Large polycyclic aromatic hydrocarbon (PAH) molecules carry the infrared (IR) emission features that dominate the spectra of most galactic and extragalactic sources. This review surveys the observed mid-IR characteristics of these emission features and summarizes laboratory and theoretical studies of the spectral characteristics of PAHs and the derived intrinsic properties of emitting interstellar PAHs. Dedicated experimental studies have provided critical input for detailed astronomical models that probe the origin and evolution of interstellar PAHs and their role in the universe. The physics and chemistry of PAHs are discussed, emphasizing the contribution of these species to the photoelectric heating and the ionization balance of the interstellar gas and to the formation of small hydrocarbon radicals and carbon chains. Together, these studies demonstrate that PAHs are abundant, ubiquitous, and a dominant force in the interstellar medium of galaxies.

Interstellar Polycyclic Aromatic Hydrocarbon Molecules

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

Empirical relations are established between the cohesive energy, surface tension, and melting temperature of different bulk solids. An expression for the size-dependent melting for low-dimensional systems is derived on the basis of an analogy with the liquid-drop model and these empirical relations, and compared with other theoretical models as well as the available experimental data in the literature. The model is then extended to understand (i) the effect of substrate temperature on the size of the deposited cluster and (ii) the superheating of nanoparticles embedded in a matrix. It is argued that the exponential increase in particle size with the increase in deposition temperature can be understood by using the expression for the size-dependent melting of nanoparticles. Superheating is possible when nanoparticles with a lower surface energy are embedded in a matrix with a material of higher surface energy in which case the melting temperature depends on the amount of epitaxy between the nanoparticles and the embedding matrix. The predictions of the model show good agreement with the experimental results. A scaling for the size-dependent melting point suppression is also proposed.

Liquid-drop model for the size-dependent melting of low-dimensional systems

Shells of atoms

Recently, films produced by depositing pre-formed mass-selected atomic clusters in the size range 1–10 nm have generated a great deal of attention. The ability to control the size, density and in some cases the morphology of the deposited particles allows unprecedented flexibility in the creation of new types of nanostructure. The technique enables research on the fundamental behaviour of matter at mesoscopic length scales and in addition has enormous potential in the creation of new materials whose properties can be widely varied. This review will describe the technology used to produce the particles and deposit them, and discuss the details of the landing on the surface and subsequent surface kinetics. The equilibrium structures and morphologies of the particles will be reviewed as also how the organisation of the particle assemblies is influenced by the particle–surface and inter-particle interactions. As a result of the reduced average atomic coordination, the quantum size effect and the modified screening response in nanoscale systems, the clusters display novel electronic and magnetic properties and these will be described for a range of elements including transition metals, noble and simple metals, and rare-earths. In addition to the native properties, the effect of the interaction with the substrate and other deposited particles will be explored. Finally, the work on coated particles and particles embedded in matrices will be reviewed and the potential to create new materials and devices will be discussed.

Nanoclusters deposited on surfaces

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

Inner-shell autoionization spectra obtained with synchrotron radiation are reported for mercury clusters ${\mathrm{Hg}}_{n}$, $n\ensuremath{\le}40$. For the small clusters $n\ensuremath{\le}12$ the spectral behavior as a function of size is interpreted in terms of an excitonic model. For larger cluster sizes a gradual deviation from van der Waals bonding is observed. The asymmetry occuring in the line profiles reflects the progressive construction of the $\mathrm{sp}$ conduction band.

Probing the transition from van der Waals to metallic mercury clusters.

Collective excitation in closed-shell potassium cluster ions

Unimolecular dissociation channels of mass-selected doubly charged sodium clusters ${\mathrm{Na}}_{\mathit{n}}^{++}$ have been determined for cluster sizes near and above the critical size, ${\mathit{n}}_{\mathit{c}}$=27, from which doubly charged clusters are detectable in mass spectra. On a time scale of 50 \ensuremath{\mu}s with respect to ionization, doubly charged clusters of size n=${\mathit{n}}_{\mathit{c}}$ are found to fission asymmetrically. For cluster sizes above ${\mathit{n}}_{\mathit{c}}$, however, evaporation of a neutral monomer is the dominant channel for delayed dissociation. An estimation of the height of the Coulombic barrier is given for ${\mathrm{Na}}_{\mathit{n}}^{++}$ near the critical size, using the framework of statistical theory.

Asymmetric fission of Nan++ around the critical size of stability.

The unimolecular dissociation of energy rich lithium cluster ions shows that Li+n dissociate by sequential atom or dimer loss. The binding energies of Li+n (n=4–42) generated in an evaporative ensemble are determined from unimolecular decay, within a well defined time window, and energy constraint. They present a sawtooth behavior vs cluster size less pronounced that it should be from a simple metal model. Odd–even alternation is superimposed on the sawtooth behavior, with odd sized cluster ions being more stable. Cohesive energies per atom of Li+n are deduced from these dissociation energies up to n=40 and from extended photo‐induced measurements up to n=95. Cohesive energies per atom of neutral clusters Lin are derived by combining these ionic cohesive energies with the literature ionization potentials. The linearity of the neutral cluster cohesive energy vs the cluster surface to volume ratio permits a volume and a surface energy to be deduced. These values are compared to the bulk values.

C. Bréchignac

Papers

Dynamics of unimolecular dissociation of sodium cluster ions

Probing the transition from van der Waals to metallic mercury clusters.

Collective excitation in closed-shell potassium cluster ions

Asymmetric fission of Nan++ around the critical size of stability.

Dissociation pathways and binding energies of lithium clusters from evaporation experiments