expansions. For helium expansions, the flow attains a steady–state condition with a terminal Mach number of 80, on a 50‐μs time scale. The transient behavior is primarily due to the mechanical action of the valve. For a 650‐μs‐wide pulse, greater than 95% of the atoms issuing from the pulsed nozzle can be described by the steady‐state parameters. However, the velocity dispersion increases at both the leading and trailing edges of the pulse. This dispersion can be quantitatively described by incorporating a time‐dependent Mach number into the standard steady‐state flux velocity distribution function.

Time-of-flight characterization of pulsed supersonic helium free-jet expansions

Results of a molecular beam electric deflection study of the polarity of gas phase clusters comprised of HNO3H2O, HNO3H2ONH3, and NH4HSNH3 are reported. No refocusing was observed for any species containing more than two molecular subunits. These results are contrasted with previous studies made in our laboratory which showed focusing for higher polymers of acetic acid; possible reasons are given for observed defocusing in the present systems where ion-pair formation is expected to occur.

Bruce D. Kay

Papers

Evidence for the existence of structures in gas-phase homomolecular clusters of water

Studies of gas-phase clusters: the solvation of HNO3 in microscopic aqueous clusters

The use of similarity profiles in studying cluster formation in molecular beams: Evidence for the role of pre-existing dimers

Time-of-flight characterization of pulsed supersonic helium free-jet expansions

A molecular beam electric deflection study of clusters: acid and salt molecules in microscopic aqueous and ammonia clusters