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 interests in basic phenomena, as well as through numerous applications of cluster research to fields ranging from catalysis to the quest for new cluster-assembled materials.

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Articles you may be interested inOxygen cluster anions revisited: Solvent-mediated dissociation of the core O4 − anion Zwitterion formation in hydrated amino acid, dipole bound anions: How many water molecules are required?

The anions of the nucleic acid bases, uracil and thymine, were studied by negative ion photoelectron spectroscopy. Both monomer anions exhibit spectroscopic signatures that are indicative of dipole bound excess electrons. The adiabatic electron affinities of these molecules were found to be 93Ϯ7 meV for uracil and 69Ϯ7 meV for thymine. No conventional ͑valence͒ anions of these molecules were observed.

Articles you may be interested inDecay dynamics of nascent acetonitrile and nitromethane dipole-bound anions produced by intracluster chargetransfer J.Low-energy photoelectron imaging spectroscopy of nitromethane anions: Electron affinity, vibrational features, anisotropies, and the dipole-bound state J. Chem. Phys. 130, 074307 (2009); 10.1063/1.3076892 Dipole bound and valence state coupling in argon-solvated nitromethane anionsConventional ͑valence͒ and dipole-bound anions of the nitromethane molecule are studied using negative ion photoelectron spectroscopy, Rydberg charge exchange and field detachment techniques. Reaction rates for charge exchange between Cs(ns,nd) and Xe(n f ) Rydberg atoms with CH 3 NO 2 exhibit a pronounced maximum at an effective quantum number of n*Ϸ13Ϯ1 which is characteristic of the formation of dipole-bound anions ͓͑CH 3 NO 2 ͒ϭ3.46 D͔. However, the breadth ͑⌬nϷ5, FWHM͒ of the n-dependence of the reaction rate is also interpreted to be indicative of direct attachment into a valence anion state via a ''doorway'' dipole anion state. Studies of the electric field detachment of CH 3 NO 2 Ϫ formed through the Xe(n f ) reactions at various n values provide further evidence for the formation of both a dipole-bound anion as well as a contribution from the valence bound anion. Analysis of the field ionization data yields a dipole electron affinity of 12Ϯ3 meV. Photodetachment of CH 3 NO 2 Ϫ and CD 3 NO 2 Ϫ formed via a supersonic expansion nozzle ion source produces a photoelectron spectrum with a long vibrational progression indicative of a conventional ͑valence bound͒ anion with a substantial difference in the equilibrium structure of the anion and its corresponding neutral. Assignment of the origin ͑vЈϭ0, vЉϭ0͒ transitions in the photoelectron spectra of CH 3 NO 2 Ϫ and CD 3 NO 2 Ϫ yields adiabatic electron affinities of 0.26Ϯ0.08 and 0.24Ϯ0.08 eV, respectively.

The reactivity pattern of small (approximately 10 to 20 atoms) anionic aluminum clusters with oxygen has posed a long-standing puzzle. Those clusters with an odd number of atoms tend to react much more slowly than their even-numbered counterparts. We used Fourier transform ion cyclotron resonance mass spectrometry to show that spin conservation straightforwardly accounts for this trend. The reaction rate of odd-numbered clusters increased appreciably when singlet oxygen was used in place of ground-state (triplet) oxygen. Conversely, monohydride clusters AlnH-, in which addition of the hydrogen atom shifts the spin state by converting formerly open-shell structures to closed-shell ones (and vice versa), exhibited an opposing trend: The odd-n hydride clusters reacted more rapidly with triplet oxygen. These findings are supported by theoretical simulations and highlight the general importance of spin selection rules in mediating cluster reactivity.

Aqueous cluster studies have lead to a reassessment of the electronic properties of bulk water, such as band gap, conduction band edge, and vacuum level. Using results from experimental hydrated electron cluster studies, the location of the conduction band edge relative to the vacuum level ͑often called the V 0 value͒ in water has been determined to be Ϫ0.12 eVрV 0 р0.0 eV, which is an order of magnitude smaller than most experimental values in the literature. With V 0 ϭϪ0.12 eV and making use of the calculated solvation energy of OH in water, the band gap of water is determined to be 6.9 eV. Again, this is smaller than many literature estimates. In the course of this work, it is shown that due to water's ability to reorganize about charge ͑1͒ photoemission thresholds of water or anionic defects in water do not determine the vacuum level, and ͑2͒ there is almost no probability of accessing the bottom of the conduction band of water with a vertical/optical process from water's valence band. The results are presented in an energy diagram for bulk water which shows the utility of exploring the conduction band of water as a function of solvent polarization.