Small Mean Free Path Research Articles

Density jump for oblique collisionless shocks in pair plasmas: physical solutions

Collisionless shocks are frequently analysed using the magnetohydrodynamics (MHD) formalism, even though MHD assumes a small mean free path. Yet, isotropy of pressure, the fruit of binary collisions and assumed in MHD, may not apply in collisionless shocks. This is especially true within a magnetized plasma, where the field can stabilize an anisotropy. In a previous article (Bret & Narayan, J. Plasma Phys., vol. 88, no. 6, 2022b, p. 905880615), a model was presented capable of dealing with the anisotropies that may arise at the front crossing. It was solved for any orientation of the field with respect to the shock front. Yet, for some values of the upstream parameters, several downstream solutions were found. Here, we complete the work started in Bret & Narayan (J. Plasma Phys., vol. 88, no. 6, 2022b, p. 905880615) by showing how to pick the physical solution out of the ones offered by the algebra. This is achieved by 2 means: (i) selecting the solution that has the downstream field obliquity closest to the upstream one. This criterion is exemplified on the parallel case and backed up by particle-in-cell simulations. (ii) Filtering out solutions which do not satisfy a criteria already invoked to trim multiple solutions in MHD: the evolutionarity criterion, that we assume valid in the collisionless case. The end result is a model in which a given upstream configuration results in a unique, or no downstream configuration (as in MHD). The largest departure from MHD is found for the case of a parallel shock.

Multi-species kinetic-fluid coupling for high-energy density simulations

Many physics problems are subject to a mix of continuum and non-equilibrium flows or conditions where one transition into the other, as a function of time and/or space. Numerically, such flows could be described with a purely kinetic method which, in the limit of small particle mean free paths, reproduces a continuum regime. However, from a computational perspective, such approaches are usually very expensive or simply not feasible, even on modern computational architectures. A solution is the construction of hybrid methods which connect the kinetic and hydrodynamic system of evolution equations. Such coupling usually requires specific boundary conditions between non-equilibrium and continuum regimes which come with their own numerical challenges and can introduce unphysical effects. Here, we present a hybrid model which uses a buffer region to transition from a kinetic into a fluid description following the original work of Degond et al. [1]. In the buffer region, both, the kinetic and hydrodynamic equations are solved simultaneously while being coupled via a so-called transition function. The latter also ensures a smooth conversion from the coupled model to either the kinetic or continuum approach at the interfaces of the buffer region. With that, the method avoids the need to find direct interface boundary conditions and allows one to localize the use of a high dimensional kinetic model only where it is needed. In our work, we extend the original method of Degond to flows with multiple particle species in 3D. We derive the coupled equations for the multispecies Vlasov-Bhatnagar-Gross-Krook (VBGK) model and its limiting Euler or Navier-Stokes hydrodynamic equations. To validate our model numerically, we simulate a Sod shock problem and study the effect of kinetic multi-species mixing in the preheat phase of a high energy-density plasma physics experiment.

Investigation on The Effect of Bismuth Concentration Towards Radiation Shielding Properties in Ag-embedded Borobismuthate Mixed Ionic Electronic Glass System

In this study, 20Li2O–xBi2O3–(79-x)B2O3–1Ag glasses for x=3,5,7,9, and 11 mol % glasses were prepared by using the melt quenching technique to investigate radiation shielding properties of the glasses using Phy-X/PSD simulation program at 15 keV to 15 MeV photon energy range. The radiation shielding parameters were carried out to determine the mass attenuation coefficient (MAC), linear attenuation coefficient (LAC), half value layer (HVL), mean free path (MFP) and effective atomic number (Zeff). The results showed that the Bi2O3 addition has improved overall radiation shielding properties. The MAC of the glass system was increased as Bi2O3 concentration increased. The HVL showed that present glass better than standard commercial concretes as lower HVL value indicates better shielding where 11 mol % required a much smaller thickness than 3 mol %. Therefore, sample with highest bismuth content has the most effective radiation protective property. Smaller MFP is most preferable as it suitable for protection materials; indicated the 11 mol % is best candidate for radiation shielding. Lastly, the higher value in Zeff was contributed by higher atomic number of Bi over B; thus, enhanced protection performance.

The Metal-Oxide Nanoparticle-Aqueous Solution Interface Studied by Liquid-Microjet Photoemission.

ConspectusThe liquid-microjet technique combined with soft X-ray photoelectron spectroscopy (PES) has become an exceptionally powerful experimental tool to investigate the electronic structure of liquid water and nonaqueous solvents and solutes, including nanoparticle (NP) suspensions, since its first implementation at the BESSY II synchrotron radiation facility 20 years ago. This Account focuses on NPs dispersed in water, offering a unique opportunity to access the solid-electrolyte interface for identifying interfacial species by their characteristic photoelectron spectral fingerprints. Generally, the applicability of PES to a solid-water interface is hampered due to the small mean free path of the photoelectrons in solution. Several approaches have been developed for the electrode-water system and will be reviewed briefly. The situation is different for the NP-water system. Our experiments imply that the transition-metal oxide (TMO) NPs used in our studies reside close enough to the solution-vacuum interface that electrons emitted from the NP-solution interface (and from the NP interior) can be detected.We were specifically exploring aqueous-phase TMO NPs that have a high potential for (photo)electrocatalytic applications, e.g., for solar fuel generation. The central question we address here is how H2O molecules interact with the respective TMO NP surface. Liquid-microjet PES experiments, performed from hematite (α-Fe2O3, iron(III) oxide) and anatase (TiO2, titanium(IV) oxide) NPs dispersed in aqueous solutions, exhibit sufficient sensitity to distinguish between free bulk-solution water molecules and those adsorbed at the NP surface. Moreover, hydroxyl species resulting from dissociative water adsorption can be identified in the photoemission spectra. An important aspect is that in the NP(aq) system the TMO surface is in contact with a true extended bulk electrolyte solution rather than with a few monolayers of water, as is the case in experiments using single-crystal samples. This has a decisive effect on the interfacial processes that can occur since NP-water interactions can be uniquely investigated as a function of pH and provides an environment allowing for unhindered proton migration. Our studies confirm that water is dissociatively adsorbed at the hematite surface and molecularly adsorbed at the TiO2 NP surface at low pH. In contrast, at near-basic pH the water interaction is dissociative at the TiO2 NP surface.The liquid-microjet measurements presented here also highlight the multiple aspects of photoemission necessary for a full characterization of TMO nanoparticle surfaces in aqueous environments. For instance, we exploit the ability to increase species-specific electron signals via resonant photoemission, so-called partial electron yield X-ray absorption (PEY-XA) spectra, and from valence photoelectron and resonant Auger-electron spectra. We also address the potential of these resonance processes and the associated ultrafast electronic relaxations for determining charge transfer or electron delocalization times, e.g., from Fe3+ located at the hematite nanoparticle interface into the aqueous-solution environment.

Simulation of Liquid Lithium Divertor Geometry Using SOLPS-ITER

Plasma-facing component (PFC) geometries are evaluated for a Fusion Nuclear Science Facility (FNSF) design to find solutions that are compatible with flowing liquid lithium components while satisfying requirements from the perspective of plasma and neutral particle transport. Flowing liquid metal (LM) divertor systems offer important advantages due to continual regeneration of the surface material, but may require modifications from standard optimized tokamak divertor designs. Scrape Off Layer Plasma Simulator for the International Thermonuclear Experimental Reactor (SOLPS-ITER) simulations are used to compare standard vertical target, open, and balanced baffled geometries. It is found that the open geometry offers little control of neutral particles and has narrow operating windows where the divertor fluxes can be lowered to acceptable levels, while maintaining the required upstream density. The addition of neutral baffles allows greater control over the divertor conditions with less impact on the upstream density. The divertor neutral pressure in the baffled geometry is slightly higher than the open divertor, but not as large with a vertical target. A first-pass coupling of the SOLPS solution with sheath and implantation/erosion models to compute the lithium emission shows that the lithium is largely confined to the near-surface region due to the small ionization mean free path and large main ion flow toward the surface. The lithium source was varied by three orders of magnitude. At the highest rate, when the sourced lithium approaches the level of the main ion recycling flux, a high core lithium concentration (~10%) is observed with only moderate power dissipation to affect the divertor plasma.

MAX-Phase Films Overcome Scaling Limitations to the Resistivity of Metal Thin Films

Metal thin films have been widely used as conductors in semiconductor devices for several decades. However, the resistivity of metal thin films such as Cu and TiN increases substantially (>1000%) as they become thinner (<10 nm) when using high-density integration to improve device performance. In this study, the resistivities of MAX-phase V2AlC films grown on sapphire substrates exhibited a significantly weaker dependence on the film thickness than conventional metal films that resulted in a resistivity increase of only 30%, as the V2AlC film thickness decreased from approximately 45 to 5 nm. The resistivity was almost identical for film thicknesses of 10-50 nm. The small change in the resistivity of V2AlC films with decreasing film thickness originated from the highly ordered crystalline quality and a small electron mean free path (11-13.6 nm). Thus, MAX-phase thin films have great potential for advanced metal technology applications to overcome the current scaling limitations of semiconductor devices.

Low lattice thermal conductivity and its role in the remarkable thermoelectric performance of newly predicted SiS2 and SiSe2 monolayers

For high-efficiency thermoelectric power conversion, not only improvement of existing materials properties but also prediction and synthesis of new thermoelectric materials are needed. Here, we have carried out a systematic investigation on the thermoelectric performance of newly predicted two dimensional (2D) semiconducting SiS2 and SiSe2 monolayers using density functional theory (DFT) and solving the Boltzmann transport equations (BTEs) for electrons and phonons. Our computed value of lattice thermal conductivity (kph) in SiSe2 monolayer is ultralow, which results in a high thermoelectric figure of merit (zT) value of 0.86 (0.83) for p-type (n-type) at 900 K in SiSe2 monolayer. While in SiS2 monolayer, zT value are 0.77 (p-type) and 0.71 (n-type) at 900 K. The values of kph are attributed to low group velocity, strong anharmonicity and phonon–phonon coupling of acoustic and low-frequency optical branches, leading to larger scattering, smaller mean free path, and shorter lifetime of phonons. It is also found that p-type doping is more effective than n-type doping to get optimal power factor (PF) and zT. Our findings suggest that newly predicted semiconducting SiSe2 and SiS2 monolayers can be very promising thermoelectric materials for the fabrication of high-efficiency thermoelectric power generators to convert waste heat into electricity.

Fabrication of new non-hazardous tungsten carbide epoxy resin bricks for low energy gamma shielding in nuclear medicine

Introduction. The main aim of this study was to fabricate a lead-free tungsten carbide epoxy resin brick that has similar shielding properties to lead brick for low energy gamma shielding in nuclear medicine. The attenuation properties of bricks were characterized by using gamma transmission principle in Single-Photon Emission Computed Tomography (SPECT) scanner. Materials and methods. In this study, various percentage of tungsten carbide epoxy resin were fabricated into brick with thickness of 0.5 cm, 1.0 cm, 1.5 cm and 2.0 cm. Tungsten carbide epoxy resin and lead bricks were irradiated with gamma rays from 99mTc to evaluate the radiation attenuation properties. A detector was used to evaluate the gamma shielding performance at 140 keV. The activity of the radioactive source was measured and recorded. The radiation attenuation of tungsten carbide epoxy resin bricks was compared with lead brick of same size and thickness. The gamma transmission was evaluated by using SPECT. Results. Results showed that tungsten carbide epoxy resin brick attenuates more radiation than a lead brick of the same thickness. This study also found that tungsten carbide epoxy resin brick is an effective radiation shielding material compared to lead. The best tungsten carbide and epoxy resin combination was found with a mixture of 90%:10% by weight, respectively. The study showed that both half-value layer and mean free path are higher at thicker samples for all materials at 140 keV. This study found that tungsten carbide and tungsten carbide epoxy resin bricks have small half-value layer and mean free path compared to lead brick. The values were 0.07 cm and 0.06 cm for lead and tungsten carbide, respectively. Conclusion. This study showed that attenuation coefficient measurement can be performed using gamma transmission principle in SPECT. Tungsten carbide epoxy resin shows high potential to replace lead as radiation shielding material.

Inhomogeneous superconductivity in LuxZr1−xB12 dodecaborides with dynamic charge stripes

We have studied the normal and superconductive state characteristics (resistivity, Hall coefficient, heat capacity and magnetization) of model strongly correlated electronic systems LuxZr1-xB12 with cooperative Jahn-Teller instability of the boron rigid cage and with dynamic charge stripes. It was found that these metals are s-wave dirty limit superconductors with a small mean free path of charge carriers l = 5 - 140 A and with a Cooper pair size changing non-monotonously in the range 450 - 4000 A. The parent ZrB12 and LuB12 borides are type-I superconductors, and Zr to Lu substitution induces a type-I - type-II phase transition providing a variation of the Ginzburg-Landau-Maki parameter in the limits 0.65 < kappa1,2 < 6. We argue in favor of the two-band scenario of superconductivity in LuxZr1-xB12 with gap values Delta_1 ~ 14 K and Delta_2 ~ 6 - 8 K, with pairing corresponding to strong coupling limit (lambda_e-ph ~ 1) in the upper band, and to weak coupling (lambda_e-ph ~ 0.1 - 0.4) in the lower one. A pseudogap Delta_ps-gap ~ 60 - 110 K is observed in LuxZr1-xB12 above Tc. We discuss also the possibility of anisotropic single-band superconductivity with stripe-induced both pair-breaking and anisotropy, and analyze the origin of unique enhanced surface superconductivity detected in these model compounds.

Theoretical and numerical study on breakdown mechanism of nitrogen spark switch

Compared with the two-electrode gas spark switch, the three-electrode gas spark switch has high controllability, low working voltage and small jitter, so the three-electrode gas spark switch is widely used in pulse power technology. The discharge of gas spark switch is high pressure gas discharge, which is characterized by high electron collision frequency (10&lt;sup&gt;12&lt;/sup&gt; Hz), small mean free path (10&lt;sup&gt;–6&lt;/sup&gt; m), short breakdown time (10&lt;sup&gt;–9&lt;/sup&gt; s), and complex physical process (including the secondary electron emission, the generation of seed electrons, the space charge effect and various collision processes between electrons and nitrogen molecules, etc). At present, it is difficult to quantitatively describe the breakdown process of the three-electrode gas switch, and the detailed theoretical research is lacking. Therefore, the breakdown mechanism of atmospheric pressure nitrogen spark switch, including two-electrode and three-electrode, is studied theoretically and numerically in this paper. The purpose of this study is to compare the simulation results of the two different gas spark switches, and obtain the characteristics of stream breakdown in different gas spark switches. Firstly, the numerical simulation and theoretical analysis of two-electrode gas spark switch are carried out. According to theoretical and numerical calculation, it can be found that for the plate-plate two-electrode switch, the stream breakdown cannot be generated under low voltage (less than 6.3 kV), while under high voltage (more than 6.3 kV), first the anode-directed streamer is formed, and then the cathode-directed streamer is created. In addition, the simulation results show that the plasma generated by the trigger can effectively reduce the breakdown voltage. Finally, the three-electrode gas spark switch is studied theoretically and numerically. It can be seen that in the breakdown process of the three-electrode gas spark switch, the breakdown first occurs between the trigger and the insulator, and then this plasma channel expands to the anode and cathode, finally forming the arc channel between the anode and the cathode. Under the calculation conditions in this paper, if the cathode-trigger and the anode-trigger are required to break down simultaneously, the applied voltage between the cathode-trigger should be greater than 1.18 kV, while the applied voltage between the anode-trigger should be greater than 3 KV. When the field emission of the trigger is considered, the breakdown threshold can be significantly reduced.

Studies on DC transport and terahertz conductivity of granular molybdenum thin films for microwave radiation detector applications

The morphological, transport, and terahertz optical properties of DC magnetron-sputtered granular molybdenum thin films with nano-grains embedded in an amorphous molybdenum/molybdenum oxide matrix have been studied in their normal and superconducting states. The superconducting transition temperatures of these films are much higher than that of bulk molybdenum. The optical properties of these thin films have been studied using terahertz time-domain spectroscopy. Their properties have been compared with those of the existing materials used for the development of radiation detectors. The films' resistivity lies in the &amp;gt;100 μΩ cm range, ideal for making highly sensitive radiation detectors. Hall measurements indicate holes as the dominant carriers with very small mean free path and mobility. In the normal state, the films are disordered bad metals. However, they have a large superfluid density and stiffness in their superconducting state. The properties of the films in the normal and superconducting states are promising for their use in cryogenic radiation detectors for microwave, terahertz, and far-infrared frequency ranges.

Reduction of dislocation, mean free path, and migration barriers using high entropy alloy: insights from the atomistic study of irradiation damage of CoNiCrFeMn

High entropy alloy has attracted extensive attention in nuclear energy due to outstanding irradiation resistance, partially due to sluggish diffusion. The mechanism from a defect-generation perspective, however, has received much less attention. In this paper, the formation of dislocation loops, and migration of interstitials and vacancies in CoNiCrFeMn high entropy alloy under consecutive bombardments were studied by molecular dynamics simulations. Compared to pure Ni, less defects were produced in the CoNiCrFeMn. Only a few small dislocation loops were observed, and the length of dislocation was small. The dislocation loops in Ni matrix were obviously longer and so was the length of dislocation. The interstitial clusters had much smaller mean free path during migration in the CoNiCrFeMn. The mean free path of 10-interstitial clusters in CoNiCrFeMn was reduced over 40 times compared to that in pure Ni. In addition, CoNiCrFeMn had a smaller difference of migration energy between interstitial and vacancy, which increased the opportunity of recombination of defects, therefore, led to less defects and much fewer dislocation loops. Our results provide insights into the mechanism of irradiation resistance in the high entropy alloy and could be useful in material design for irradiation tolerance and accident tolerance materials in nuclear energy.

A meshfree method for the BGK model for rarefied gas dynamics

In this paper, we have applied semi-Lagrangian schemes with meshfree interpolation, based on a moving least squares method, to solve the BGK model for rarefied gas dynamics. Sod’s shock tube problems are presented for a large range of mean free paths in one-dimensional physical space and three-dimensional velocity space. In order to validate the solutions obtained from the meshfree method, we have used the piecewise linear spline interpolation. Furthermore, we have compared the solutions of the BGK model with the solutions obtained from direct simulation Monte Carlo method. In the case of a very small mean free path, the numerical solutions are compared with the exact solutions of the compressible Euler equations. Overall, we found that the meshfree interpolation gives better approximation than the piecewise linear spline interpolation.

Thermal conductivity of amorphous silica nanoparticles

Understanding the heat transfer through confined systems at nanometer scale is of both scientific interest and technical importance. We report in this paper, a case study on the thermal conductivity of amorphous silica nanoparticles. It is found that the solid-state thermal conductivity of amorphous silica nanoparticles is size-dependent and can be quantitatively calculated by using a phenomenological theory. The observed size effect on thermal conductivity of silica nanoparticles is however not significant, mostly due to the very small phonon mean free path of amorphous silica, l0 = 0.59 nm. It is also found that the phonon-defect scattering at the nanoparticle surfaces has a predominate effect on the involved heat transfer process. The calculated thermal conductivity of 5-nm silica nanoparticles is about 0.539 W/(mK).

Electrochemical Nucleation and Growth of Cobalt in Presence of Dioxime Additives

The concerns about copper (Cu) interconnects have been increasing because the Cu line resistance will not linearly scale with the dimension in 7 nm technology and beyond1. Instead, Cu resistivity in small features increases significantly because of the electron scattering at grain boundaries and interfaces2. To solve this problem, alternative metals such as cobalt (Co) is considered as a promising material because of its high melting point and small mean free path. High melting temperature lows the risk of electromigration3 and small mean free path decreases the contribution from electron scattering at boundaries and interfaces. Recently, defect-free filling of Co in nanoscale features has been achieved using proprietary additives4-6. The additives play an important role in such processes creating differential plating rates between the feature bottom and field regions. Different functional groups in organic additives were found to influence electrodeposition kinetics. For instance, additives with a conjugated pair of oxime groups displayed strong suppression effects on Co deposition and a suppression breakdown occurs upon the reduction of adsorbed Co-dioxime chelates7. This talk presents a study, where we used several dioxime additives like dimethylglyoxime (DMG), cyclohexane dioxime (CHD), and furil dioxime (FD) to investigate their impacts on Co nucleation process. Figures 1 (a) and (b) show the chronoamperometries during the Co nucleation in presence of DMG and CHD, respectively. Two current peaks were observed, which are ascribed to a unique two-step nucleation behavior involving free cobalt cation and complexed cobalt chelates. Figure 1(c) shows the cyclic voltammetry of Co deposition with FD, where a concentration dependent suppression effect was observed. Figure 1(d) shows the normalized nucleation current transients numerically deconvoluted from the total currents. A progressive nucleation was observed. The results will be discussed in detail in conjunction with microscopic observations. REFERENCES Steinhögl, W.; Schindler, G.; Steinlesberger, G.; Engelhardt, M., Size-dependent resistivity of metallic wires in the mesoscopic range. Physical Review B 2002, 66 (7), 075414.Wu, W.; Brongersma, S.; Van Hove, M.; Maex, K., Influence of surface and grain-boundary scattering on the resistivity of copper in reduced dimensions. Applied physics letters 2004, 84 (15), 2838-2840.Adelmann, C.; Wen, L. G.; Peter, A. P.; Siew, Y. K.; Croes, K.; Swerts, J.; Popovici, M.; Sankaran, K.; Pourtois, G.; Van Elshocht, S. In Alternative metals for advanced interconnects, Interconnect Technology Conference/Advanced Metallization Conference (IITC/AMC), 2014 IEEE International, IEEE: 2014; pp 173-176.Rigsby, M. A.; Brogan, L. J.; Doubina, N. V.; Liu, Y.; Opocensky, E. C.; Spurlin, T. A.; Zhou, J.; Reid, J. D., Superconformal Cobalt Fill through the Use of Sacrificial Oxidants. ECS Transactions 2017, 80 (10), 767-776.Wafula, F.; Wu, J.; Branagan, S.; Suzuki, H.; Gracias, A.; van Eisden, J. In Electrolytic Cobalt Fill of Sub-5 nm Node Interconnect Features, 2018 IEEE International Interconnect Technology Conference (IITC), IEEE: 2018; pp 123-125.Kelly, J.; Chen, J.-C.; Huang, H.; Hu, C.; Liniger, E.; Patlolla, R.; Peethala, B.; Adusumilli, P.; Shobha, H.; Nogami, T. In Experimental study of nanoscale Co damascene BEOL interconnect structures, Interconnect Technology Conference/Advanced Metallization Conference (IITC/AMC), 2016 IEEE International, IEEE: 2016; pp 40-42.Lyons, T.; Huang, Q., Effects of Cyclohexane-Monoxime and Dioxime on the Electrodeposition of Cobalt. Electrochimica Acta 2017, 245, 309-317. Fig. 1 (a) Co nucleation process with the addition of 100 ppm DMG (b) Co nucleation process with the addition of 10 ppm CHD (c) The CV results with the addition of different concentration of FD (d) Normalized current transients with 10 ppm FD. Figure 1

Diffusion limit for kinetic Fokker-Planck equation with heavy tails equilibria: The critical case

This paper is devoted to the diffusion and anomalous diffusion limit of the Fokker-Planck equation of plasma physics, in which the equilibrium function decays towards zero at infinity like a negative power function. We use probabilistic methods to recover and extend the results obtained in [22]. We prove in particular, in the critical case where the classical diffusion coefficient is no more defined, that the small mean free path limit gives rise to a diffusion equation, with an anomalous time scaling and with a variance breaking.

Wave nature of conduction electrons in amorphous Co90Sc10 and Fe90Sc10 alloys

The resistivity of amorphous Fe90Sc10 and Co90Sc10 alloys can be described well in terms of a simple model based on the wave character of electrons and their associated tunnelling over the temperature ranges ~1.9 K to 135 K and ~1.9 K to 12 K respectively. The extended range of agreement between experiment and theory for amorphous Fe90Sc10 is linked with its relatively small mean free path of = 0.32 nm, thus allowing electron waves to tunnel between clusters. On the other hand the restricted region of tunnelling of electron waves for amorphous Co90Sc10 alloys is linked with its relatively large mean free path of = 0.48 nm which restricts the ability for tunnelling between clusters while enabling electron waves to tunnel between different regions with a cluster.

Diffusion Limit of 1-D Small Mean Free Path of Radiative Transfer Equations in Bounded Domain

Diffusion Limit of 1-D Small Mean Free Path of Radiative Transfer Equations in Bounded Domain

Ambient Pressure Photoelectron Spectroscopy: Opportunities in Catalysis from Solids to Liquids and Introducing Time Resolution

AbstractPhotoelectron spectroscopy is an excellent technique to explore chemically complex systems in catalysis. However, due to a small mean free path of photoelectrons in gas, liquid, and solid media, the study of the gas–solid, liquid–gas, solid–liquid interfaces, as well as liquid homogeneous systems are a serious challenge. With differentially pumped analyzers this limitation ceases to exist and no longer restricts photoelectron spectroscopy only to ultra‐high vacuum conditions. Presently, photoemission studies at tens of mbar of pressure are possible. To reach atmospheric pressure and even higher, membrane‐covered closed cells have been developed. Graphene membranes are impermeable to molecules and almost transparent to photoelectrons. They are used as a pressure barrier between the enclosed cell at atmospheric pressure and the electron analyzer at vacuum while allowing transmission of photoelectrons. By accessing to atmospheric pressure range, with this kind of cell, photoemission studies will become a versatile in situ and operando tool for catalysis. Studies involving liquids in static conditions are an important aspect in this direction, which can be extended to homogeneous catalytic systems. Liquids are presently accessible using micro‐jets. Another aspect which is of paramount interest to investigate catalysts is time resolution. By improving the time resolution of photoemission measurements to the sub‐second regime it is possible to follow the kinetic changes that are crucial of a catalytic reaction, the changes that occur during catalyst pretreatment and activation, and notably to differentiate active species from spectator ones, which may be the dominating species in a classical experiment.

Efficient Stochastic Asymptotic-Preserving Implicit-Explicit Methods for Transport Equations with Diffusive Scalings and Random Inputs

For linear transport and radiative heat transfer equations with random inputs, we develop new generalized polynomial chaos based asymptotic-preserving stochastic Galerkin schemes that allow efficient computation for the problems that contain both uncertainties and multiple scales. Compared with previous methods for these problems, our new method uses the implicit-explicit time discretization to gain higher order accuracy, and by using a modified diffusion operator based penalty method, a more relaxed stability condition---a hyperbolic, rather than parabolic, CFL stability condition---is achieved in the case of a small mean free path in the diffusive regime. The stochastic asymptotic-preserving property of these methods will be shown asymptotically and demonstrated numerically, along with a computational cost comparison with previous methods.