Tuning out vibrational levels in molecular electron energy-loss spectra

The phenomenon whereby features associated with certain vibrational levels in molecular states of mixed electronic character disappear under specific scattering conditions in electron energy-loss spectra is investigated. In particular, using a combination of experimental measurements and coupled-channel calculations, anomalous vibrational intensities in the mixed valence-Rydberg 1Π u←X1Σg+ transition of N 2 are explained. A single parameter, i.e., the ratio of the generalized electronic transition moments to the diabatic valence and Rydberg components of the mixed states, dependent on the experimental scattering conditions, is found to be essentially capable of describing all observed relative vibrational intensities, including the near disappearance of the b1Π u(v=5) feature for momentum-transfer-squared values K2 ≈ 0.3 a.u. This result highlights the interesting possibility of experimental control of molecular quantum-interference effects in electron energy-loss spectra, something that is not possible in optical spectra

Absolute differential cross sections for the electron impact excitation of the 12S → 22S + 22P levels of atomic hydrogen at 50 and 100 eV

Absolute experimental differential cross sections for the electron impact excitation of the 12S → 22S + 22P levels of H at 50 and 100 eV incident energy are obtained using an application of the method of mixtures and available accurate He (n = 2) experimental electron impact excitation differential cross sections. The determination of the number density composition of the mixed beam is made from energy loss measurements of the mixed beam at 200 eV and 25° scattering angle using accurate H and He theoretical differential cross sections obtained from the distorted-wave Born approximation [D. H. Madison (private communication)] and convergent close coupling [I. Bray and A. Stelbovics, Phys. Rev. A 46, 6995 (1992); D. V. Fursa and I. Bray, Phys. Rev. A 52, 1279 (1995)]

Polarization correlations for electron-impact excitation of the resonant transitions of Ne and Ar at low incident energies

The electron-polarized-photon coincidence method is used to determine linear and circular polarization correlations in vacuum ultraviolet (VUV) for the differential electron-impact excitation of neon and argon resonance transitions at impact energies of 25 and 30 eV at small scattering angles up to 40. The circular polarization correlation is found to be positive in the case of Ne at 25 eV and supports the prediction of the present B-spline R-matrix theory concerning the violation of a long-established propensity rule regarding angular momentum transfer in electron-impact excitation of S→P transitions. Comparisons with the results from the present relativistic distorted-wave approximation and an earlier semirelativistic distorted-wave Born model are also made. For the case of Ar, at 25 and 30 eV, the circular polarization measurements remain in agreement with theory, but provide limited evidence as to whether or not the circular polarization at small scattering angles is also positive. For the linear polarizations, much better agreement with theory is obtained than in earlier measurements carried out by S. H. Zheng and K. Becker

Clean ultrahigh vacuum system with single-structure diffusion pumps

A practical example of the use of single-structure diffusion pumping groups in an experiment requiring hydrocarbon-free, ultrahigh vacuum is presented. With the use of proper vacuum procedures and careful foreline design and operation, but with no exotic hardware or liquid nitrogen, we have demonstrated that such a pump could provide vacuum that was clean, i.e., which had no residual pump-oil contamination to a partial pressure below 10-8 mbar. Their use is thus compatible, at least in peripheral vacuum chambers, with standard UHV-surface science processes. We discuss our vacuum system in detail, and present a review of the literature on the production of clean vacua with diffusion pumps

Electron-impact ionization of atomic hydrogen at incident electron energies of 15.6, 17.6, 25, and 40 eV

Absolute doubly differential cross sections for the electron-impact ionization of atomic hydrogen have been measured from near threshold to intermediate energies. The measurements are calibrated to the well-established, accurate differential cross section for electron-impact excitation of the atomic hydrogen transition H(12S⃗22S+22P). In these experiments background secondary electrons are suppressed by moving the atomic hydrogen target source to and from the collision region. Measurements cover the incident electron energy range of 14.6–40 eV, for scattering angles of 10°–120° and are found to be in very good agreement with the results of the most advanced theoretical models—the convergent close-coupling model and the exterior complex scaling model

STAT4-associated natural killer cell tolerance following liver transplantation

Objective: Natural killer (NK) cells are important mediators of liver inflammation in chronic liver disease. The aim of this study was to investigate why liver transplants (LTs) are not rejected by NK cells in the absence of human leukocyte antigen (HLA) matching, and to identify a tolerogenic NK cell phenotype.Design: phenotypic and functional analyses on NK cells from 54 LT recipients were performed, and comparisons made with healthy controls. Further investigation was performed using gene expression analysis and donor:recipient HLA typing.Results: NK cells from non-HCV LT recipients were hypofunctional, with reduced expression of NKp46 (p&lt;0.05) and NKp30 (p&lt;0.001), reduced cytotoxicity (p&lt;0.001) and interferon (IFN)-? secretion (p&lt;0.025). There was no segregation of this effect with HLA-C, and these functional changes were not observed in individuals with HCV. Microarray and RT-qPCR analysis demonstrated downregulation of STAT4 in NK cells from LT recipients (p&lt;0.0001). Changes in the expression levels of the transcription factors Helios (p=0.06) and Hobit (p=0.07), which control NKp46 and IFN? expression, respectively, were also detected. Hypofunctionality of NK cells was associated with impaired STAT4 phosphorylation and downregulation of the STAT4 target microRNA-155. Conversely in HCV-LT NK cell tolerance was reversed, consistent with the more aggressive outcome of LT for HCV.Conclusions: LT is associated with transcriptional and functional changes in NK cells, resulting in reduced activation. NK cell tolerance occurs upstream of major histocompatibility complex (MHC) class I mediated education, and is associated with deficient STAT4 phosphorylation. STAT4 therefore represents a potential therapeutic target to induce NK cell tolerance in liver disease.<br/

Human CD49a+ lung NK cell cytotoxicity in response to Influenza A Virus

Influenza A virus (IAV) is a major global public health burden due to its routine evasion of immunisation strategies. Natural Killer (NK) cells are innate cytotoxic cells with important antiviral activity in the human body, yet the function of these cells in the control of IAV infection is unclear. The aim of this study was to determine the role of lung NK cell cytotoxic responses to IAV. Human lung explants were infected ex vivo with IAV and lung NK cell activation was analysed by flow cytometry. Cytotoxic responses of NK cell subsets against IAV-infected macrophages were measured by flow cytometry and ELISA. Despite reports of hypofunctionality in the pulmonary environment, human lung-associated NK cells responded rapidly to ex vivo IAV infection, with upregulation of surface CD107a 24 h post-infection. The lung NK cell phenotype is similar in maturity and differentiation to NK cells of the peripheral blood but a unique CD56brightCD49a+CD103+CD69+ NK cell population was identified in the lung, indicating NK cell residency within this organ. In response to ex vivo IAV infection a greater proportion of resident CD56brightCD49a+ NK cells expressed surface CD107a compared to CD56brightCD49a- NK cells, suggesting a hyperfunctional NK cell population may be present within human lung tissue and could be the result of innate immunological training. Furthermore NK cells provided significant anti-viral, cytotoxic activity following contact with influenza infected cells, including the production and release of IFN-γ and Granzyme-B resulting in macrophage cell death. These results suggest that a resident, memory NK cell population are present in the human lung and may provide early and important control of viral infection. A greater understanding of this resident mucosal population may provide further insight into the role of these cells in controlling viral infection and generating appropriate adaptive immunity to IAV

Coherence Parameter Measurements for Electrons Scattering off Heavy Noble Gas Targets

Electron impact excitation of the resonance levels of Ne, Ar, Kr and Xe has been studied for electron scattering angles up to 50' and impact energies between 30 and 80 eV. The P1 and P4 Stokes parameters have been measured in each case so that the influence of spin in the excitation process could be studied through evaluation of rho 00, the relative spin-flip cross section. After careful account was taken of various depolarizing effects due particularly to the finite volume of the interaction region and, in the cases of Kr and Xe, to nuclear spin, very good agreement has been found with theoretical predictions thus resolving a previously reported discrepancy. No evidence has been found for spin-flip under the experimental parameters used in this study, even for the heaviest target studied

Electron impact excitation of X 1Σg+(v′′=0)X\,{^1\Sigma_g^+}(v''=0) to the a′′ 1Σg+a''\,{^1\Sigma_g^+}, b 1Πub\,{^1\Pi_u}, c3 1Πuc_3\,{^1\Pi_u}, o3 1Πuo_3\,{^1\Pi_u}, b′ 1Σu+b'\,{^1\Sigma_u^+}, c4′ 1Σu+c_4'\,{^1\Sigma_u^+}, G 3ΠuG\,{^3\Pi_u}, and F 3ΠuF\,{^3\Pi_u} states of molecular nitrogen

Measurements of differential cross sections (DCSs) for electron-impact excitation of the a″ Σg+1, b Πu1, c3 Πu1, o3 Πu1, b′ Σu+1, c4′ Σu+1, G Πu3, and F Πu3 states in N2 from the X Σg+1 (v″ =0) ground level are presented. The DCSs were o