Investigation of precision grinding process for production of silicon diaphragms

The application of precision grinding for the formation of a silicon diaphragm is investigated. The test structures involved 2-6 mm diam diaphragms with thicknesses in the range of 25-150 //m. When grinding is performed without supporting the diaphragm, bending occurs due to nonuniform removal of the silicon material over the diaphragm region. The magnitude of bending depends on the µNal thickness of the diaphragm. The results demonstrate that the use of a porous silicon support can significantly reduce the amount of bending, by a factor of up to 300 in the case of 50 m thick diaphragms. The use of silicon on insulator (SOI) technology can also suppress or eliminate bending although this may be a less economical process. Stress measurements in the diaphragms were performed using x-ray and Raman spectroscopies. The results show stress of the order of 1 X107-! X108 Pa in unsupported and supported by porous silicon diaphragms while SOI technology provides stress-free diaphragms. Results obtained from finite element method analysis to determine deterioration in the performance of a 6 mm diaphragm due to bending are presented. These results show a 10% reduction in performance for a 75 µm thick diaphragm with bending amplitude of 30 fim, but negligible reduction if the bending is reduced to

Theory of the near K-edge structure in electron energy loss spectroscopy

Arguments are given that lead to a formalism for calculating near K-edge structure in electron energy loss spectroscopy (EELS). This is essentially a one electron picture, while many body effects may be introduced at different levels, such as the local density approximation to density functional theory or the GW approximation to the electron self-energy. Calculations are made within the all electron LMTO scheme in crystals with complex atomic and electronic structures, and these are compared with experiment. (c) 2004 Elsevier B.V. All rights reserved.

On the solvation of L-aspartic acid

We use molecular statics and dynamics to study the stability of L-aspartic acid both in vacuo and solvated by polar and non-polar molecules using density functional theory in the generalized gradient approximation. We find that structures stable in vacuo are unstable in aqueous solution and vice versa. From our simulations we are able to come to some conclusions about the mechanism of stabilisation of zwitterions by polar protic solvents, water and methanol.

Importance of the H- channel in Ps-H scattering

It is shown that virtual H- formation has a profound effect upon low-energy Ps(1s)-H(1s) scattering, yet H- formation only accounts for about 10% of the total cross section just above threshold. Infinite series of Rydberg resonances converging on to the H- formation threshold are seen.

Second-order distorted wave calculation for electron impact ionization of hydrogen

The triple differential cross sections for ionization of atomic hydrogen by electron impact are analysed in the case of coplanar, asymmetric geometry within the framework of second- order distorted wave theory. Detailed calculations are performed without making any approximations (other than numerical) in the evaluation of the second-order amplitude. The present results are compared with experimental measurements and other theoretical calculations for incident energies of 250, 150 and 54.4 eV. It is found that the second-order calculations represent a marked improvement over the results obtained from first-order theories for impact energies of 150 eV and higher. The close agreement between the present second-order plane wave calculation and those of Byron et al calculated using the closure approximation at an incident energy of 250 eV implies that the closure approximation is valid for this energy. The large difference between the present second-order distorted wave calculations and experiment at an incident energy of 54.4 eV suggests that higher order effects are important for incident energies less than 100 eV.

Fully differential cross sections for transfer ionization - a sensitive probe of high level correlation effects in atoms

The transfer ionization process offers a unique opportunity to study radial and angular electron correlations in the helium atom. We report calculations for the multiple differential cross sections of the transfer ionization process p + He --> H + He++ + e(-). The results of these calculations demonstrate the strong sensitivity of the fully differential cross sections to fine details of electron correlation in the target atom. Specifically, angular electron correlation in the ground state of helium results in a broad peak in the electron emission spectra in the backward direction, relative to the incoming beam. Our model explains some of the key effects observed in measurements of multiple differential cross sections using the COLTRIMS technique.

On the observation of the fine structure effect in non- relativistic (e, 2e) processes

The spin asymmetry arising in an (e,2e) process using spin- polarized incoming electrons with non-relativistic energies is shown to be dominated by the fine structure effect if a suitable kinematical regime is chosen. Calculations in the distorted wave Born approximation (DWBA) for both the triple differential cross-section and the spin asymmetry are presented for the inner shell ionization of argon. This process would provide an accessible target for existing experimental set-ups.

Spin asymmetries in spatially resolved processes of ionization of heavy atoms by relativistic electrons

The triple-differential cross section for ionization of a heavy atom is shown to depend on the spin of the incident electron even if this is polarized entirely parallel or antiparallel to its direction of propagation, the atom is unpolarized, and the spins of the ejected electrons are not resolved. Quantitative predictions for the spin asymmetry are presented in a relativistic distorted-wave Born approximation. Simple physical models are introduced to understand both these results and further symmetry properties involving the reversal of a spatial momentum component also.

The electron impact ionization of magnesium (2p)

A systematic study of the triple differential cross section for the electron impact ionization of magnesium is presented. Complete sets of theoretical results using both the first Born and the distorted wave Bom approximation are given for a range of asymmetric kinematical regimes. How the physical significance of the different approximations enter the character of the cross sections will be explicitly demonstrated. Comparison is made with experiments of the Maryland group and suggestions are made for new experiments.

On the electron impact ionization of multi-charged ions: a theoretical investigation of relativistic effects

The electron impact ionization of highly charged ions is investigated. Using a relativistic distorted wave Born approximation, we explore the possible effects that should be observable in a high-energy electron impact ionization (e, 2e) coincidence experiment involving multi-charged ions. We present calculations of triple-differential cross sections. We will focus on relativistic and distortion effects and consider geometries where these may be easily observed.

Positron scattering by the negative hydrogen ion

Results are presented for e(+) scattering by H- in the impact energy range 0less than or equal toE(0)less than or equal to10 eV. These include integrated cross sections for Ps formation in the 1s, 2s, and 2p states, as well as in an aggregate of states with ngreater than or equal to3, and for direct ionization. Differential cross sections for Ps formation in the 1s, 2s, and 2p states are also exhibited. The calculations are based on a coupled pseudostate approach employing 19 Ps pseudostates centered on the e(+). It is found that Ps formation in the 2p state dominates that in the 1s or 2s states below 8 eV, that formation in states with ngreater than or equal to3 exceeds the sum of the n=1 and n=2 cross sections above 2.5 eV, and that direct ionization outstrips total Ps formation above 6.3 eV. The threshold law (E-0-->0) for exothermic Ps formation, which includes the cases Ps(1s), Ps(2s), and Ps(2p), is shown to be 1/E-0.

Revealing the effect of angular correlation in the ground-state He wavefunction: a coincidence study of the transfer ionization process

A joint theoretical-experimental study of the transfer ionization process p + He -> H-0 + He2+ + e(-) is presented. For the first time all particles in the final state have been detected in triple coincidence. This fully differential measurement is in good agreement with a theoretical model where the target is described by a wavefunction containing both radial and angular correlation terms.

Fragmentation of positronium

The impulse approximation is used to calculate cross sections for fragmentation of Ps(1s) in collision with He, Ne, Ar, Kr, and Xe. Triple, double, single, and total cross sections are evaluated. Reasonably good agreement is found with the measurements of Armitage [Phys. Rev. Lett. 89, 173402 (2002)] on Ps(1s)+He(1(1)S) scattering. These absolute measurements comprise the total Ps ionization cross section and the cross section differential with respect to the longitudinal energy of the ejected positron. Characteristics of free electron and free positron scattering are explored in the double and triple differential cross sections for Ps(1s)+Xe scattering.

Total cross sections for transfer ionization in fast ion-helium collisions

The effects of electron correlation and second-order terms on theoretical total cross sections of transfer ionization in collisions of the helium atom with fast H+, He2+ and Li3+ ions are studied and reported. The total cross sections are calculated using highly correlated wavefunctions with expansion of the transition amplitude in the Born series through the second order. The results of these calculations are in sensible agreement with experimental data.