From collisions to clusters: first steps of sulphuric acid nanocluster formation dynamics

The clustering of sulphuric acid with base molecules is one of the main pathways of new-particle formation in the Earth's atmosphere. First step in the clustering process is likely the formation of a (sulphuric acid)₁(base)₁(water)_n cluster. Here, we present results from direct first-principles molecular dynamics collision simulations of (sulphuric acid)₁(water)_{0, 1} + (dimethylamine) → (sulphuric acid)₁(dimethylamine)₁(water)_{0, 1} cluster formation processes. The simulations indicate that the sticking factor in the collisions is unity: the interaction between the molecules is strong enough to overcome the possible initial non-optimal collision orientations. No post-collisional cluster break up is observed. The reasons for the efficient clustering are (i) the proton transfer reaction which takes place in each of the collision simulations and (ii) the subsequent competition over the proton control. As a consequence, the clusters show very dynamic ion pair structure, which differs from both the static structure optimisation calculations and the equilibrium first-principles molecular dynamics simulations. In some of the simulation runs, water mediates the proton transfer by acting as a proton bridge. In general, water is able to notably stabilise the formed clusters by allocating a fraction of the released clustering energy.     

Superfluidity and quantum melting of p-H2 clusters

Structural and superfluid properties of p-H2 clusters of size up to N=40 molecules, are studied at low temperature (0.5 K相似文献   

The relation between crystal structure and the formation and mobility of protonic charge carriers in perovskite-type oxides: A case study of Y-doped BaCeO3 and SrCeO3

Proton conductivity phenomena in 10% Y-doped barium and strontium cerate are investigated experimentally and by quantum molecular dynamics simulations. In particular the impact of deviations from the cubic perovskite structure on the formation and mobility of protonic charge carriers is investigated. For Y: SrCeO₃, which shows a larger deviation from the ideal cubic perovskite structure, the concentration and mobility of protonic defects is significantly lower than for Y: BaCeO₃. The first is due to the decay of the oxygen position into two sites, only one of which is involved in the formation of protonic defects. The symmetry reduction also leads to the formation of different one-dimensional proton diffusion paths, and unfavourable jumps between such paths are supposed to control the macroscopic proton diffusion coefficient in Y: SrCeO₃. The analysis suggests the formation of strong but transient hydrogen bonds and inter-octa-hedra proton transfer between vertices for SrCeO₃ in contrast to just intra-octahedra proton transfer for BaCeO₃. Whereas for BaCeO₃ the proton transfer step is identified to be rate-limiting at T= 1000 K, for SrCeO₃ both proton transfer and reorientation are found to be of similar magnitude.     

Mesoscopic and macroscopic dipole clusters: Structure and phase transitions

A two dimensional (2D) classical system of dipole particles confined by a quadratic potential is studied. This system can be used as a model for rare electrons in semiconductor structures near a metal electrode, indirect excitons in coupled quantum dots etc. For clusters of N ≤ 80 particles ground state configurations and appropriate eigenfrequencies and eigenvectors for the normal modes are found. Monte-Carlo and molecular dynamic methods are used to study the order-disorder transition (the “melting” of clusters). In mesoscopic clusters (N < 37) there is a hierarchy of transitions: at lower temperatures an intershell orientational disordering of pairs of shells takes place; at higher temperatures the intershell diffusion sets in and the shell structure disappears. In “macroscopic” clusters (N > 37) an orientational “melting” of only the outer shell is possible. The most stable clusters (having both maximal lowest nonzero eigenfrequencies and maximal temperatures of total melting) are those of completed crystal shells which are concentric groups of nodes of 2D hexagonal lattice with a number of nodes placed in the center of them. The picture of disordering in clusters is compared with that in an infinite 2D dipole system. The study of the radial diffusion constant, the structure factor, the local minima distribution and other quantities shows that the melting temperature is a nonmonotonic function of the number of particles in the system. The dynamical equilibrium between “solid-like” and “orientationally disordered” forms of clusters is considered.     

Molecular-dynamics simulation of the heat capacity for nickel and copper clusters: Shape and size effects

We have investigated the heat capacity of ideal Cu and Ni fcc clusters with diameters from 2 to 6 nm in the temperature range 200–800 K by the molecular-dynamics method using a modified tight-binding potential. Our analysis has shown consistency with the experimental results at temperatures of 200–300 K. The data obtained are also indicative of several regularities that are in agreement with the analytical calculations. We have concluded from the results of our computer simulations that the heat capacity in the case of isolated free clusters can exceed that of a bulk material, with this difference decreasing as the nanoparticle grows proportionally to the reduction in the fraction of surface atoms. The excess of the heat capacity for ideal copper and nickel nanoclusters with D = 6 nm at T = 200 K has been found to be 10% and 13%, respectively. Consequently, the large heat capacities of copper and nickel nanostructures observed in some real experiments cannot be related to the characteristics of free clusters. We hypothesize that these properties of a nanomaterial depend on the degree of agglomeration of its constituent particles, i.e., the surfaces and interphase boundaries of interconnected nanoclusters can have a strong effect. To test this hypothesis, we took nickel and copper clusters of various sizes (4000–7200 atoms) produced through the simulation of condensation from the gas phase. At high temperatures, we failed to adequately assess the role of the interphase boundaries in calculating the heat capacity of nanoparticles. The reason was the mass diffusion of Ni or Cu atoms to impart an energetically more favorable shape and structure to the synthesized clusters. At low temperatures, the heat capacity of such clusters exceeded that of clusters with an ideal shape and structure by a value from 3.2% to 10.6%. We have concluded that the Ni and Cu clusters produced in real experiments cannot be applied in devices using the thermal energy of such clusters without a preliminary optimization stage, because their external shape and interior structure are nonideal.     

Quasifree proton scattering on halo nuclei as a tool for studying the neutron-halo structure

An experimental method is proposed for investigating the structure of the two-neutron halo in quasifree proton scattering on clusters of halo nuclei. This scattering process is studied in inverse kinematics by using a ⁶He beam incident to a stack of track emulsions. Preliminary data on the reaction ⁶He + p → ⁴He + p + X are compared with the results of simple kinematical calculations for quasifree proton scattering on the clusters forming the halo of the ⁶He nucleus.     

A thermodynamic approach to mechanical stability of nanosized particles

Thermodynamic stability conditions for nanoparticles (resulting from non-negativity of the second variation of the free energy) have been analyzed for two cases: (i) a nonvolatile nanosized particle with the size-dependent surface tension; (ii) the limiting case of larger objects when the surface tension takes its macroscopic value. It has been shown that the mechanical stability of a nanoparticle, i.e. its stability relative to the volume fluctuations, is defined by an interplay between the excess (“surface”) free energy and the volumetric elastic energy. According to the results obtained, noble gas clusters and metal nanoparticles satisfy the mechanical stability condition. At the same time, water nanodrops, as well as nanoparticles presented by nonpolar organic molecules, correspond to the stability limit. Among the investigated systems, the stability condition is not carried out for n-Pentane clusters.     

Study of ω-meson production in pp collisions at ANKE

The production of ω-mesons in the pp → ppω reaction has been investigated with the COSY-ANKE spectrometer for excess energies of 60 and 92MeV by detecting the two final protons and reconstructing their missing mass. The large physical background was subtracted using an event-by-event transformation of the proton momenta between the two energies. Differential distributions and total cross-sections were obtained after careful studies of possible systematic uncertainties in the overall ANKE acceptance. The results are compared with the predictions of theoretical models. Combined with data on the φ-meson, a more refined estimate is made of the Okubo-Zweig-Iizuka rule violation in the φ/ω production ratio.     

Quantum chemical study of electronic and dynamic properties of metal and mixed non-stoichiometric clusters

The aim of this contribution is to show that quantum chemistry has suitable tools to extract the specific properties of small metallic and mixed non-stoichiometric clusters which cannot be obtained by extrapolation from the bulk properties to the atom. For this purpose, first the main features of the methods used for the calculations of the ground and excited states of clusters valid at zero temperature (T=0) will be sketched and the factors determining accuracy of results will be pointed out. The structural and optical response properties of cationic Na _n ⁺ clusters as a function of size will be presented and compared with experimental data. The series of non-stoichiometric alkali-halide clusters containing single and multiple excess electrons will serve as prototypes to study a possible “metal-insulator transition” and “segregation into metallic and ionic parts” in finite systems. Second, an outline of ab initio molecular dynamics methods based on gradient corrected density functional approach with gaussian basis used for determination of temperature dependent ground state properties will be presented. Different temperature behavior of distinct type of structures will be illustrated on an example of Li ₉ ⁺ cluster. Presented by V. Bonačić-Koutecky at the International Conference on “Atomic Nuclei and Metallic Clusters”, Prague, September 1–5, 1997. This work has been supported by the Deutsche Forschungsgemeinschaft (SFB 337, Energy transfer in molecular aggregates) and the Consiglio Nationale delle Ricerche (CNR, Rome).     

Small InAsN and InN clusters: electronic properties and nitrogen stability belt

Electronic properties of several small non-stoichiometric InAsN and InN clusters derived from the symmetry elements of the zincblende InAs and wurtzite InN bulk lattices have been studied by the first-principle, many-body field theoretical methods. Clusters’ nucleation conditions reflected those in quantum confinement and “vacuum”. Electronic properties of such clusters can be tuned both by the use of quantum confinement and doping, which provide for symmetry breaking and realization of excitations optically forbidden in tetrahedral and hexagonal symmetry clusters. Doping with nitrogen enhances stability and allows tailoring the optical transition energy of the clusters from ultraviolet to infrared. The obtained results closely correlate with available experimental data.     

Water alignment and proton conduction inside carbon nanotubes

First-principles molecular dynamics simulations have been carried out to investigate the structure, electronic properties, and proton conductivity of water confined inside single-walled carbon nanotubes. The simulations predict the formation of a strongly connected one-dimensional hydrogen-bonded water wire resulting in a net electric dipole moment directed along the nanotube axis. An excess proton injected into the water wire is found to be significantly stabilized, relative to the gas phase, due to the high polarizability of the carbon nanotube.     

Structure theorem for covariant bundles on quantum homogeneous spaces

The natural generalization of the notion of bundle in quantum geometry is that of bimodule. If the base space has quantum group symmetries, one is particularly interested in bimodules covariant (equivariant) under these symmetries. Most attention has so far been focused on the case with maximal symmetry — where the base space is a quantum group and the bimodules are bicovariant. The structure of bicovariant bimodules is well understood through their correspondence with crossed modules. We investigate the “next best” case — where the base space is a quantum homogeneous space and the bimodules are covariant. We present a structure theorem that resembles the one for bicovariant bimodules. Thus, there is a correspondence between covariant bimodules and a new kind of “crossed” modules which we define. The latter are attached to the pair of quantum groups which defines the quantum homogeneous space. We apply our structure theorem to differential calculi on quantum homogeneous spaces and discuss a related notion of induced differential calculus. Presented at the 10th International Colloquium on Quantum Groups: “Quantum Groups and Integrable Systems”, Prague, 21–23 June 2001. This work was supported by a NATO fellowship grant.     

Surface self-diffusion of a Pt adatom on cuboctahedral and truncated decahedral clusters,size dependence

The self-diffusion of single Pt adatom on the surface of cuboctahedral and truncated decahedral clusters with 561–10 179 atoms are studied within the context of the many-body potentials obtained via the embedded atom method. The minimum energy diffusion path and the corresponding energy barrier for adatom diffusion on the cuboctahedral and truncated decahedral clusters surfaces are determined through a combination of the quenched molecular dynamics and the nudged elastic band method. The calculated energy barriers are consistent with the available experimental data. The dependence of energy barrier for adatom diffusion across the step edge on the cluster size is found. For the larger cuboctahedral and truncated decahedral clusters, the simulations show that the movement of the adatom is confined to a central region, and the adatom may escape from the center region only at elevated temperatures. In addition, we also find that the truncated decahedral structure is more favorable over the cuboctahedral structure for smaller clusters. The cluster growth experiments support our results.     

Local superfluidity of parahydrogen clusters

We study by quantum Monte Carlo simulations the local superfluid response of small (up to 27 molecules) parahydrogen clusters, down to temperatures as low as 0.05 K. We show that at low temperature superfluidity is not confined at the surface of the clusters, as recently claimed by Khairallah et al. [Phys. Rev. Lett. 98, 183401 (2007)10.1103/PhysRevLett.98.183401]. Rather, even clusters with a pronounced shell structure are essentially uniformly superfluid. Superfluidity occurs as a result of long exchange cycles involving all molecules.     

Generalized Fock spaces,new forms of quantum statistics and their algebras

We formulate a theory of generalized Fock spaces which underlies the different forms of quantum statistics such as ‘infinite’, Bose-Einstein and Fermi-Dirac statistics. Single-indexed systems as well as multi-indexed systems that cannot be mapped into single-indexed systems are studied. Our theory is based on a three-tiered structure consisting of Fock space, statistics and algebra. This general formalism not only unifies the various forms of statistics and algebras, but also allows us to construct many new forms of quantum statistics as well as many algebras of creation and destruction operators. Some of these are: new algebras for infinite statistics,q-statistics and its many avatars, a consistent algebra for fractional statistics, null statistics or statistics of frozen order, ‘doubly-infinite’ statistics, many representations of orthostatistics, Hubbard statistics and its variations.     

Structural transformations and melting in neon clusters: quantum versus classical mechanics

The extraordinary complexity of Lennard-Jones (LJ) clusters, which exhibit numerous structures and "phases" when their size or temperature is varied, presents a great challenge for accurate numerical simulations, even without accounting for quantum effects. To study the latter, we utilize the variational Gaussian wave packet method in conjunction with the exchange Monte Carlo sampling technique. We show that the quantum nature of neon clusters has a substantial effect on their size-temperature "phase diagrams," particularly the critical parameters of certain structural transformations. We also give a numerical confirmation that none of the nonicosahedral structures observed for some classical LJ clusters are favorable in the quantum case.     

Electronegativity and hardness profiles of a chemical process: Comparison between quantum fluid density functional theory andab initio SCF method

Temporal evolution of electronegativity and hardness associated with a collision process between a Be atom and a proton has been studied within a quantum fluid density functional framework. In the presence of a third collisional partner to take away excess energy, this collision may lead to a chemical reaction producing a BeH⁺ molecule. For comparisonab initio SCF level calculation (with 6–31G** basis set) on BeH⁺ molecule with different geometries have been performed. Electronegativity equalization and maximum hardness principles are analyzed.     

Path-integral Monte Carlo simulations of electron localization in water clusters

Path-integral Monte Carlo simulations are used to investigate the possibility of binding of an excess electron to water clusters. Model potentials are used to characterize the interaction between the excess electron and a water monomer and between two water molecules. The simulations reveal that two water molecules can bind an electron. In addition, it is found that the excess electron can be trapped in the field of three water molecules arranged in a linear configuration. The results are used to comment on recent molecular beam experiments.     

Decoherence within a single atom

An “almost diagonal” reduced density matrix (in coordinate representation) is usually a result of environment induced decherence and is considered the sign of classical behavior. We show that the proton of a ground state hydrogen atom can indeed possess such a density matrix. This example demonstrates that the “almost diagonal” structure may be derived from an interaction with a low number of degrees of freedom which play the role of the environment. We also show that decoherence effects in our example can only be observed if the interaction with the measuring device is significantly faster than the interaction with the environment (the electron). In the opposite case, when the interaction with the environment is significant during the measurement process, coherence is maintained. Finally, we propose a neutron scattering experiment on cold He atoms to observe decoherence which shows up as an additional positive contribution to the differential scattering cross section. This contribution is inversely proportional to the bombarding energy.