High Vacuum In Order Research Articles

GRASP: The Future?

The theoretical foundations of relativistic electronic structure theory within quantum electrodynamics (QED) and the computational basis of the atomic structure code GRASP are briefly surveyed. A class of four-component basis set is introduced, which we denote the CKG-spinor set, that enforces the charge-conjugation symmetry of the Dirac equation. This formalism has been implemented using the Gaussian function technology that is routinely used in computational quantum chemistry, including in our relativistic molecular structure code, BERTHA. We demonstrate that, unlike the kinetically matched two-component basis sets that are widely employed in relativistic quantum chemistry, the CKG-spinor basis is able to reproduce the well-known eigenvalue spectrum of point-nuclear hydrogenic systems to high accuracy for all atomic symmetry types. Calculations are reported of third- and higher-order vacuum polarization effects in hydrogenic systems using the CKG-spinor set. These results reveal that Gaussian basis set expansions are able to calculate accurately these QED effects without recourse to the apparatus of regularization and in agreement with existing methods. An approach to the evaluation of the electron self-energy is outlined that extends our earlier work using partial-wave expansions in QED. Combined with the treatment of vacuum polarization effects described in this article, these basis set methods suggest the development of a comprehensive ab initio approach to the calculation of radiative and QED effects in future versions of the GRASP code.

End-to-end image analysis pipeline for liquid-phase electron microscopy.

SummaryLiquid phase transmission electron microscopy allows the imaging of materials in liquid environments. The sample is encapsulated within electron‐beam transparent windows and hence protected by the ultrahigh vacuum necessary within the electron gun. Such an approach allows to study biological and soft materials in their natural environment and offers the possibility of accessing their dynamic nature. Yet, the electron beam scattering from the windows and solvent increases the image noise and blur. Herein, we propose a pipeline to both de‐noise and sharpen images obtained by liquid transmission electron microscopy. We develop the workflow in a way that it does not require any human interference, nor introduce artefacts, but actually unveils features of the imaged samples covered by the noise and the blur.Lay DescriptionTransmission Electron Microscopy TEM is one of the most powerful techniques for structural determination at the nanoscale, with the ability to image matter down to the atomic level. TEM is only possible by keeping the electron beam under high vacuum in order to avoid undesired scattering events in the beam path. High vacuum means that the TEM samples must conventionally be in solid‐state. Thus, samples in liquid form or containing liquids, like water, need special preparation techniques which tend to alter the structure and chemical nature of the sample. Such alterations are particularly critical for biological and soft organic materials where the structures are controlled by the presence of water and/or other liquids. The development of new cameras, materials and sample holders have made possible for TEM to be performed on liquid samples. Liquid Phase Transmission Electron Microscopy (LTEM) offers the possibility to investigate nanoscopic structures in liquid state and monitor dynamic processes. However important limitations come from the liquid nature of samples in the imaging process such as the low contrast afforded by organic and biological materials and additional noise and blur introduced by the liquid sample and its thickness. Existing image analysis algorithms for TEM result inadequate for LTEM. The end‐to‐end image analysis method herein has the ability to recover the original images together with their sharpness, without introducing any artefacts. The proposed algorithms offer the great advantage of unveiling image details which are not usually seen during imaging, thus allowing a better understanding of the nature, structure and ultimately the function of the investigated structures. The fully automatised analysis method allows to efficiently process dozens of images in few hours, improving dramatically the performance of LTEM imaging

Relativistic corrections to vacuum polarization contributions in muonic hydrogen

The method proposed by Kinoshita and Nio to compute higher order vacuum polarization contributions to the Coulomb potential in muonic hydrogen is generalized to obtain relativistic corrections to their results.

Discharge Characteristics of Plasma Display Panels with SrCaO Protective Layer Manufactured Using “All-in-Vacuum” Process

Plasma display panels (PDPs) with a SrCaO protective layer of high secondary electron emission coefficient were manufactured using the “all-in-vacuum” process, whereby the protective layer deposition and panel sealing were performed continuously in high vacuum in order to keep the surface of the protective layer as clean as possible. The PDP with the SrCaO protective layer manufactured using the “all-in-vacuum” process (“all-in-vacuum” SrCaO-PDP) for Ne­20 vol% Xe exhibited a luminance 2.1 times larger, a luminous efficacy 1.7 times larger, and the same discharge time lag, compared with the PDP with the MgO protective layer manufactured using the conventional process (conventional MgO-PDP) for Ne­4 vol% Xe, which had almost the same discharge voltage. [doi:10.2320/matertrans.M2011181]

Correlation between muonic levels and nuclear structure in muonic atoms

A method that deals with the nucleons and the muon unitedly is employed to investigate the muonic lead, with which the correlation between the muon and nucleus can be studied distinctly. A “kink” appears in the muonic isotope shift at a neutron magic number where the nuclear shell structure plays a key role. This behavior may have very important implications for the experimentally probing the shell structure of the nuclei far away from the β-stable line. We investigate the variations of the nuclear structure due to the interaction with the muon in the muonic atom and find that the nuclear structure remains basically unaltered. Therefore, the muon is a clean and reliable probe for studying the nuclear structure. In addition, a correction that the muon-induced slight change in the proton density distribution in turn shifts the muonic levels is investigated. This correction to muonic level is as important as the Lamb shift and high order vacuum polarization correction, but is larger than anomalous magnetic moment and electron shielding correction.

Investigation of the dwell period’s influence on the fatigue crack growth of a titanium alloy

The dwell effect, which is known to induce a reduction in the fatigue life of titanium alloys at room temperature, is related to early crack initiation. The present results support faster crack growth rates. The governing mechanisms are identified by mean of scanning electron microfractographic observations. The potential role of the atmosphere is examined through comparative testing performed in air and in high vacuum in order to distinguish the specific contributions of cold creep and environment assistance.

Structural behaviour of aeronautical tungsten carbide/carbon‐coated titanium ball screws under space thermal‐vacuum conditions

ABSTRACTIn view of the pressing need for improvements in space mechanisms and tribology research work, a thermal high‐vacuum mechanism test facility has been constructed at Rome ‘La Sapienza’ University. The purpose of this facility is to provide fundamental data on the performance of mechanical components and materials in high vacuum in order to test the validity of space mechanism designs. The facility also makes it possible to examine the behaviour of both mechanisms and materials under combined environmental factors, such as ultraviolet radiation, atomic oxygen and temperature. This paper reports on the behaviour of titanium recirculating ball screws provided by Umbra Cuscinetti SpA (Foligno, Italy) under thermal‐vacuum conditions. It is to be noted that these ball screws—well known in the aeronautical field—may be particularly attractive as a new generation of space mechanisms. A solid multilayer film of tungsten carbide/carbon (WC/C) was deposited on screw surfaces as a lubricant and was then compared with the case of no lubrication, which presently appears promising for many mechanisms. In order to compare experimental results, traditional stainless steel mechanisms were also tested and analysed. The requisites imposed by the space environment, as well as by contact mechanical stresses, have given useful information on a preliminary geometrical configuration and choice of materials, both of which are presented in this paper. Thermal‐vacuum effects on overall ball screw efficiency and on ball screw coating friction and wear are also reported and commented on herein as well as numerical contact simulations of crack mechanisms.

Evaporation of Interstellar Organic Materials in the Solar Nebula

A series of evaporation experiments was performed on interstellar organic analogs under high vacuum in order to study the distribution of interstellar organic materials in the solar nebula. Temperature dependences of the weight loss of the samples and the elemental composition of the evaporation residue were examined. The chemical composition of evaporated gases during heating was also investigated. It was found on the basis of the experimental results and the astrophysical model for the solar nebula that there would be molecular cloud and diffuse cloud organic materials at heliocentric distances larger than 2.7 and 2.1 AU, respectively, in the late evolution stage of the disk. Very reductive gas was evaporated at around 2.4 AU during the evaporation of diffuse cloud organic materials, and this causes very reduced condition in the solar nebula. We discuss a redox state of chondrites based on our experimental result and conclude that ordinary chondrites would form at heliocentric distances of less than 2.2 AU, enstatite chondrites at around 2.4 AU, and carbonaceous chondrites at heliocentric distances larger than 3.0 AU. From the comparison of carbon and nitrogen contents in evaporation residues of interstellar organic analogs with those in carbonaceous chondrites, it is concluded that the C and N contents in carbonaceous chondrites cannot be explained by the heating of interstellar organic dusts before accretion to parent bodies.

Self-organized InGaAs quantum dots grown on GaAs (3 1 1)B substrate studied by conductive atomic force microscope technique

We have used conductive atomic force microscope (AFM) in a high vacuum in order to investigate the electronic properties of self-organized In x Ga 1– x As quantum dots (QDs) on GaAs (3 1 1)B substrates. The QDs were fabricated by atomic H-assisted molecular beam epitaxy, and Si AFM tips coated with Au, which warrants electrical conductivity were used to measure both the topographic and current images of QDs surface simultaneously. The conductive AFM measurements were performed in vacuum at room temperature and at lowered temperatures. With this technique, the current–voltage ( I– V) characteristics of QDs of varying sizes, and of any other arbitrary positions on the QDs surface can also be studied by using the same conductive AFM tip. It was found that the center of a QD is more conductive than its periphery, and the surface in between the QDs is highly resistive. The differences in the conductance are thought to be due to the local modification of surface bending associated with the surface states. Further, we have shown that the conductance becomes spatially uniform at all points over the packed and ordered QDs at low temperatures, which could be explained by lateral coupling of these strained QDs.

Fatigue behavior of a high strength steel in vacuum, in air and in 3.5% NaCl solution under cathodic protection

Abstract Fatigue resistance of the high strength steel SE702 (A517 type) in air and in 3.5% NaCl solution under cathodic protection is compared with the fatigue resistance in high vacuum in order to identify the effects of the environment on the fatigue damage processes. Fatigue lifetimes, obtained in air and in vacuum, reveal a strong influence of the gaseous environment and a significant effect of the test frequency in air. Experiments performed in aqueous environment under cathodic protection exhibit an additional reduction of the fatigue resistance relative to results obtained in air. Interrupted fatigue tests show that both ambient air and NaCl solution reduce initiation and propagation stages compared with vacuum. The gaseous environment tends to accelerate transgranular cracking whereas the NaCl environment strongly favors ‘intergranular’ propagation, enhancing the crack growth rate relative to air. In some conditions, the air appears to be more detrimental than the NaCl solution with cathodic protection on the initiation stage. This work confirms the key role of hydrogen in NaCl solution and the main role of water vapor in air. But it also shows that the role of oxygen can not be neglected in air and especially for the crack initiation stage.

Simple way to generate high order vacuum graphs

We describe an efficient practical procedure for enumerating and regrouping vacuum Feynman graphs of a given order in perturbation theory. The method is based on a combination of Schwinger-Dyson equations and the two-particle-irreducible ("skeleton") expansion. The regrouping leads to skeletons containing only free propagators, together with "ring diagrams" containing all the self-energy insertions. As a consequence, relatively few diagrams need to be drawn and integrations carried out at any single stage of the computation and, in low dimensions, overlapping ultraviolet/infrared subdivergences can be cleanly isolated. As an illustration we enumerate the graphs contributing to the 4-loop free energy in QCD, explicitly in a continuum and more compactly in a lattice regularization.

Compressing and focusing a short laser pulse by a thin plasma lens.

We consider the possibility of using a thin plasma slab as an optical element to both focus and compress an intense laser pulse. By thin we mean that the focal length is larger than the lens thickness. We derive analytic formulas for the spot size and pulse length evolution of a short laser pulse propagating through a thin uniform plasma lens. The formulas are compared to simulation results from two types of particle-in-cell code. The simulations give a greater final spot size and a shorter focal length than the analytic formulas. The difference arises from spherical aberrations in the lens which lead to the generation of higher-order vacuum Gaussian modes. The simulations also show that Raman side scattering can develop. A thin lens experiment could provide unequivocal evidence of relativistic self-focusing.

CEMS studies of thermally treated Fe/TiN coatings on Si(1,0,0)

Thin layers of 25 nm 57 Fe have been evaporated on Si-wafers in high vacuum in order to enhance the sensitivity of Conversion Electron Mossbauer Spectroscopy (CEMS). On the 57 Fe layer, 25 nm of TiN has been deposited using plasma activated reactive sputtering (Sputron). CEMS enables nondestructive characterisation and determination of physico-chemical properties of iron-containing phases in depth up to 300 nm. Results of Mossbauer spectroscopy have been applied to determine the mechanism for interfacial bonding and the influence of thermal treatment of the samples in UHV up to 873 K. Temperature dependence of the development of different iron-nitrides at the interface Fe/TiN and diffusion of nitrogen into the Fe-layer have been investigated.

Radiative corrections in strong Coulomb fields

Contributions to the electron Lamb shift in highly charged ions are summarized. Recent theoretical developments as well as current experimental results are considered. Special emphasis is laid on higher-order vacuum polarization corrections as well as on the nuclear size effect on the electron self energy.

Vacuum polarization and the electric charge of the positron.

We show that higher-order vacuum polarization would contribute a measurable net charge to atoms, if the charges of electrons and positrons do not balance precisely. We obtain the limit \ensuremath{\Vert}${\mathit{Q}}_{\mathit{e}}$+${\mathit{Q}}_{\mathit{e}\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}$\ensuremath{\Vert}${10}^{\mathrm{\ensuremath{-}}18}$e for the sum of the charges of electron and positron. This also constitutes a new bound on certain violations of PCT invariance.

Higher-order vacuum polarization charge densities for spherical symmetric external potentials

The vacuum polarization contribution of order α(Zα) n withn≧3 on electron binding energies has been calculated following the prescription of Wichmann and Kroll. The numerical techniques for the evaluation of the vacuum polarization charge density are discussed. Results are presented for atoms withZ=73, 82 and 170, respectively.

The microchannel plate as detector for VUV photons and metastable atoms in the 1-0.1 Pa pressure region

The problem of detecting extremely low count rates of VUV photons or metastable atoms, at the maximum possible target gas density and opening angle, can conveniently be solved by using a double channel plate detector. In an electron-impact experiment, the first of two channel plates has been operated as a separative wall between the target gas region (0.1 Pa) and a high vacuum in order to achieve an opening angle of 0.8 sr.

Vacuum Polarization in Heavy-Ion Collisions

The results of a study on vacuum polarization, orders $\ensuremath{\alpha}{(Z\ensuremath{\alpha})}^{n}$, $n>~3$, for large-$Z$ systems encountered in heavy-ion collisions are presented. It is shown that the higher-order vacuum polarization cannot prevent the $1{S}_{\frac{1}{2}}$ state from reaching the lower continuum, $E=\ensuremath{-}{m}_{e}{c}^{2}$, for some critical charge ${Z}_{\mathrm{cr}}\ensuremath{\sim}170$. In addition, the stability and localization of a heliumlike system for $Z>{Z}_{\mathrm{cr}}$ is demonstrated.

Superheavy elements and nonlinear electrodynamics

Maxwell’s equations, which underlie electrodynamics, are linear equations. Nonlinear effects, such as photon-photon scattering, are known to arise in quantum electrodynamics, and one might expect them to become important in the case of strong external fields. We investigate the consequences of a class of nonlinear Lagrangians, which includes that of Born and Infeld and whose common property is that they lead to upper limits for the electric-field strength (somewhat analogous to the upper limit for the velocity of a particle in special relativity). These nonlinear Lagrangians also lead to a finite electromagnetic selfenergy for the electron, unlike the case of Maxwellian electrodynamics. The importance of nonlinear effects of course depends upon the size of the upper limit to the electric-field strength. If this upper limit is determined by requiring that the mass of the electron is of an entirely electromagnetic origin, nonlinear effects become very important in determining the eigenvalues of electrons bound to superheavy nuclei. For example, theIs energy eigenvalue is raised by 270 keV forZ= 164. The Lagrangians considered here do not lead to an absolute gap between bound states and the states of the lower continuum; the Iδ energy eigenvalue becomes equal to —0.511 MeV, where the lower continuum begins, forZ=215. An analogy between nonlinear electrodynamics and higherorder vacuum polarization corrections is studied.

Higher-Order Vacuum Polarization Corrections in Muonic Atoms

Higher-Order Vacuum Polarization Corrections in Muonic Atoms