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

We present the results of extensive computer simulations of several sodium microclusters, using the Car–Parrinello method (unified density‐functional theory and molecular dynamics). Dynamical simulated annealing strategies are adopted in the search for low‐energy minima of the potential energy surface. A detailed analysis of the results for both structural and electronic properties at temperatures in the 0–600 K range is carried out, which allows us for the first time to gain insight into the structural ‘‘growth’’ pattern, the extent of the validity of (spherical, spheroidal, and ellipsoidal) jellium models, and the effects of temperature. In particular, new and unforeseen structures are discovered for n=10, 13, 18, and 20 and we emphasize the constant presence of arrangements with local pentagonal symmetry for the low‐energy isomers as well as the similarity of the structural pattern with that of Lennard‐Jones systems. Shape transformations with increasing temperature are observed, ‘‘rigidity’’ and ‘‘nonrigidity’’ of the individual clusters examined, and the presence of distinct isomers is identified for the smaller ones. Closing of electronic shells is confirmed for Na8 and Na20 and—to a minor extent only—for Na18. Hybridization of cluster states of different angular momenta, which represents a deviation from the spherical shell model, is discovered in several cases and discussed in detail, also in correspondence with the presence of anisotropy of the electronic potential. In most cases, this hybridization is observed to increase with increasing temperature, in parallel with the increase of the eccentricity of the cluster shape. In spite of the relatively high atomic mobility, our results do not support a spherical liquid‐droplet picture for the atomic distribution.

Structural and electronic properties of sodium microclusters (n=2–20) at low and high temperatures: New insights from ab initio molecular dynamics studies

We investigate the unimolecular dissociation dynamics of energy‐rich sodium cluster ions, Na+n (5≤n≤40) by measuring the time evolution of their sequential monomer or dimer evaporative cooling The experimental technique, tandem time‐of‐flight mass spectroscopy, measures the relative rate of competing dissociation channels from metastable ion clusters selected during an initial sampling time interval immediately following the creation of the ion cluster ensemble Pulsed laser UV photoionization converts the distribution of neutral clusters emerging from a free‐jet expansion to the distribution of ion clusters from which the initial selection takes place For the smaller clusters, 3≤n≤14, we compare the measured dissociation rates with those calculated from a modified version of the RRK theory of unimolecular dissociation In applying the theory we use monomer and dimer binding energies determined from theoretical calculation For larger clusters, 15≤n≤40, the binding energies are not known, and we invert 

Dynamics of unimolecular dissociation of sodium cluster ions

Observation of electronic shells and shells of atoms in large Na 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

The decomposition of metastable photoionized mass‐selected alkali clusters is investigated using a tandem time‐of‐flight spectrometer. Na+n and K+n are found to decompose mainly by the evaporation of either a single neutral atom or a neutral dimer in a time scale of about 10 μs. The predominant fragmentation channels are found to follow the adiabatic dissociation channels associated with the lowest energies accordingly to our CI calculations. In the light of our experimental and theoretical results a comparison between the different calculations of the absolute atomization energies available in the literature is presented.

Adiabatic decomposition of mass‐selected alkali clusters

Neutral potassium clusters of two different mass distributions generated either by a sonic or a conical nozzle were photoionized at several ionizing energies. By combining different nucleation conditions with several photoionization situations, variations of ion mass spectral patterns reflecting either neutral or ionic stabilities were obtained. The spectra show peaks or steps for those neutral and ionized clusters which contain 8, 20, and 40 electrons. These can be understood in terms of electronic shell structure for both neutral and ionized potassium clusters. However local irregularities appearing for the small cluster sizes reveal the influence of their geometrical structure. Moreover ionizing laser power dependencies taken at various photon energies were used to study fragmentation effects following ionization.

Photoionization of potassium clusters: Neutral and ionic cluster stabilities

Photofragmentation studies are described for size‐selected ionized potassium‐atom clusters, generated in a supersonic expansion and analyzed with a tandem time‐of‐flight system. In the range n=3 to 41 the relative ion intensities in photofragmentation spectra reflect the relative stabilities of the ion products. Moreover the photon‐energy dependence of the fragmentation patterns suggests that photodissociation proceeds rather via a fast sequential evaporation of neutral monomers than via a fission process. A value can be deduced for the mean binding energy per atom which is increasing with the cluster size.

Photodissociation of size‐selected K+n clusters

Laser ionization spectra and electronic structure of Kn and NaKn−1 clusters

The preferential dissociation channels of singly and doubly charged antimony clusters have been determined from the unimolecular dissociation of energy rich cluster ions, using an ion stopping technique. It is found that singly charged Sb+n clusters with 5≤n≤80 dissociate by loss of neutral molecules. Neutral dimer loss is observed for n=5, 6, 7 whereas for n≥8, Sb+n loses Sb4. The fragmentation of doubly charged Sb++n has been investigated above the critical size n++c=24 from which doubly charged clusters are detectable in mass spectra. On the time scale of the experiment, which is 1 μs≤t≤100 μs with respect to photoionization, the delayed Coulombic fission into two singly charged clusters competes with the evaporation of Sb4. It is shown that for the smaller Sb++n clusters with 26≤n≤36 the fission to two singly charged clusters is of relatively asymmetrical character, leading to the detachment of five and seven atom cationic fragments. In larger clusters n≥40 the fission is of more symmetrical character...

J. Ph. Roux

Papers

Adiabatic decomposition of mass‐selected alkali clusters

Photoionization of potassium clusters: Neutral and ionic cluster stabilities

Photodissociation of size‐selected K+n clusters

Laser ionization spectra and electronic structure of Kn and NaKn−1 clusters

Coulombic fission and evaporation of antimony cluster ions