Hydrogen adsorption in a copper ZSM5 zeolite: an inelastic neutron scattering study

Currently microporous oxidic materials including zeolites are attracting interest as potential hydrogen storage materials. Understanding how molecular hydrogen interacts with these materials is important in the rational development of hydrogen storage materials and is also challenging theoretically. In this paper, we present an incoherent inelastic neutron scattering (INS) study of the adsorption of molecular hydrogen and hydrogen deuteride (HD) in a copper substituted ZSM5 zeolite varying the hydrogen dosage and temperature. We have demonstrated how inelastic neutron scattering can help us understand the interaction of H-2 molecules with a binding site in a particular microporous material, Cu ZSM5, and by implication of other similar materials. The H-2 molecule is bound as a single species lying parallel with the surface. As H-2 dosing increases, lateral interactions between the adsorbed H-2 molecules become apparent. With rising temperature of measurement up to 70 K (the limit of our experiments), H-2 molecules remain bound to the surface equivalent to a liquid or solid H-2 phase. The implication is that hydrogen is bound rather strongly in Cu ZSM5. Using the simple model for the anisotropic interaction to calculate the energy levels splitting, we found that the measured rotational constant of the hydrogen molecule is reduced as a consequence of adsorption by the Cu ZSM5. From the decrease in total signal intensity with increasing temperature, we were able to observe the conversion of para-hydrogen into ortho-hydrogen at paramagnetic centres and so determine the fraction of paramagnetic sites occupied by hydrogen molecules, ca. 60%. (c) 2006 Elsevier B.V. All rights reserved.

Relationship between phonons and thermal expansion in Zn(CN)2 and Ni(CN)2 from inelastic neutron scattering and ab initio calculations

Zn(CN)2 and Ni(CN)2 are known for exhibiting anomalous thermal expansion over a wide temperature range. The volume thermal expansion coefficient for the cubic, three dimensionally connected material, Zn(CN)2, is negative (alpha(V) = −51  10(-6) K-1) while for Ni(CN)2, a tetragonal material, the thermal expansion coefficient is negative in the two dimensionally connected sheets (alpha(a) = −7  10(-6) K-1), but the overall thermal expansion coefficient is positive (alpha(V) = 48  10(-6) K-1). We have measured the temperature dependence of phonon spectra in these compounds and analyzed them using ab initio calculations. The spectra of the two compounds show large differences that cannot be explained by simple mass renormalization of the modes involving Zn (65.38 amu) and Ni (58.69 amu) atoms. This reflects the fact that the structure and bonding are quite different in the two compounds. The calculated pressure dependence of the phonon modes and of the thermal expansion coefficient, alpha(V), are used to understand the anomalous behavior in these compounds. Our ab initio calculations indicate that phonon modes of energy approx. 2 meV are major contributors to negative thermal expansion (NTE) in both the compounds. The low-energy modes of approx.8 and 13 meV in Zn(CN)2 also contribute significantly to the NTE in Zn(CN)2 and Ni(CN)2, respectively. The measured temperature dependence of the phonon spectra has been used to estimate the total anharmonicity of both compounds. For Zn(CN)2, the temperature-dependent measurements (total anharmonicity), along with our previously reported pressure dependence of the phonon spectra (quasiharmonic), is used to separate the explicit temperature effect at constant volume (intrinsic anharmonicity).

GLUON SATURATION AND THE TOTAL PROTON-AIR CROSS SECTION

In this work we propose a simple model for the total proton-air cross section, which is an improvement of the minijet model with the inclusion of a window in the p(T)-spectrum associated to the saturation physics. Our approach introduces a natural cutoff for the perturbative calculations which modifies the energy behavior of this component. The saturated component is calculated with a dipole model. The results are compared with experimental cross sections measured in cosmic ray experiments.

An experimental and theoretical investigation into positron and electron scattering from formaldehyde

We report on measurements of total cross sections (TCSs) for positron scattering from the fundamental organic molecule formaldehyde (CH(2)O). The energy range of these measurements was 0.26-50.3 eV, whereas the energy resolution was similar to 260 meV. To assist us in interpreting these data, Schwinger multichannel level calculations for positron elastic scattering from CH(2)O were also undertaken (0.5-50 eV). These calculations, incorporating an accurate model for the target polarization, are found to be in good qualitative agreement with our measured data. In addition, in order to compare the behaviour of positron and electron scattering from this species, independent atom model-screened additivity rule theoretical electron TCSs, now for energies in the range 1-10 000 eV, are also reported.

Dipole polarizability and Rayleigh light scattering by the hydrated electron

Assuming the existence of a confined state of the electron in bulk water the polarizability of the hydrated electron is analyzed. Statistically uncorrelated supermolecular structures composed of seven water molecules (first solvation shell) with an extra electron were extracted from classical Monte Carlo simulation and used in quantum mechanical second-order Moller-Plesset calculations. It is found that the bound excess electron contributes with 274 a.u. to the total dipole polarizability of 345 a.u. for (H(2)O)(7)(-). From the calculated polarizabilities the Rayleigh elastic light scattering properties are inferred and found to considerably enhance activity and light depolarization. (C) 2009 Elsevier B.V. All rights reserved.

Breakup effects in fusion and total reaction cross sections of weakly bound systems

We use a new technique to investigate the systematic behavior of near barrier complete fusion, total fusion and total reaction cross sections of weakly bound systems. A dimensionless fusion excitation function is used as a benchmark to which renormalized fusion data are compared and dynamic breakup effects can be disentangled from static effects. The same reduction procedure is used to study the effect of the direct reaction mechanisms on the total reaction cross section.

Universal trend for heavy-ion total reaction cross-sections at energies above the Coulomb barrier

Heavy-ion total reaction cross-section measurements for more than 1100 reaction cases covering 61 target nuclei in the range (6)Li-(238)U and 158 projectile nuclei from (2)H to (84)Kr (mostly exotic ones) have been analyzed in a systematic way by using an empirical, three-parameter formula that is applicable to the cases of projectile kinetic energies above the Coulomb barrier. The analysis has shown that the average total nuclear binding energy per nucleon of the interacting nuclei and their radii are the chief quantities that describe the cross-section patterns. A great amount of cross-section data (87%) has been quite satisfactorily reproduced by the proposed formula; therefore, the total reaction cross-section predictions for new, not yet experimentally investigated reaction cases can be obtained within 25% (or much less) uncertainty.

Total reaction cross-sections for light weakly bound systems

We have measured the elastic scattering cross-section for (8)Li + (9)Be and (8)Li + (51)V systems at 19.6 MeV and 18.5 MeV, respectively. We have also extracted total reaction cross sections from the elastic scattering analysis for several light weakly bound systems using the optical model with Woods-Saxon and double-folding-type potentials. Different reduction methods for the total reaction cross-sections have been applied to analyze and compare simultaneously all the systems.

Elastic scattering of a proton-halo nucleus: (8)B+(58)Ni

The elastic channel of the (8)B + (58)Ni system has been measured at energies around the Coulomb barrier. An optical potential fi to the experimental angular distributions is obtained. The total reaction cross section consistent with the obtained potential is reported and possible deviations from normal behaviour are discussed.

Positronium scattering by a hydrogen molecule including exchange

We perform a three-positronium (Ps) state [Ps(ls,2s,2p)] coupled-channel calculation of Ps-H-2 scattering including the effect of electron exchange. At medium energies, higher excitations and ionization of Ps are treated within the framework of the first Born approximation. In both cases exchange is included using a recently proposed nonlocal model exchange potential which is free of non-orthogonality problems common in the usual antisymmetrization scheme. The present total cross sections at low and medium energies are in encouraging agreement with experiment.

Effect of electron correlation on positronium formation in positron-helium scattering

A three-parameter correlated wave function for the helium ground state is used to study the scattering reaction e(+) + He --> He+ + Ps, where Ps stands for positronium atom. An exact analytical expression is constructed for the first Born scattering amplitude for Ps formation from helium. Based on this numerical results are presented for both differential and total cross-sections. It is demonstrated that the inner electronic correlation of the target atom plays a crucial role in explaining the discrepency between theory and experiment.

Positronium-positronium interaction: resonance, scattering length, and Bose-Einstein condensation

The low-energy scattering of ortho positronium (Ps) by ortho Ps has been studied in a full quantum mechanical coupled-channel approach. In the singlet channel (total spin s(T) = 0) we find S- and P-wave resonances at 3.35 eV (width 0.02 eV) and 5.05 eV (width 0.04 eV), respectively, and a binding of 0.43 eV of Ps(2). The scattering length for s(T) = 0 is 3.95 Angstrom and for s(T) = 2 is 0.83 Angstrom. The small s(T) = 2 scattering length makes the spin-polarized ortho Ps atoms an almost noninteracting ideal gas which may undergo Bose-Einstein condensation. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Positronium scattering by atoms and molecules at low energies

The recent theoretical and experimental activities in positronium (Ps) scattering by atoms and molecules are reviewed with special emphasis at low energies. We critically compare the results of different groups - theoretical and experimental. The theoretical approaches considered include the R-matrix and close-coupling methods applied to Ps-H, Ps-He and Ps-Li scattering, and a coupled-channel approach with a nonlocal model potential for Ps scattering by H, He, H-2, Ne, Ar, Li, Na, K, Rb, Cs and Ps and for pickoff quenching in Ps-He scattering. Results for scattering lengths, partial. total and differential cross-sections as well as resonance and binding energies in different systems are discussed. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Positronium-hydrogen-atom scattering in a five-state model

The scattering of orthopositronium (Ps) by hydrogen atoms has been investigated in a five-state coupled-channel model allowing for Ps(1s)H(2s,2p) and Ps(2s,2p)H(1s) excitations using a recently proposed electron-exchange model potential. The higher (n greater than or equal to 3) excitations and ionization of the Ps atom are calculated using the first Born approximation. Calculations are reported of scattering lengths, phase shifts. elastic, Ps and H excitation, and total cross sections. Remarkable correlations are observed between the S-wave Ps-H binding energy and the singlet scattering length, effective range, and resonance energy obtained in various model calculations. These correlations suggest that if a Ps-H dynamical model yields the correct result for one of these four observables, it is expected to lead to the correct result for the other three. The present model, which is constructed so as to reproduce the Ps-H resonance at 4.01 eV, automatically yields a Ps-H bound state at - 1.05 eV that compares well with the accurate value of - 1.067 eV. The model leads to a singlet scattering length of 3.72a(0) and effective range of 1.67a(0), whereas the correlations suggest the precise values of 3.50a(0) and 1.65a(0) for these observables, respectively. [S1050-2947(99)07703-3].

Positronium atom scattering by H-2 in a coupled-channel framework

The scattering of ortho-positronium (Ps) by H-2 has been investigated using a three-Ps-state (Ps(1s,2s, 2p)H-2(X (1)Sigma(g)(+))) coupled-channel model and using the Born approximation for higher excitations and ionization of Ps and B (1)Sigma(u)(+) and b (3)Sigma(u)(+) excitations of H-2. We employ a recently proposed time-reversal-symmetric non-local electron-exchange model potential. We present a calculational scheme for solving the body-frame fixed-nuclei coupled-channel scattering equations for Ps-H-2, which simplifies the numerical solution technique considerably. Ps ionization is found to have the leading contribution to target-elastic and all target-inelastic processes. The total cross sections at low and medium energies are in good agreement with experiment.