Noble Gas Clusters Research Articles

Quantum mechanics and phase transitions in small noble-gas clusters

The quantum-statistical properties of Argon clusters are calculated applying a Path-Integral Monte-Carlo (PIMC) method. Results for pair correlation functions and kinetic-and configurational energy expectations for various cluster sizes and temperatures are presented. Physical phenomena such as ground-state kinetic energy and quantum admixture of different atomic configurations are studied and experiences with the method are given. In particular, the relation between metastable cluster states and phase transitions is discussed.

The dependence of the surface energy of regular clusters and small crystallites on the particle size

AbstractWhereas in the case of liquids the theories on the dependence of the surface energy on the particle size are uniform in general, describing a decrease of σ with decreasing particle size, in the case of small solid particles also a dependence with the inverse sign has been assumed in the literature.Equations of the size dependence of σ and γ are derived for icosahedral and cuboctahedral clusters of noble gases considering interactions only with next neighbours. It will be shown that the character of the size dependence is dominated by the choice of the position of the dividing surface. Compared to that, such equations are derived also for ion crystallites. The resulting differences can be explained by the differences in structure and bonding.

Ionized cluster beam deposition

An experimental cluster beam facility of the type used by Takagi was developed and tested. Since the cluster generation rate (of silver clusters in graphite crucibles) was small and not consistently observable, a basic study of the generation process was undertaken. Theoretical evaluation has revealed that clusters are formed on crucible internal surfaces and not in the vapor stream which passes out of the crucible through a nozzle, as had been assumed earlier. Experiments have confirmed this heterogeneous cluster growth model, showing that clusters occur consistently only when a temperature distribution is maintained within the crucible which favors cluster formation on surfaces near the crucible nozzle. A remaining problem is the low rate of cluster generation. This rate needs to be raised if metal clusters are to be used in practical film deposition apparatus. An alternative process which involves gaseous rather than metallic clusters is also under study. It is well known that isentropic expansion of gases, pressurized to a few atmospheres, results in homogeneous nucleation and can produce appreciable quantities of clusters with thousands of atoms. In order to utilize such gaseous clusters for film deposition applications, an assist mode is used in which films are grown by simultaneous vapor deposition and bombardment with energetic noble gas clusters. So far, a suitable cluster beam source has been developed, and deposition of gold films on GaAs substrates has been initiated, whereby ionized argon cluster beams provide the energy.

Noble-gas bubbles in metals: Molecular-dynamics simulations and positron states.

A theoretical treatment of atomic structure and positron states in noble-gas bubbles in metals is presented. The Al-He and Cu-Kr systems are considered as specific examples. For large bubbles (radii above a few tens of angstroms) a calculational scheme is developed combining molecular-dynamics results for the metal--noble-gas interface with positron calculations. It is demonstrated that a positron is trapped at the surface of a noble-gas bubble, i.e., at the metal-gas interface. The annihilation rate with metal electrons is similar to that at a clean surface, while simultaneously there is a significant annihilation rate with gas-atom electrons. This enables relationships between the gas density and the positron lifetime to be obtained for the systems considered. Experimental evidence supports the theoretical relations. In the molecular-dynamics simulations a trend towards close-packed layer-by-layer ordering of the gas atoms close to the metal-gas interface is found even in the cases where the bulk gas is in a fluid phase. The positron-state calculations also cover the case of adsorbed noble-gas layers at surfaces. For vacancy--noble-gas clusters containing up to 13 vacancies complementary positron results obtained with the calculational method developed by Puska and Nieminen are presented.

Complex-forming reactions in neutral noble gas clusters

The complex-forming reaction between CH3F and HCl imbedded in medium–large argon clusters has been monitored using molecular beam infrared spectroscopy. A ‘‘pickup’’ technique has been used which consists of exposing the (CH3F)mArn clusters, formed by expansion of a dilute mixture of CH3F in Ar from a supersonic nozzle, to a cross flux of HCl molecules. The clusters are characterized, with and without the reactant flow, by bolometric photoevaporation IR spectroscopy using line tunable CO2 lasers. Our data show that the products can be clearly distinguished from the reactant species. A plot of reactivity vs nozzle pressure, which is directly related to cluster size, indicates that, within our range of measurements, the HCl diffusion into the clusters is completed in less than 100 μs.

Rare gas dimers in a nozzle beam

Supersonic expansion from a nozzle produces noble gas clusters under various source conditions. The characteristics of atoms and dimers in a nozzle beam are examined. A study of the dimer collision cross-section suggests the existence of a temperature-dependent limiting oven pressure (PL) for the observation of pure dimers. This is further supported by the dependence of the beam intensity on stagnation pressure. The reduced pressure-temperature coordinates for noble gas dimers behave in accordance with the model of corresponding jets. The velocity distribution of atoms and dimers in a beam corresponding to stagnation pressure on either side ofPL is measured to determine the effect of condensation on the distribution pattern.

Evaporation of small clusters of noble gases by ionization

We have studied by molecular dynamics simulation the evolution of rare-gas clusters after their ionization. We find that, assuming the formation of a molecular dimer ion in the cluster, complete dissociation of small clusters occurs, in agreement with recent experimental results, and with mass spectra of Ar and Xe.

Evaporation of small clusters of noble gases by ionization

Abstract We have studied by molecular dynamics simulation the evolution of rare-gas clusters after their ionization. We find that, assuming the formation of a molecular dimer ion in the cluster, complete dissociation of small clusters occurs, in agreement with recent experimental results, and with mass spectra of Ar and Xe.

Calculated ground-state structures of 13-molecule clusters of carbon dioxide, methane, benzene, cyclohexane, and naphthalene

The ground-state structures of 13-molecule clusters of carbon dioxide, methane, benzene, cyclohexane, and naphthalene have been calculated by potential energy minimization, employing -12-6-1 nonbonded atom–atom potential functions. In addition, seven-molecule clusters were calculated for carbon dioxide and benzene. In all cases, noncrystallographic pentagonal motifs were found to stabilize the structures. The coordination polyhedra of the 13-molecule clusters are close to regular icosahedra, much the same as has been reported earlier for noble-gas clusters. The factors, determining the ability of the clusters to adopt this conformation, are discussed.