Multi-atom Cluster Research Articles

Generation of multi-atom cluster states via an unconventional geometric phase shift in cavity QED

This paper proposes two schemes to generate the multi-atom cluster states. The first scheme is based on the interaction of atoms with a highly detuned cavity mode and a classical field, the second scheme is based on the interaction of atoms with a cavity mode, strongly driven by a resonant classical field.

Effects of nanosecond-duration laser prepulses on solid targets

A critical issue in high-intensity laser-solid interactions is the effect of the laser prepulse on the target, but the experimental details of these lower-intensity interactions are often difficult to measure due to the subsequent high-intensity pulse. We have performed target experiments using a 0.5-ns duration, 800-nm wavelength laser pulse, specifically designed to mimic the typical amplified spontaneous emission (ASE) prepulse from a high-power Ti:Sapphire laser. Using this “artificial” ASE prepulse, we find that the threshold for relevant changes to typical solid targets occurs at a fluence of ∼0.1J∕cm2, or ∼108W∕cm2, well below the plasma formation threshold. Notably, the results are not consistent with simple surface vaporization, and suggest that the ASE prepulse causes multiatom clusters to be ejected from the target surface. In a full high-intensity experiment, this ablated material would then strongly interact with the subsequent primary laser pulse.

Identification of isolated Pt atoms in H-mordenite

The state and dispersion of Pt supported on mordenite have been studied using FTIR spectroscopy, CO Chemisorption, and H–D isotope exchange of cyclopentane. Under conditions for which other criteria indicate the presence of monatomic Pt0, a characteristic IR band of adsorbed CO is identified at 2123 cm–1. The position of this band does not change with CO coverage, indicating an interaction between CO and highly dispersed Pt. A significant blue shift of the band with respect to that of CO on multiatomic Pt clusters and the disappearance of this band upon neutralizing the acidic OH groups show that the Pt atoms are electron-deficient and interact with zeolite protons. These FTIR data thus confirm the presence of extremely small Pt clusters, presumably [Pt1—Hz]z+ adducts, in H-mordenite.

A novel viewpoint on the Cu-Au phase diagram: The interplay between fixed ising energies and elastic effects

Theoretical models of temperature-composition phase diagrams of binary A-B systems have traditionally been based on various approximate solutions to the same broad class of physical Hamiltonian—the generalized Ising model. Common to all such approaches is the description of the interaction energies between sites, pairs or multi-atom clusters of A and B by some fixed (volume-independent) parameters. Despite extensive attempts to improve on the method of solution to the statistical problem (quasi chemical approach, cluster variation method i.e. CVM, high-temperature expansions, Monte Carlo), or to increase the ranged of the interaction to further neighbors, numerous qualitative discrepancies with real phase diagrams (briefly reviewed here) remain. We show that the source of the difficulty is not in the statistical method or range of interaction, but rather in the physical content of the interaction energies used. Recognizing that all successful classical packing models of solids require a description of the competition between fixed-volume “chemical” energies (representable as fixed Ising interaction parameters) and size-mismatch “elastic” energies (not representable by fixed energies), we include both on the same footings in a generalized Ising-like approach. Calculating both terms from first-principles self-consistent band theory we show through CVM calculations on the prototype Cu-Au phase diagram how many of the deficienciesof the “chemical-only” Ising models can be cured. This reveals the dominant role of lattice relaxation in determining many of the thermodynamic properties and the phase diagram.

Formation of silicon and water cluster ions in pulsed-laser stimulated field desorption

Multiatom cluster ions of 3 to 16 atoms/cluster have been observed in pulsed-laser stimulated field desorption of silicon emitters. In a time-of-flight mass spectrum fine structures can be found in the mass line of every ion species, which are produced by isotope mixing in the clusters, and which will facilitate a correct identification of the charge state of the ions. The most abundant cluster ion species are Si2+4, Si2+5, and Si2+6. The numbers 4–6 may be called the magic numbers of the desorption process since they correspond to highly symmetric small units of atoms existing in Si surface layers. Most of the energy distributions of the ion species contain an exponentially decaying low energy tail which is produced by spontaneous dissociation of larger cluster ions, possibly due to photodissociation of larger cluster ions by the same laser pulses used for desorption. Water cluster ions of the form (H2O)mH+ (m=1–12) can be observed in pulsed-laser field desorption of ‘‘field condensed’’ water on a metal emitter surface. The appearance energy decreases monotonically with the cluster size, and also depends on the condition of the surface.

A potential energy scaling Monte Carlo simulation of thin film nucleation and growth

A Monte Carlo computer simulation employing a potential energy scaling technique has been used to model the initial stages of thin film growth. The model monitors variations in the interaction potential that occur as a result of the arrival or departure of adatoms or impurities at sites in a 20 x 20 array. Boltzmann-ordered statistics are used to simulate fluctuations in vibrational energy at each site in the array, and the resulting site energy is compared with threshold levels of possible atomic events. In addition to adsorption, desorption and surface migration of adatoms, the simulation considers adatom incorporation and diffusion of a substrate atom to the surface. The effect of lateral interaction of nearest, second-nearest and third-nearest neighbors is also considered. A series of computer experiments were conducted for the Ge-Fe(100) system to illustrate the behavior of the model and to show that the surface migration of monomers, the formation of multiatom clusters and the coalescence of growing clusters behave in an accurate physical manner. Further, the model was found to be especially useful in the study of growth around different types of surface defects.

Recent developments in the vibrational spectroscopies (infrared, Raman, electron energy loss etc.) as applied to the structural analysis of species chemisorbed on metal surfaces

An account is given of the methods of vibrational spectroscopy that are nowadays used to obtain structural information about species adsorbed on metal catalyst surfaces. The discussion deals separately with experiments involving a) oxide-supported metal catalysts in the finely divided form, and b) specific surfaces of metal single crystals. A discussion is also given of the advantages and limitations of each method, e.g. infrared transmission and Raman spectroscopy in the case of finely-divided metals, and electron energy loss spectroscopy and reflection-absorption infrared spectroscopy as used for metal single crystals. Infrared and Raman spectroscopic studies of ligands attached to multi-atom metal clusters are shown to lead to convincing identifications of species adsorbed on single crystal surfaces. The latter results in turn contribute to the understanding of the overlapping spectra from several different adsorbed species that commonly occur on finely-divided metal catalysts.

Cluster formation in amorphous and polycrystalline thin films

Ions field evaporating from thin films r.f. sputter deposited onto molybdenum and tungsten tips in the presence of hydrogen are identified by employing an energy-focused atom probe field ion microscope. The existence of multiatomic clusters is unequivocally established. While the abundance of clusters for a well-developed tungsten tip is about 0.01%, polycrystalline sputtered metal films (nickel, gold and platinum) yield 2–5% cluster ions. For polycrystalline NiO x films and amorphous germanium and silicon films, the yield of clusters increases up to about 20% and the cluster size also increases. The abundance of cluster ions is thus directly related to the degree of disorder of the materials. Study of cluster ion formation may shed light on the bonding mechanisms of thin film materials.

The solubility and trapping of hydrogen in vanadium

Solid solutions of hydrogen in vanadium are affected greatly by the presence of other interstitials carbon, nitrogen and oxygen. The solutions depart drastically from randomness at low temperatures and the solvus line is pushed to higher equilibrium hydrogen concentration as the concentration of these interstitials is increased. Utilization of internal friction and resistivity measurements permits this effect to be measured quantitatively. Two explanations are examined to interpret the results: (1) an effect of mechanical constraint of the matrix on the precipitate which acts to retard its growth. (2) an effect of clusters of hydrogen with C, N or O to alter the equilibrium concentration of hydrogen in the free lattice, thereby altering the effective supersaturation of hydrogen. The first is considered to be inadequate to explain the full magnitude of the effect. The second can explain the effect in detail if the multi-atom clusters are assumed to exist in relatively high concentration, that is, if the binding energy per hydrogen atom is very much greater than kT.

Multiatomic clusters emerging from a metal surface under ion bombardment

[Ag n ]− ions emerging from a silver target under bombardment with energetic krypton ions have been measured forn ranging up to 60. The relative intensities of ions with odd numbers are strikingly favoured. The numbersn=8 and 20 mark minima in the abundance of ions with even numbers. The observations are briefly discussed.