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Notes: We have measured the initial silane and polysilane product yields from disilane decomposition in rf and dc discharges, at 25 and 250 °C and 20 Pa (0.15 Torr) pressure as typically used for a-Si:H film deposition. From analyses of these yields we conclude that the initial Si2H6 fragmentation pattern is SiH3+SiH2+H (91±9%) and H3SiSiH+2H (9±9%), that the primary product of the H+Si2H6 reaction is SiH4+SiH3, and that SiH3 is the dominant radical causing film growth. We have measured a radical-surface reaction probability of 0.34±0.03, very similar to that observed for SiH3 in SiH4 discharges. We report a spatial distribution of emission indicative of a γ-regime discharge. From deposition on glass fibers strung between the electrodes, we find that highly strained a-Si:H film is produced everywhere except on or near the electrodes, suggesting that energetic ion impact is necessary to yield useful films in disilane discharges.

Notes: Time-dependent production of higher-silane gases and a-Si:H film are measured relative to decomposed silane in rf and dc, hot and cold cathode, static-gas discharges. From the absence of higher-silane production in very low silane partial-pressure discharges, it is inferred that most higher silanes are produced by gas-phase SiH2-initiated reactions. The higher silanes are thus tracers of SiH2, while the film production traces the fraction of H, SiH, and SiH3 in the initial decomposition. From the measured stable product yields, we deduce that SiH4→SiH2+2H is the dominant electron-collisional dissociation channel.

Notes: A detailed kinetic study of silane-germane glow discharges is presented. Stable gas decomposition and production rates have been measured using mass spectrometry and a kinetic model for the plasma chemistry is developed. It is found that germane depletes about four times faster than silane, nearly independently of their relative fractions. Germane is found to be much more reactive than silane with silylene, germalyn, and atomic hydrogen, and the silylene-germane reaction leads in large part to film rather than stable gases. The spatial characteristics of the discharge are studied using optical emission and fiber deposition profiles. From these it is deduced that the present, low-power discharge operates in a "hybrid'' α-γ regime, and that ion effects are important near the electrodes.

Notes: The stable gas products of germane dissociation and subsequent radical reactions have been measured in pure germane glow discharges characteristics of the initial germane fragmentation are inferred from these data. The spatial distribution of discharge optical emission, and of film deposition on glass fibers, have also been measured. Finally, the surface reaction probability β of depositing neutral radicals has been measured to be 0.61±0.09 on the grounded electrode. Major differences between germane and silane discharges occur in all these observables. Possible explanations of these differences are given, but much less chemical data exists for germane, thereby precluding definitive judgments. A probable cause of the normally much poorer semiconductor quality of a-Ge:H films, compared to a-Si:H, is suggested. This is based on the thermodynamics of the H2 release reaction at the growing surface.

Notes: A comparative study of the various model assumptions in Monte Carlo simulations of low-pressure sputter-atom transport is presented. The few-collision conditions and actual "racetrack'' magnetron geometry, typical of low-pressure magnetron sputtering, are emphasized. For the gas phase scattering problem, a comparison is made between hard sphere, Lennard–Jones 6-12, and Abrahamson Thomas–Fermi–Dirac [Phys. Rev. 178, 76 (1969)] interatomic potentials. The hard sphere potential results in both a significantly lower energy distribution and a more diffuse angular distribution for the depositing flux, as compared with the more realistic "softer'' potentials. Because energy-dependent cross sections are obtained when using the 6-12 and Abrahamson potentials, an "energy filtering'' effect is observed, i.e., high-energy particles arrive at the substrate preferentially to those at low energy. It is concluded that the hard sphere model will lead to serious errors in both the energy and angular distributions of the arrival flux, and that the 6-12 and Abrahamson potentials yield results that are similar to each other. For the nascent sputter distribution, fractal TRIM (transport of ions in matter) simulations are compared to the analytic Thompson distribution. While both distributions give nearly identical results for the angle-integrated fluxes, the fractal TRIM distribution shows a strong angular dependence of the energy distribution. The implications of this effect for finite geometry systems are discussed.

Notes: A superfluid-helium-tight optical fiber feedthrough has been developed. Heat shrinkable Kynar provides a removable seal around a plastic optical fiber. The seal preserves the continuity of the fiber and is reliable after repeated thermal cycles. © 1998 American Institute of Physics.

Notes: It is highly unlikely that chronic infection with H. pylori could occur in the absence of adhesin–host cell interactions. Also, there is no evidence that any of the serious outcomes of H. pylori infection such as gastric and duodenal ulcers, gastric cancer or mucosa-associated lymphoid tissue (MALT) lymphoma could occur without prior colonization of the gastric epithelium mediated by H. pylori adhesins. H. pylori is highly adaptable, as evidenced by the fact that it can occupy a single host for decades. An important facet of this adaptability is its ability to physically interact with various types of host cells and also with host mucins and extracellular matrix proteins using a number of different adhesins displaying a variety of unique receptor specificities. Thus it is highly unlikely that any one particular H. pylori adhesin will ever be proven responsible for a particular outcome such as duodenal ulcer, MALT lymphoma, or adenocarcinoma. Also, while the search for additional H. pylori adhesins should and certainly will continue, we suggest that the scope of this effort should be expanded to include investigations into the patterns of expression and interaction between individual outer membrane proteins. Which of the numerous H. pylori outer membrane proteins (OMPs) actually function as adhesins (i.e., have receptor-binding sites) and which OMPs are simply necessary for optimal display of the adhesive OMPs? There are many other important questions about H. pylori adhesins waiting to be answered. For example, which adhesins are responsible for loose adherence to host cells and which adhesins are responsible for intimate, or membrane-to-membrane, adherence, and do these adhesins normally work in concert or in a sequential fashion? Also, is a specific type of adhesin necessary for type IV protein translocation into host cells and, if so, is adhesin expression coregulated with the effector protein export?

Notes: Background. Our goal was to test the idea that Helicobacter pylori genotypes vary from one population to another.Methods. Analysis of Sau3A and Hinf I restriction fragment–length polymorphism (RFLP) in a 375-bp polymerase chain reaction amplicon of hpaA was used to compare 31 H. pylori isolates from a relatively small and genetically homogeneous population (Goteborg, Sweden) with those of large, genetically heterogeneous populations located in two different countries (50 isolates from Houston, TX, and 69 isolates from Minas Gerais, a state in the southeastern region of Brazil).Results. Five different Sau3A and three different Hinf I restriction patterns were found; different combinations of these comprise 10 different RFLP types, I through X. The RFLP types found in the United States and Brazil collections were very similar, except for two Brazil isolates belonging to type VIII and five Brazil isolates belonging to type X, neither type found in the United States. The overall profile of H. pylori isolates from Sweden was remarkably different, with 18 of 31 (58%) having a new Sau3A restriction pattern, termed gS; 10 of these 18 isolates had Hinf I restriction pattern E (RFLP type VIII), and 8 had Hinf I restriction pattern F (RFLP type IX). No isolates from Sweden belonged to RFLP type III or type X.Conclusions. RFLP typing of a 375-bp polymerase chain reaction–amplified DNA fragment of H. pylori hpaA revealed that H. pylori genotypes can and do vary from one population to another. We conclude that the unique RFLP profile shown by the group of H. pylori isolates from Goteborg is the result of a cohort effect in this relatively small, stable, genetically homogeneous population. Also, the overall similarity between RFLP profiles of the H. pylori isolates from Texas and Minas Gerais coincides with the fact that although geographically distanced, these populations are similar in being large, dynamic, and genetically heterogeneous.

Notes: CagA is transported into host target cells and subsequently phosphorylated. Clearly this is a mechanism by which Helicobacter pylori could take control of one or more host cell signal transduction pathways. Presumably the end result of this interaction favors survival of H. pylori, irrespective of eventual damage to the host cell. CagA is noted for its amino acid (AA) sequence diversity, both within and outside the variable region of the molecule. The primary purpose of this review is to examine how variation in the type and number of CagA phosphorylation sites might determine the outcome of infection by different strains of H. pylori. The answer to this question could help to explain the widely disparate results obtained when H. pylori CagA status has been compared to type and severity of disease outcome in different populations, that is in different countries. Analysis of all available CagA sequences revealed that CagA contains both tyrosine phosphorylation motifs (TPMs) and cyclic-AMP-dependent phosphorylation motifs (CPMs). There are two potential CPMs near the N-terminus of CagA and at least two in the repeat region; these are not all equally well conserved. We also defined a 48-residue AA sequence, which includes the N-terminal TPM at tyrosine (Y)-122, which distinguishes between Eastern (Hong Kong-Taiwan-Japan-Thailand) H. pylori isolates and those from the West (Europe-Africa-the Americas-Australia). All 28 of the Eastern type CagA proteins have a functional N-terminal TPM whereas 11 of 47 (23.4%) of the Western type contain an inactive motif, with threonine (T) replacing the critical aspartic acid (D) residue. Only 13 of 24 (54%) known CagA sequences have an active TPM in the repeat region and only one has two TPMs in this region. The potential TPM near the C-terminus of CagA is not likely to be important since only 3 of 24 (12.5%) sequences were found to be intact. Protein database searches revealed that the AA sequence immediately following the TPM at Y-122 in CagA is homologous with a pair of PDZ domains which are common in signal transducing proteins, particularly tyrosine phosphatases. This provides a theoretical link between CagA and many of the observed responses of host cells to H. pylori. In summary, not all CagA proteins are equal in their potential for initiating host cell responses via signal transduction pathways. The degree of functional diversity of this protein depends upon which phosphorylation motifs are critical to the biological activity of CagA.