Effect of desorbed gas on microwave breakdown on vacuum side of dielectric window

The gas desorbed from the dielectric surface has a great influence on the characteristics of microwave breakdown on the vacuum side of the dielectric window. In this paper, the dielectric surface breakdown is described by using the electromagnetic particle-in-cell-Monte Carlo collision(PIC-MCC) model. The process of desorption of gas and its influence on the breakdown characteristics are studied. The simulation results show that, due to the accumulation of desorbed gas, the pressure near the dielectric surface increases in time, and the breakdown mechanism transitions from secondary electron multipactor to collision ionization. More and more electrons generated by collision ionization drift to the dielectric surface, so that the amplitude of self-organized normal electric field increases in time and sometimes points to the dielectric surface. Nevertheless, the number of secondary electrons emitted in each microwave cycle is approximately equal to the number of primary electrons. In the early and middle stages of breakdown, the attenuation of the microwave electric field near the dielectric surface is very small. However, the collision ionization causes a sharp increase in the number density of electrons,and the microwave electric field decays rapidly in the later stage of breakdown. Compared with the electromagnetic PIC-MCC simulation results, the mean energy and number of electrons obtained by the electrostatic PIC-MCC model are overestimated in the later stage of breakdown because it does not take into account the attenuation of microwave electric field. The pressure of the desorbed gas predicted by the electromagnetic PIC-MCC model is close to the measured value,when the number of gas atoms desorbed by an incident electron is taken as 0.4.

Size Effects of Atomically Precise Gold Nanoclusters in Catalysis

The emergence of ligand-protected,atomically precise gold nanoclusters(NCs)in recent years has attracted broad interest in catalysis due to their well-defined atomic structures and intriguing properties.Especially,the precise formulas of NCs provide an opportunity to study the size effects at the atomic level without complications by the polydispersity in conventional nanoparticles that obscures the relationship between the size/structure and properties.Herein,we summarize the catalytic size effects of atomically precise,thioate-protected gold NCs in the range of tens to hundreds of metal atoms.The catalytic reactions include electrochemical catalysis,photocatalysis,and thermocatalysis.With the precise sizes and structures,the fundamentals underlying the size effects are analyzed,such as the surface area,electronic properties,and active sites.In the catalytic reactions,one or more factors may exert catalytic effects simultaneously,hence leading to different catalytic-activity trends with the size change of NCs.The summary of literature work disentangles the underlying fundamental mechanisms and provides insights into the size effects.Future studies will lead to further understanding of the size effects and shed light on the catalytic active sites and ultimately promote catalyst design at the atomic level.

Phylogenetic and genetic characterization of a 2017 clinical isolate of H7N9 virus in Guangzhou,China during the fifth epidemic wave

Pathogenic H7N9 influenza viruses continue to pose a public health concern. The H7N9 virus has caused five outbreak waves of human infections in China since 2013. In the present study, a novel H7N9 strain(A/Guangdong/8 H324/2017) was isolated from a female patient with severe respiratory illness during the fifth wave of the 2017 H7N9 epidemic. Phylogenetic analysis showed that the H7N9 viruses collected during the fifth wave belong to two different lineages: the Pearl River Delta lineage and the Yangtze River Delta lineage. The novel isolate is closely related to the Pearl River Delta H7N9 viruses, which were isolated from patients in Guangdong Province. The novel H7N9 isolate has an insertion of three basic amino acids in the cleavage site of hemagglutinin(HA), which may enhance virulence in poultry. The 2017 isolate also possesses an R292 K substitution in the neuraminidase(NA)protein, which confers oseltamivir resistance. This study highlights the pandemic potential of the novel H7N9 virus in mammals;thus, future characterization and surveillance is warranted.

Single-electron charging and ultrafast dynamics of bimetallic Au_(144−x)Agx(PET)_(60) nanoclusters

Alloying is an important strategy in tailoring the functionality of materials.In metal nanoclusters(NCs),the introduction of a heterometal leads to alloy nanoclusters that often outperform the homometal ones in terms of the physical and chemical properties.In this work,a series of four M_(144)(PET)60 alloy NCs(where,M=Au/Ag,PET=–SCH_(2)CH_(2)Ph)are synthesized and characterized.The silver doping into the homogold template(Au_(144))leads to more prominent optical absorption features in the steady-state spectrum in the visible range.Femtosecond transient absorption spectroscopy reveals the effect of Ag doping on the electronic relaxation dynamics compared to Au_(144)and the pump fluence independent dynamics.Electrochemical results reflect a narrowing of HOMO–LUMO gap(E_(g))induced by Ag doping.A temperature dependence of the single-electron charging is also observed for the series of alloy NCs,in which the E_(g)values of the alloy NCs enlarge as the temperature decreases,which is characteristic of semiconducting behavior.