Cross sections for charge transfer, neutralization and excitation in H-H and H- -H+ collisions are presented in the energy range 1-30 keV. The semiclassical close-coupling approach is used with a two-centre two-electron atomic state expansion and frozen total spin. All details of the two-centre atomic state expansion scheme are given. The basis is expanded onto the bound state of H- and the two-centre H(1s)-H(nlm ) states, where n covers the K, L and M shells. A good agreement with experiments is achieved for charge transfer, neutralization and excitation to H(2s) in H-H collisions. Disagreement with previously published close-coupling calculations and experimental data is noted, especially for two-electron charge transfer in H- -H+ .

A technique to probe the interior of three-dimensional dynamic granular systems is presented. Positron emission particle tracking (PEPT) allows a single tracer particle to be followed around a three dimensional vibrofluidized granular bed for periods up to six hours. At present the technique is able to resolve the position of the grains to +/-4 mm, with an average temporal resolution of about 7 ms. Packing fraction profiles are calculated by making use of the ergodicity of the system, and granular temperature profiles are obtained, in the dilute case, from the short time behavior of the mean squared displacement. At longer times, the mean squared displacement shows a range of behavior which can be explained by the presence of strong gradients in the packing fraction. Convection currents were observed, but were sufficiently small in magnitude to be ignored during the analysis of grain motion. The system was modeled using the Smoluchowski equation, which was solved numerically, and the results compared with the experimentally determined displacement probability density functions. Good agreement between experiment and numerical results was achieved using Brownian motion relationships modified to accommodate differences between granular systems and thermal systems.

Exploring the WEP with a pulsed cold beam of antihydrogen View the table of contents for this issue, or go to the journal homepage for more 2012 Class. Quantum Grav. 29 184009

%'e have used optical microscopy to perform direct measurements of the fractal volume and surface dimensions of sandstone samples on scales from 0.5 -200 pm. The dimensions are evaluated by box-counting techniques on the digitized representation of the microscope pictures. The connection between the fractal dimensions and rock permeability is discussed.The relation between porosity and permeability in rocks is a subject of scientific as well as practical interest.In general, no simple functional relationship has been found between these quantities. This is basically due to the fact that most porous rocks are very complex systems which may consist of a range of irregular grain sizes. Several analytical models have been suggested'to predict rock permeability from basic statistical quantities connected to the rock. However, a general problem of many of these models is that they are based on quantities (e.g. , "mean channel length" ), which are difficult to measure in real rocks.Various studies suggest that the surfaces of rock grains and of whole rock samples are fractal. In particular, Wong et al. have used small-angle neutron scattering (SANS) to show that the microscopic geometry of sandstone surfaces can be characterized by a fractal dimension D, in the range from 2.55 to 2.95. The fractal interpretation of the surface geometry, combined with some other basic rock or fluid properties may turn out to give a fundamental description and understanding of relative permeability of simultaneous two-phase flow in the rock. However, the absolute rock permeability is determined by the structure of the pore space and surface area on "grain-size" scales, which are usually from 1 pm and larger.At present there are relatively few direct measurements of possible fractal scaling of both pore surface and volume of these scales. Katz and Thompson (KT) used scanning electron microscopy and optical data to suggest that these quantities are given by the same fractal dimension for certain sandstones. However, this claim has later been disputed both in connection with the SANS experiment as well as on theoretical grounds. In fact, it may be easily shown that this can only be true in special cases. A simple example showing that this is not true is a threedimensional (3D) model porous medium consisting of closed, uniform Koch curves.The surface dimension here will be D, =2.26, while the pore volume will have D"= 3, the Euclidean dimension.In this paper we report direct measurements of the geometry of sandstone samples from the Oseberg North Sea oil reservoir by analyzing optical micrographs. The samples were polished thin sections (thickness approximately 30 pm) bound to a glass substrate. By using an inverted optical microscope in transmitted-light illumination mode, it was possible to focus on the twodimensional (2D) cuts of the grains. The images were digitized using a video frame grabber with 512&512-pixel resolution and 256 grey levels. Five different microscope magnifications were used setting the relevant scales from 0.5 to 200 p...

A theoretical investigation of the experimentally observed (Stolterfoht N et al 2001 Phys. Rev. Lett. 87 023201) interference effects in the double differential cross sections for ionization of the hydrogen molecule by fast ion impact is reported. The H2/2H cross section ratios as a function of the ejected electron velocity show an oscillating pattern, for which Stolterfoht et al propose a formula C + G sin(k D)/(k D), where k is the electron momentum and D the internuclear separation in H2. Our analysis in its simplest form leads instead to a formula C + G sin(k|| D)/(k|| D) where k|| is the component of k parallel to the projectile velocity. The presented theoretical model thus explains why at 90° the interference pattern will be strongly suppressed. In addition to the simplified analysis a numerical evaluation of a more accurate model is presented, confirming the latter qualitative prediction.

The ionization of H(1s) in superintense, high-frequency, attosecond pulses is studied beyond the dipole approximation. We identify a unique nondipole 3rd lobe in the angular distribution of the ejected electron and show that this lobe has a well-defined classical counterpart. The ionization is likely to occur in the direction opposite to the laser propagation direction, which is fully understood from an analysis of the classical dynamics.

We calculate ionization probabilities for the H 2 + molecule subject to a linearly polarized infrared laser field.The time-dependent Schrödinger equation is solved numerically for all electronic degrees of freedom while the nuclei are fixed at arbitrary separations and orientations with respect to the direction of the field. The ionization signal displays a strong dependence on the degree of alignment between the laser polarization and the internuclear vector. At perpendicularly aligned geometry the signal is suppressed due to a symmetry selection rule that excludes an important intermediate resonance.

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