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

▪ Abstract This review provides a historical context for our understanding of the hydration shell surrounding halide ions and illustrates how the cluster systems can be used, in combination with theory, to elucidate the behavior of water molecules in direct contact with the anion. We discuss how vibrational predissociation spectroscopy, carried out with weakly bound argon atoms, has been employed to deduce the morphology of the small water networks attached to anions in the primary steps of hydration. We emphasize the importance of charge-transfer in the binary interaction, and discuss how this process affects the structures of the larger networks. Finally, we survey how the negatively charged water clusters (H2O)n− are providing a molecular-level perspective on how diffuse excess electrons interact with the water networks.

Molecular aspects of halide ion hydration: the cluster approach.

This article provides a review of recent work in the field of helium nanodroplet spectroscopy with an emphasis on the dynamical aspects of the interactions between molecules in helium as well as their interaction with this unique quantum solvent. Emphasis is placed on experimental methods and studies introducing recent new approaches, in particular including time-resolved techniques. Corresponding theoretical results on the energetics and dynamics of helium droplets are also discussed.

http://iopscience.iop.org/article/10.1088/0953-4075/39/8/R01/pdf

Spectroscopy and dynamics in helium nanodroplets

In this paper we address some of the unique, novel and basic features of clusters, which involve the physical and chemical consequences of their large surface/volume ratio and the size dependence of the properties of large finite systems Energetic, quantum, electronic, spectroscopic, and electrodynamic size effects in clusters were quantified by cluster size equations (CSEs), which describe the gradual “transition” from the large finite cluster to the infinite bulk system, with increasing the cluster size Some progress was also accomplished in the description of the “transition” from cluster microsurfaces to macrosurfaces, which can be described in terms of surface CSEs The analysis implies a nonuniversality principle for cluster size effects, with different physical properties being described in terms of distinct CSEs The CSEs provide a quantitative answer to a central question in the area of cluster chemical physics: What is the minimal cluster size for which its properties become size invariant and do not differ in any significant way from those of the macroscopic sample of the same material? A unified (but not universal) description is advanced for the merging between the properties of microscopic large finite systems and those of a macroscopic bulk material

Cluster size effects

We present the photoelectron‐spectra of Cl−, Br−, and I−, solvated in water clusters‐(H2O)n, where n is 1–7, 1–16, and 1–60, respectively, taken with 7.1 eV photon energy. The vertical binding energies of the solvated anions are used to extract the solvent electrostatic stabilization energies of the anion. The photoelectron spectra of the solvated I− indicate the formation of the first solvation layer with a coordination number of six. Ab initio calculations support solvation shell closure at n=6. This conclusion is not born‐out by current molecular dynamics calculations. These calculations favor structures with a surface solvated anion (coordination number of 3–4) and reproduce (within 0.2 eV) our vertical binding energies. The fitting of the experimental binding energies of large I−(H2O)n to the models of classical electrostatic solvation is consistent with surface solvation. In the size range n=34–40 we have detected special cluster structures, with very low electrostatic stabilization.

Photoelectron spectroscopy of Cl−, Br−, and I− solvated in water clusters

We have measured the photoabsorption spectra of mass selected Ar+n clusters, n=3–40, from 355–1064 nm. The smaller clusters, n<15, display a visible photoabsorption spectrum similar to Ar+3, i.e., a broad, intense band peaking near 520 nm. From n=15–20 this photoabsorption band shifts smoothly to a longer wavelength, peaking near 600 nm for Ar+20. This band does not change appreciably as n increases from 20 to 40. These results clearly demonstrate that the Ar+n clusters have photophysical properties quite different from those of Ar+2. We have also studied the photoabsorption and subsequent photofragmentation of Ar+n cluster ions, n=3–60, at selected visible wavelengths. The ionic photofragment distributions both indicate that photofragmentation proceeds through the loss of individual Ar atoms and place an upper bound of 90 meV on the cluster ion binding energy in the large cluster limit.

Photoabsorption and photofragmentation studies of (Ar+)n cluster ions

Photodissociation of Mass-Selected (CO2)n– Clusters: Evaporation Leading to Magic Numbers in Fragment-Ion Distributions

Photofragmentation studies of mass selected Br−2⋅(CO2)n, n≤24, cluster ions are presented. The heterogeneous cluster ions are synthesized in a supersonic expansion crossed with an electron beam. These ionic clusters are mass analyzed in a time‐of‐flight mass spectrometer prior to photodissociation of the Br−2 absorber in the cluster. Mass analysis of the ionic photofragments shows that in some cases the photodissociated Br−2 has recombined and in other cases a Br atom has escaped from the cluster. The branching ratio for recombination depends on both the size of the initial cluster ion and the photodissociation wavelength. The photofragmentation data are consistent with a recombination mechanism dominated by attractive forces in the smaller clusters and repulsive interactions in the larger clusters.

Recombination of Br−2 photodissociated within mass selected ionic clusters

http://jila.colorado.edu/lineberger/sites/default/files/uploads/%28CO2%29n%2B%20Photodestruction%20Chem%20Phys%20Letters1984.pdf

Photodestruction cross sections for mass-selected ion clusters: (CO2)n+

Ionic photofragmentation patterns of (CO2)+n, 2 13.Ionic photofragmentation patterns of (CO2)+n, 2 13.

Michael L. Alexander

Papers

Photoabsorption and photofragmentation studies of (Ar+)n cluster ions

Photodissociation of Mass-Selected (CO2)n– Clusters: Evaporation Leading to Magic Numbers in Fragment-Ion Distributions

Recombination of Br−2 photodissociated within mass selected ionic clusters

Photodestruction cross sections for mass-selected ion clusters: (CO2)n+

Photofragmentation of (CO2)+n: Simple energy dependence for the number of neutrals ejected, 13<n<26