Hydrogen Clusters Research Articles

Slowing down of hydrogen clusters in thin foils

We present experimental results for the energy loss in carbon of 10 to 120 keV/proton H n + clusters (up to n=25). Special attention is given to the ratio between the total energy loss of a H n + cluster and the total energy loss of n individual protons of the same velocity. At low cluster energies this ratio is less than unity, while it is larger than unity at high energies. Saturation effects with cluster size are observed at least for lower cluster energies. A theoretical model describing these effects is developed, and the theoretical results compare well with experimental data.

Comparison of the stability of small clusters of hydrogen near a Pd(001) or a Ni(001) surface

Experimental data show that the H/Pd(001) system presents an ordered phase while a more diffuse diagram is observed for H/Ni(001). Using Green-function properties, we developed recently a formalism to describe the electronic structure of finite size hydrogen clusters near a metallic surface. Any atomic configuration can be considered in this real-space tight-binding framework. Multi-atom interactions are obtained as global quantities, getting round the explicit computation of differences between large numbers. Using this formalism, we present a comparison of the stability of small clusters of hydrogen near a Pd and a Ni surface. The stages of the growth in the case of H in Pd are described, and very good agreement with experimental results is obtained. In the case of H in Ni, the multi-atom interaction energies are very small, as observed experimentally.

Ab initio and Monte Carlo study of the structure and stability of hydrogen clusters (H3+(H2)n) (n = 3-16)

The authors study the stability and molecular structure of hydrogen cluster ions of the general form H{sub 3}{sup +}(H{sub 2}){sub n}. They employ a monte carlo procedure. They find that the ionic interaction between H{sub 3}{sup +} and H{sub 2} is the primary interaction leading to stability. The hydrogen molecular interactions tend to keep the molecules apart, but seems to have little effect on stability.

Large hydrogen cluster stopping in carbon

We have investigated theoretically the stopping of clusters impinging on a target at high velocities. All energy losses are assumed to be due to electronic interaction which is described within the linear dielectric formalism. We focus on hydrogen clusters consisting of ten or more H 2 molecules, moving in a carbon target. Two different models are employed to specify the target properties, namely the Drude and the Mermin dielectric functions. Interference effects, quantified as usual by the vicinage function, are identified as intra- and inter molecular contributions. The influence of different approximations for the atomic pair correlation function is also studied. Results are given for the stopping power per atom as a function of cluster size and velocity.

Radiation damage and impurity behavior in the superconducting tokamak TRIAM-1M under steady-state operation

Abstract Radiation damage of plasma facing materials and the behavior of impurities in the scrape-off layer, under steady-state discharge of superconducting tokamak TRIAM-1M, was examined with using a rotatable collector probe. TEM observations of metallic probe specimens exposed to the discharges showed that numerous radiation induced defects such as dislocation loops and hydrogen clusters were formed. These are typical defects formed by low energy (< a few keV) hydrogen bombardment. Mo, Fe and Cr, which originated from Mo fixed limiters and the 304SS vacuum vessel walls, were deposited on the specimens. The deposition profile depended on the geometrical relationship with the main plasma and affected the internal damage of the specimens. It is worth noting that the density of the metallic impurities in the scrape-off layer was almost constant during the steady-state discharge and had no adverse effects on plasma confinement in the TRIAM-IM.

Vicinage Effects in Secondary Electrons Yields from the (001) Surface of SnTe Induced by MeV Hydrogen Clusters

Yields of secondary electrons from the (001) surface of SnTe are studied at the impact of 0.3-0.8 MeV/amu H+, H2+ and H3+ ions. The yield per ion upon impact of the Hn+ ion is more than n times as large as that of isotachic proton impact. The observed molecular effects in the yields are explained as being the result of increased stopping power for fragment protons and of electrons released from the molecular ions upon their dissociation. It is also shown that the scaling relation, γ3=2γ2-γ1, remains roughly constant at oblique incidence angles up to 10° to the surface, where γn is the yield at Hn+ incidence.

Structure and energetics of hydrogen clusters. Structures of H+11 and H+13. Vibrational frequencies and infrared intensities of the H+2n+1 clusters (n=2–6)

A b initio self-consistent-field (SCF) Hartree–Fock and configuration interaction (CI) calculations have been carried out for H+2n+1 (n=1–6) clusters using a triple-zeta plus polarization basis set. Fully optimized structures and energies of H+11 and H+13 are presented. These structures can be thought as the addition of H2 molecules to a deformed H+9. Dissociation energies as a function of cluster size follow the pattern established experimentally by Hiraoka and Mori. Nevertheless, our energy results on the biggest clusters suffer from the lack of size consistency of CI with single and double substitutions (CISD) calculations. Analytic gradient techniques have been used to locate stationary point geometries and to predict harmonic vibrational frequencies and infrared intensities at the two levels of theory SCF (n=1–6) and CISD (n=1–4) both with triple-zeta polarization basis sets. Of special interest are the new vibrational modes of H+11 and H+13, which have no counterpart in the H+9 cluster. Our predicted frequencies compare fairly well with the experimental results of Okumura, Yeh, and Lee.

Interaction of hydrogen Hn+ clusters with thin carbon foils.

A Monte Carlo computer program was developed to calculate the angular distribution of fragments resulting from the breakup of fast hydrogen clusters ${\mathrm{H}}_{\mathit{n}}^{+}$ when traversing thin amorphous carbon foils. The three-dimensional relative coordinates of each proton in the cluster are given as input. The code begins with a generation of a random orientation of clusters. Next, the foil is divided into slabs and the effects of Coulomb explosion, multiple scattering, and energy loss on each fragment are calculated for successive slabs. The distributions of separations and relative velocities at the exit surface as well as the angular distributions of fragments are then obtained. Therefore, the cluster-stopping-power ratio is given. The use of the code is illustrated in the case of 30-, 40-, 60-, and 80-keV/p clusters, with n=2,3 to 21 odd, impinging on a 2-\ensuremath{\mu}g/${\mathrm{cm}}^{2}$ carbon foil, where experimental results are available.

Cluster stopping power for an electron gas at finite temperatures: calculations for hydrogen and water clusters

A formalism for the stopping power of atomic clusters moving in an electron gas at finite temperature is presented. Calculations for diatomic, triatomic and larger clusters are performed, for cluster velocities smaller than the Fermi velocity of the target electrons. The dependence of the cluster stopping power on cluster orientation is more significant for small clusters at low temperatures. The interference effects on the stopping power of large clusters are described in terms of the pair correlation function of the cluster components. Specific results are given for clusters of hydrogen and water molecules. The authors obtain an enhancement of the cluster stopping power, which varies with temperature due to changes in the screening conditions. For large molecular clusters the intermolecular vicinage function is negative, and at low temperatures diminishes the stopping power enhancement due to intramolecular contributions.

Multiatom interactions between hydrogen atoms near a metallic surface

An electronic calculation of the interactions between hydrogen clusters near a metallic surface is presented. For the first time, tetramers of impurities are considered. The interaction energies are determined in a self-consistent way within the tight-binding formalism. This approach is applied to clusters of hydrogen near a Pd(001) surface. Agreement with experimental results is good: for an adsorbed layer, multiatom interactions stabilize the observed C(2 × 2) structure. In the bulk the HH distance is greater than 2.1 Å. The determination of the multiatom interactions is needed for the overlayer phase diagram calculation. Such cluster interactions cannot be reduced strictly to a combination of pair energies and lead to an asymmetrical phase diagram.

Formation of extended hydrogen complexes in silicon.

We have examined the nucleation and growth of hydrogen clusters using pseudopotential total-energy calculations. A double-layer {111} H platelet, resulting from clustering of diatomic ${\mathrm{H}}_{2}^{\mathrm{*}}$ complexes, is lower in energy by 0.15 eV per H pair than interstitial ${\mathrm{H}}_{2}$ molecules. This result explains two experimental observations: (1) predominance of Si-H bonding observed in Si containing high H concentrations and (2) the formation of H {111} platelets. The H vibrational frequencies, H-H separations, and other properties of the platelet are in general agreement with experiment.

Ab initio structure calculations of hydrogen ionic clusters

Ab initio molecular electronic-structure theory has been used in an attempt to characterize the low-lying stationary points on the potential energy hypersurface of positive ionic hydrogen clusters. Using triple-zeta-plus-polarization basis sets, self-consistent-field Hartree-Fock calculations, configuration-interaction calculations with single + double substitutions and fourth-order Møller-Plesset calculations have been carried out for H + n n = 3 to 9 (odd). Dissociation energies as a function of cluster size follow the most recent experimental data. To our knowledge, the present study represents the best theoretical prediction for H + 7 and H + 9.

Structure of small molecular hydrogen clusters

The ground state energies and structural properties of small (H2) N ,N=2−7, are calculated using the variational Monte Carlo method. These wavefunctions include both short- and long-range correlation effects that are important in the binding of van der Waals clusters. We have investigated these clusters using shadow wavefunctions and found that the coupling to shadow variables raises the energy in all cases, implying that the ground states of these small clusters are properly described as quantum liquids rather than solid structures.

Multiatom interactions between hydrogen atoms near a metallic surface

An electronic calculation of the interactions between hydrogen clusters near a metallic surface is presented. For the first time, tetramers of impurities are considered. The interaction energies are determined in a self-consistent way within the tight-binding formalism. This approach is applied to clusters of hydrogen near a Pd(001) surface. Agreement with experimental results is good: for an adsorbed layer, multiatom interactions stabilize the observed C(2 × 2) structure. In the bulk the HH distance is greater than 2.1 Å. The determination of the multiatom interactions is needed for the overlayer phase diagram calculation. Such cluster interactions cannot be reduced strictly to a combination of pair energies and lead to an asymmetrical phase diagram.

“Boson” peak in Raman spectra of hydrogenated amorphous silicon

Hydrogenated amorphous silicon films were prepared by a dc magnetron sputtering method. Broad background signal observed in Raman spectra near the excitation line which is recently attributed to the recombination of nonthermal electron with nonthermal holes was decomposed from the phonon bands. The Stokes/ anti-Stokes ratio, depolarization ratio and the shape of the background signal have the characteristics typical for the “boson” peak in glasses. The appearance of the “boson” peak in a-Si:H is discussed in a model of the non-continuous structure of amorphous silicon, composed of the blobs of strongly connected silicon atoms placed around hydrogen clusters.

NMR study of the distribution of aromatic molecules in NaY zeolite

Local and macroscopic distributions of adsorbed benzene, 1,3,5-trimethylbenzene, and hexamethylbenzene (HMB) molecules among the intracrystalline cavities of NaY zeolite samples have been investigated by {sup 129}Xe and multiple-quantum NMR. Xenon-129 NMR, a sensitive probe of macroscopic distributions of molecules in zeolites at room temperature, demonstrates the importance of sample heat treatment in distributing an adsorbate homogeneously throughout a collection of zeolite particles. NaY samples containing organic guests adsorbed at temperatures near the bulk species melting point produce multiple or broad xenon lines characteristic of macroscopic (i.e., interparticle) adsorbate concentration gradients. After thorough heat treatment of the samples at elevated temperatures, the xenon spectrum collapses to a single sharp line, consistent with a macroscopically uniform distribution. For HMB adsorbed in NaY, counting hydrogen clusters by multiple-quantum NMR reveals intraparticle HMB distributions consistent with one molecular per cavity at low loadings (average bulk loading {le} 1 molecule per cavity) and two molecules per cavity at higher loadings.

The role of hydrogen clusters in the Staebler-Wronski effect of amorphous silicon as elucidated by optically detected electron nuclear double resonance

Since the discovery of the Staebler-Wronski effect (SWE) the correlation with hydrogen has been pointed out by many workers. In this work, the optically detected electron nuclear double resonance (ODENDOR) experiment has been carried out for the purpose of elucidating microscopically the electron-hole recombination process and the role of hydrogen in the mechanism of the SWE. The SWE is suggested to correlate with the Si-H bond clusters.

Semiempirical study of rare gas and rare gas–hydrogen ionic clusters: R+n, (RnH)+, and (RnH2)+ for R≡Ar, Xe

The ionic rare gas clusters Ar+n and Xe+n and rare gas–hydrogen clusters (ArnH)+, (ArnH2)+, (XenH)+ and (XenH2)+ are studied by the semiempirical diatomics-in-ionic-systems (DIIS) method. The Ar+n clusters (n&amp;gt;3) are seen to have a structure of a linear Ar+3 core surrounded by n−3 neutral or almost neutral Ar atoms. For Xe+n (n&amp;gt;3), a symmetrical Xe+4 ionic core with the geometry of regular pyramid is formed. The rare gas–hydrogen clusters with one H atom have a simple Rk(RH)+ structure with k neutral rare gas atoms attracted to the (RH)+ molecule by polarization forces. Two H atoms can bind with Ar atoms to form quasistable clusters ArnH+2 which dissociate to (n−1)Ar+H+(ArH)+ through a high barrier of roughly 0.75 eV. Two H atoms and one Xe+ ion are shown to form a collinear valence-bound (XeHH)+ cluster whose dissociation energy is 0.46 eV.

Formation of SnO 2 gels from dispersed sols in aqueous colloidal solutions

Transparent SnO 2 gels were obtained from SnCl 4 aqueous solution. The sol formation from tin oxihydroxy peptization in different concentrations and by electrolyte addition in solution was measured. It was verified that the residual presence of chloride ions compromises the colloidal system stability. The sol-gel transition was investigated as a function of the quantity of solid particles in the aqueous environment and of aging time at 60°C by infrared spectroscopy and rheological measurements. The transition from plastic to pseudoplastic flow observed with the increase in loading suggests that a continuous and three-dimensional network formation is closely related to hydrogen bridges and/or hydrogen clusters, culminating in the gel formation.

Emission of hydrogen clusters from insulating films under fast heavy ion bombardment

Emission of hydrogen clusters from insulating films was investigated under bombardment with fission fragments from a 252Cf source and with a 32S beam at energies 25–90 MeV by using a tandem accelerator. Results were compared with the predictions of different models.