Noble Gas Clusters Research Articles

Rotational analysis of the ground state of the Benzene·Ne van der Waals cluster cation by resolved high Rydberg spectroscopy

Series of resolved high n (40< n<110) Rydberg states of the benzene·Ne van der Waals cluster are measured by delayed field ionization after double resonance excitation with Fourier-transform limited UV laser pulses. A cross–correlation analysis yields the first rotationally resolved spectrum of an aromatic molecule noble gas cluster ion. Its analysis leads to accurate values for the adiabatic ionization energy ( AIE=74497.512(30) cm −1) of the benzene·Ne cluster and the rotational constants of the cluster ion. The average van der Waals bonding length of the Ne atom from the (benzene) +-plane is 3.320(2) Å.

Structural transitions in metal ion-doped noble gas clusters: Experiments and molecular dynamics simulations

We present the size distributions of metal ion-doped noble gas clusters of the form M+Xn (M=Mg, K and X=Ar, Kr, Xe) studied with time-of-flight mass spectrometry. All the recorded spectra exhibit magic number patterns, which change gradually from the familiar icosahedral sequence N=n+1=13,19,23,26,29,32 to another one that exhibits the magic numbers N=9,10,11,17,21,24,26,27,30, as the atomic size ratio of the metal ion to the noble gas atom decreases. Furthermore, as the cluster size N increases, the new sequence seems to convert again to the icosahedral one at some critical cluster size. Molecular dynamics simulations using pairwise additive Lennard-Jones potentials are performed in order to investigate the stability and the geometrical structure of these systems as a function of radii ratio, interaction energy, and cluster size. The results obtained are in very good agreement with the experimental ones and indicate that when the size of the dopant is comparable to that of the noble gas atoms then the clusters exhibit icosahedral geometries, while for smaller ratios, clusters having a geometry based on a capped square antiprism (CSA) are more stable.

Explosion of atomic clusters heated by high-intensity femtosecond laser pulses

We have experimentally and theoretically studied the high-intensity $(&gt;{10}^{16}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}),$ femtosecond photoionization of inertially confined noble-gas clusters. We have examined the energies of electrons and ions ejected during these interactions and found that particles with substantial kinetic energy are generated. Electrons with energies up to 3 keV and ions with energies of up to 1 MeV have been observed. These experimental observations are well explained by a theoretical model of the cluster as a small plasma sphere that explodes following rapid electron collisional heating by the intense laser pulse.

High resolution spectroscopy of Ne and Ar containing noble gas clusters

Detailed studies of the first rotational spectra of mixed noble gas trimers and tetramers containing Ne and Ar atoms are described. Transitions of various isotopomers of Ne2Ar, NeAr2, Ne3Ar, Ne2Ar2, and NeAr3 were measured. Accurate values for rotational constants and centrifugal distortion constants were obtained from the spectral analyses. The spectroscopic parameters are consistent with isosceles triangular geometries of the trimer systems, and with distorted tetrahedral arrangements for the tetramers. Further evidence for the geometries was found in the effects of spin statistics in the spectra of isotopomers with C2v symmetry and of those with C3v symmetry. The simplicity of these fundamental systems makes them well suited for the determination and characterization of many-body nonadditive interactions. The signal strengths observed in these first high resolution spectroscopic studies of noble gas tetramers indicate the possibility of detecting the spectra of larger noble gas clusters.

High Energy Ion Explosion of Atomic Clusters: Transition from Molecular to Plasma Behavior

We have studied the high-intensity femtosecond photoionization of inertially confined noble-gas clusters. The explosion of the resulting highly ionized, high-temperature microplasma ejects ions with substantial kinetic energy. We have observed Xe ions with kinetic energy up to 1 MeV and charge states as high as ${40}^{+}$. This ion energy is over three orders of magnitude higher than has previously been observed in the Coulomb explosion of molecules or clusters of any kind and indicates that there is a fundamental shift in the nature of intense laser-matter interactions between molecules and large clusters.

Temperature measurement from scattering spectra of clusters: theoretical treatment

Scattering spectra from X-ray, electron or neutron diffraction experiments are sufficient to describe the phase behaviour of noble gas clusters and to determine their temperature. Using classical Monte Carlo simulations combined with optimized data analysis and Path Integral Monte Carlo calculations as “idealized experiments” we obtain scattering spectra of Ar- and Ne-clusters. Starting from the classical and quantum mechanical hypervirial theorems we devise a method to estimate the temperature and the caloric curves (which describe the phase behaviour of the noble gas clusters) directly from these scattering spectra using an interatomic potential function as input. As applications we studied for Ar-clusters the effect of different model potentials on the temperature estimate thus contributing to the intricate question of what experimentally is the temperature of an isolated cluster. For Ne-clusters we investigate the differences between classical and quantum mechanical treatment.

High-energy ions produced in explosions of superheated atomic clusters

Efficient conversion of electromagnetic energy to particle energy is of fundamental importance in many areas of physics. A promising avenue for producing matter with unprecedented energy densities is by heating atomic clusters, an intermediate form of matter between molecules and solids1, with high-intensity, ultra-short light pulses2–4. Studies of noble-gas clusters heated with high-intensity (>1016Wcm–2) laser pulses indicate that a highly ionized, very high temperature micro-plasma is produced. The explosion of these superheated clusters ejects ions with substantial kinetic energy3–5. Here we report the direct measurement of the ion energy distributions resulting from these explosions. We find, in the case of laser-heated xenon clusters, that such explosions produce xenon ions with kinetic energies up to 1 MeV. This energy is four orders of magnitude higher than that achieved in the Coulomb explosion of small molecules6, indicating a fundamental difference in the nature of intense laser–matter interactions between molecules and clusters. Moreover, it demonstrates that access to an extremely high temperature state of matter is now possible with small-scale lasers.

Ion‐Size Effects in the Growth Sequences of Metal‐Ion‐Doped NobleGas Clusters

We have studied the stability and the structure of doped noble gas cluster ions of the type M+Xn, (M=In, Al, Na, X=Ar, Kr, Xe) by systematically changing the composition M/X and observing changes in the magic number patterns appearing in the mass spectra. When the metal ion radius is comparable to the radius of the noble gas atom, the mass spectra show the familar icosahedral magic numbers n+ 1=13,19,23,26,29,32, 46, 55,… In constrast, for metal ions with radii significantly smaller than the noble gas atoms, we observe a new series of magic numbers n + 9, 11, 17, 21,24, 26,… This series converts into the icosahedral one for larger clusters. Using a simple hard sphere packing model, we show that this new series of magic numbers is consistent with a cluster growth sequence which is based on a capped square antiprism (CSA) geometrical structure of the clusters.

Survival of noble gas clusters scattering from hot metal surfaces

We present classical trajectory calculations of Ar n ( n = 1000−400) colliding with a hot Pt(111) surface. Large cluster fragments are found to survive a surface collision, and the fraction of atoms remaining in the fragment is concluded to increase with initial cluster size and surface temperature, and decrease with incident velocity above 100 m/s. Up to 52% of the initial cluster is found to survive as one unit in the most favorable case of Ar 4000 scattering from a surface at 1500 K. The implications of the results for new experimental investigations are discussed.

Shapes of 3He clusters.

It has been established that a dominant factor controllingthe ground-state properties and shapes of disparate finitefermion systems ~such as nuclei and simple-metal clusters,whose nature of bonding and cohesion are of very differentorigins with widely differing characteristic energetic andspatial scales! are the shell effects resulting from level de-generacies in conjunction with the Pauli principle ~e.g.,for nuclei see Ref. 1; for simple metal clusters see Refs. 2and 3!.In contrast to heavier noble-gas clusters,

Studies on the production and stability of large C N+ and M x+R N (M = C, Si, Ge and R = Ar, Kr) clusters

We present mass spectra obtained using a simple cluster source based on the mixing of the plasma produced from the laser ablation of a solid target with the supersonic expansion of a noble gas. Depending on the formation conditions, this source produces several distinct types of cluster, such as the strongly bonded covalent C N + and the weakly bonded C +/Si +/Ge +-doped noble gas clusters. From the analysis of the observed time-of-flight spectra as a function of the source conditions, we are able to infer the possible microscopic mechanisms responsible for the formation of these clusters.

Noble gas clusters as matrices for infrared spectroscopy. From small clusters to the bulk-matrix limit: SF6Arn, SF6Krn, and SF6Xen with 100≲n≲10 000

It is demonstrated that matrix-like spectroscopy may be carried out in the gas phase using molecular beams of clusters and the conditions under which bulk-matrix-like behavior is achieved are illustrated. At the same time, we obtain information on the structural evolution of noble gas clusters as a function of their size. Infrared spectra for SF6 attached to noble gas clusters of argon, krypton, and xenon were recorded using a free jet cluster source and a laser photofragmentation detection technique. When a dilute mixture of the chromophore in Ar and Kr is expanded at relatively low pressures, the clusters spectra show a feature characteristic of the SF6 solvated in a defective, unannealed matrix. This feature disappears at higher source pressures (larger sizes) at which the chromophore prefers to reside on the surface of the cluster. This can be demonstrated by producing neat clusters and depositing the chromophore on them. However, on producing still larger clusters, a different absorption appears which is accurately located at the same position as the main absorption in a well-annealed matrix of Ar or Kr. This behavior is related to the transition of clusters from a Mackay icosahedral structure, shown to be the most stable for smaller clusters, to the face-centered-cubic (fcc) structure which is observed in the bulk phase. This structural transition occurs at a nozzle stagnation pressure which corresponds to an average cluster size of about 2000 atoms for both Ar and Kr. Scattering studies performed on argon clusters suggest that the fcc-type clusters correspond to the largest sizes in the cluster size distribution present in the beams. A similar structural transition for xenon cluster was not established as the SF6 appears to solvate only slightly in Xe in the size range studied here.

Many-body trial wave functions for atomic systems and ground states of small noble gas clusters

Clusters of sizes ranging from two to five are studied by variational quantum Monte Carlo techniques. The clusters consist of Ar, Ne, and hypothetical lighter (‘‘1/2-Ne’’) atoms. A general form of trial function is developed for which the variational bias is considerably smaller than the statistical error of currently available diffusion Monte Carlo estimates. The trial functions are designed by a careful analysis of long- and short-range behavior as a function of inter-atomic distance; at intermediate distances, on the order of the average nearest neighbor distance, the trial functions are constructed to have considerable variational freedom. A systematic study of the relative importance of n-body contributions to the quality of the optimized trial wave function is made with 2≤n≤5. Algebraic invariants are employed to deal efficiently with the many-body interactions.

Noble gas clusters in model zeolite cavities

Noble gas atoms trapped in the intracrystalline cavities of zeolites may form clusters. A classical-mechanical isoenergetic molecular dynamics simulation is performed to simulate the dynamical behavior of noble gas clusters in zeolite cavities. To implement the simulation, a model is adopted of a homogeneous spherical cavity with Morse interaction between the noble gas atoms and cavity walls. The results for Ar6 clusters indicate that the noble gas clusters in the cavity undergo the same solid/liquid phase changes as in free space, and, at high enough energies, a rapid exchange between atoms adsorbed on the inner surface and thosein the interior of the cavity. Mathematical quenching is used to investigated the multidimensional potential surface of Ar clusters in the cavity.

Registration Method for Metastable Decompositon of Benzene–Ar n and Toluene–Ar n Cluster Ions in RETOF Instruments

A simple method for the study of metastable organic molecule – noble gas cluster cations in a RETOF mass spectrometer, based on the change in the direction of the flight path of charged particles, is presented.

Cluster Molecular Dynamics on Massively Parallel Computers

AbstractWe report the results of computer simulations of phase transitions in noble-gas clusters. The calculations were performed on a MasPar MP-l massively parallel computer with 8,192 processing elements (PE's). We discuss the efficient implementation of molecular dynamics algorithms for small clusters on this type of architecture. The simulations are based on a classical Lennard-Jones pair potential and follow the temporal evolution of the system by numerically integrating Newton's equations of motion using the Gear algorithm. Because the number of particles is much smaller than the number of PE's, optimal partitioning of the processing element array is an essential and non-trivial task.

Scattering of noble gas clusters with energies in the keV range

Noble gas clusters with an average size of several thousand atoms and with energies in the keV range were directed at metal surfaces under small angles of incidence. The scatter products were examined with respect to size, energy, and recoil angle. All observations fit an impact model in which the tangential momentum is preserved during impact, while the normal kinetic energy is converted to cluster heat. The associated temperature rise results in the evaporation of cluster atoms. Small clusters evaporate completely; large clusters are reduced in size. The evaporated atoms continue in a forward direction (together with the residual cluster fragments) and form a well-collimated and energetic atom beam.

Close-packing structure of small barium clusters.

Barium clusters are produced by the inert-gas condensation technique. The observed size distribution shows large peaks at n=13, 19, 23, 26, 29, 32, as well for singly charged clusters as for doubly and triply charged clusters. These results can be interpreted by icosahedral close-packing structures, similar to those observed in noble-gas clusters.

Quantum mechanics and the structure of noble-gas clusters

Quantum mechanics and the structure of noble-gas clusters

Ab initio study of electronic properties in free and matrix-isolated iron dihalides.

Ab initio Hartree-Fock calculations are performed in the molecular $^{5}\mathrm{\ensuremath{\Delta}}_{\mathrm{g}}$, $^{5}\mathrm{\ensuremath{\Pi}}_{\mathrm{g}}$, and $^{5}\mathrm{\ensuremath{\Sigma}}_{\mathrm{g}}^{+}$ states of the dihalides ${\mathrm{FeCl}}_{2}$ and ${\mathrm{FeBr}}_{2}$ at experimental and geometry-optimized atomic separations. Molecular-term energies, ionization energies, bond energies, electric-field gradients (EFG), and electron densities \ensuremath{\rho}(o) (the latter two quantities at the iron nucleus) are evaluated and compared with experiment. Electron-correlation corrections and basis-set effects on the energies, EFG, and \ensuremath{\rho}(0) are studied. The relative isomer shift of noble-gas-matrix-isolated ${\mathrm{FeCl}}_{2}$ and ${\mathrm{Fe}}^{0}$ is analyzed on the basis of electron densities calculated in ${\mathrm{FeCl}}_{2}$--noble-gas and ${\mathrm{Fe}}^{0}$--noble-gas clusters.