State Number Densities Research Articles

Exploration of applying Cesium–Xenon system as a sensitive broadband frequency up-conversion photodetecting medium

Alkali metal vapor was applied as sensitive frequency-conversion photodetectors, however, their application was limited by their natural narrow (∼100 MHz) sensing frequency range. Here, Cs–Xe excimer is used to break this limitation. Demonstration of Cs–Xe frequency up-conversion photodetector is given. Using excimer transition, 550–552 nm light is converted to 541 nm fluorescence, this conversion wavelength band is much larger than a typical atom energy level transition. The frequency up-conversion background is mainly energy pooling fluorescence, which varies with Cs 6P3/2 state number density on roughly a cubic function. Atom collision generated Cs 12DJ, 9FJ levels are measured and discussed. By switching excimer transitions selected, e.g. using Cs nPJ (n = 7, 8, …) related upward excimer transitions, it is possible to detect light with other wavelength from infrared to terahertz band. The concept in this work may get application in photon detection with high sensitivity.

Three-Dimensional Radiation Analysis of the Hypersonic Jules Verne Reentry Spectra

The automatic transfer vehicle (ATV) Jules Verne, developed by the European Space Agency, played a crucial role in resupply missions to the International Space Station from 2008 to 2015. Following its resupply missions, Jules Verne was programmed to execute a planned destructive reentry into Earth’s atmosphere. To better understand the spectra generated by the Jules Verne, three-dimensional hypersonic flowfield and radiation analyses are performed at the 75 km trajectory point of the ATV. Results are compared to measurements obtained at 78 and 73 km by a miniature Echelle spectrograph operated from an airborne platform. Incorporating three-dimensional effects into the radiation analysis results in an overall reduction of the total radiation spectra and a decrease in atomic and molecular features, leading to a closer alignment between the observed and simulated values. Comparable effects are observed by making minor adjustments to the internal mode temperatures and modifying the model for the population of excited state number densities.

Collisional-radiative modeling of shock-heated nitrogen mixtures

A three-temperature collisional-radiative model for shock-heated nitrogen–argon mixtures is developed to facilitate the study of nonequilibrium electronic excitation and ionization behind strong shock waves. Model predictions accurately reproduce measurements of N2 dissociation for mixtures of 2%–10% N2 in argon, with some discrepancies observed for 20% N2 mixtures. Potential causes of the discrepancies are discussed. Net dissociation in mixtures containing 20% N2 is significantly impacted by the dissociation of N2(A), the first excited electronic state of N2, indicating that molecular electronic excitation can affect net dissociation in shock-heated nitrogen flows. The collisional-radiative model successfully predicts the three-stage behavior and induction time observed in concentration measurements of atomic nitrogen in its fourth excited state, the 3s4P level, behind reflected shocks. Mechanisms for the observed behavior are discussed, which deviate from those inferred using a simpler kinetic model. Excited state number density predictions are strongly influenced by the modeling of radiation self-absorption and the inclusion of the measured non-ideal pressure rise. At higher N2 concentrations, the measured data indicate increased efficiency of atomic nitrogen electronic excitation in collisions with N as compared to collisions with N2 and Ar. A global sensitivity analysis of the excited state predictions is then performed, identifying the processes in the kinetic model that most sensitively influence the predicted excited state time history and further clarifying the dominant mechanisms affecting the experimental observables.

Impact of nanoparticle agglomeration on thermal conductivity of molten salt based nanofluids: Insights from molecular dynamics and lattice Boltzmann methods

The nanoparticle aggregation has significant influence on heat transfer properties. The contradictions between different researches exist and the underlying mechanisms of heat conduction remain unclear. To address these issues, the thermal conductivity of solar salt-SiO2 nanofluids in the non-aggregated and aggregated states was investigated, based on the combination of the advantages of molecular dynamics (MD) in revealing physical insights and lattice Boltzmann method (LBM) in simulating properties for complex structures. LBM models were simplified on the basic of MD results. Different heat transfer physics in nanofluids at nanoscale and mesoscale were compared. The results show that the thermal conductivity of aggregated nanofluid is higher than the non-aggregated nanofluid. By analyzing the diffusion coefficient, number density and vibrational density of states, we found that the previous hypotheses (such as Brownian motion, interfacial layer and interfacial thermal resistance) cannot explain the enhanced thermal conductivity induced by the nanoparticle agglomeration. The heat transfer mechanisms of aggregated nanofluid were further analyzed from new perspectives, i.e., the contributions of different material components and fluctuation modes to heat flux. It is found that the nanoparticle aggregation leads to the nanoparticle contribution increase, indicating the formation of low thermal resistance channels within the aggregates. The heat conduction contributed by collision decreases in the aggregated state, while that contributed by potential energy increases. Moreover, the thermal conductivity of aggregated nanofluid is negatively correlated with temperature, aggregate size and fractal dimension of agglomerates, and positively correlated with mass fraction of nanoparticles, degree of agglomeration, and backbone length of each aggregate.

Multi-skyrmion states in the Skyrme model with a false vacuum potential* *Y. L. M. was supported in part by the National Science Foundation of China (11875147, 12147103).

We study multi-skyrmion states using the Skyrme model with a false vacuum potential up to baryon number using the product ansatz. It is found that both the false and true vacuum potentials can result in a cluster structure for the multi-skyrmion states. We also analyze the effect of explicit chiral breaking on the masses and contour surfaces of the baryon number density of the multi-skyrmion states.

Metavalent bonding impacts charge carrier transport across grain boundaries

Understanding the mechanisms underpinning the charge carrier scattering at grain boundaries is crucial to design thermoelectrics and other electronic materials. Yet, this is a very challenging task due to the complex characteristics of grain boundaries and the resulting difficulties in correlating grain boundary structures to local properties. Recent advances in characterizing charge transport across grain boundaries are reviewed, demonstrating how the microstructure, composition, chemical bonding and electrical properties of the same individual grain boundary can be correlated. A much higher potential barrier height is observed in high-angle grain boundaries. This can be ascribed to the larger number density of deep trapping states caused by the local collapse of metavalent bonding. A novel approach to study the influence of the local chemical bonding mechanism around defects on the resulting local properties is thus developed. The results provide insights into the tailoring of electronic properties of metavalently bonded compounds by engineering the characteristics of grain boundaries.

A numerical procedure for understanding the self-absorption effects in laser induced breakdown spectroscopy.

Optical emission spectroscopic techniques, such as laser-induced breakdown spectroscopy (LIBS), require an optimal state of plasma for accurate quantitative elemental analysis. Three fundamental assumptions must be satisfied in order for analytical results to be accurate: local thermodynamic equilibrium (LTE), optically thin plasma, and stoichiometric ablation. But real-life plasma often fails to satisfy these conditions, especially the optical thinness of plasma, resulting in the reabsorption of emitted radiation called self-absorption. To study the self-absorption effect, we simulated optically thick emission spectrum at typical laser-produced plasma conditions. The simulation of the spectrum involves four stages, including the estimation of the ratio of the number density of various ionisation states in the plasma using the Saha-Boltzmann equation, the peak intensity of a spectral line using the Boltzmann equation, the full-width half maxima of each spectral line using the Stark broadening method, and the generation of full spectra by providing a Lorentzian profile for each spectral line. Then self-absorption is applied to the simulated spectrum. We investigated the dependence of the self-absorption coefficient on the plasma temperature, optical path length, and element concentration in the sample. Self-absorption decreases with increased plasma temperature, whereas it increases with increasing optical path length and analyte concentration. We also investigated the role of self-absorption in quantitative analysis by calibration-free LIBS with and without resonance lines of the binary alloy (Mg 50% & Ca 50%). We observed a drastic reduction in error from 27% to 2% in the composition estimation when excluding resonance lines.

Temporal studies of titanium ionised density fraction in reactive HiPIMS with nitrogen admixture

Temporal evolutions of the ground state number densities of titanium atoms and ions and the ionised density fraction during pulse on-time are experimentally investigated for the high-power impulse magnetron sputtering process in an Ar and Ar/N2 atmosphere. For the study, two distinct pulses with the same pulse length at a constant average power but different pulse off-times were selected. In both conditions, four representative points within the hysteresis curve were chosen, and the time evolutions of sputtered species densities were investigated in both the target and the substrate regions. At the pulse beginning, a high density of residual particles sputtered during the previous pulse is present in both studied regions. In the target region, in conditions of longer pulse off-time and thus higher peak discharge current, 95% of sputtered particles are ionised in metallic, transition and compound regimes. With shorter pulse off-time conditions, and thus a lower peak discharge current, the ionised density fraction is the lowest in the metallic regime; in the transition and compound regimes, it exceeds 80%. In the substrate region, the temporal evolution of sputtered species densities differs from that obtained in the target region. After the pulse ignition, titanium atoms and ions remaining near the substrate from the previous pulse are pushed away. In the metallic regime, the sputtered atoms refill the substrate region already during the pulse on-time in both the low- and high-current conditions. In the high-current conditions, the titanium ions arrive at the substrate together with the sputtered atoms; however, in the low-current conditions, the titanium ion density decreases during the pulse on-time, and the titanium ions arrive later during the pulse off-time. In the transition and compound regimes, both densities steadily decrease during the pulse on-time and the titanium atoms and ions arrive at the substrate during the pulse off-time.

Testing mean-field theory for jamming of non-spherical particles: contact number, gap distribution, and vibrational density of states.

We perform numerical simulations of the jamming transition of non-spherical particles in two dimensions. In particular, we systematically investigate how the physical quantities at the jamming transition point behave when the shapes of the particle deviate slightly from the perfect disks. For efficient numerical simulation, we first derive an analytical expression of the gap function, using the perturbation theory around the reference disks. Starting from disks, we observe the effects of the deformation of the shapes of particles by the n-th-order term of the Fourier series [Formula: see text]. We show that the several physical quantities, such as the number of contacts, gap distribution, and characteristic frequencies of the vibrational density of states, show the power-law behaviors with respect to the linear deviation from the reference disks. The power-law behaviors do not depend on n and are fully consistent with the mean-field theory of the jamming of non-spherical particles. This result suggests that the mean-field theory holds very generally for nearly spherical particles.

An improved collision - radiation model of the OH spectrum in the ultraviolet band

In order to calculate the spectral radiation characteristics of OH(A2∑) molecules more accurately, an improved collision-radiation (C-R) model is proposed to determine the excited states OH(A) number density. The six-components air model (CO, CO2, H2, H2O, N2, and O2) instead of the two-components air model (N2 and O2) are used as other plume products in the excitation/de-excitation radiation reaction, and the rate of the four reactions are modified to improve the accuracy and applicability of the number density. Effects of temperature and other plume products on the number density of excited states are analyzed in the experiment, and the spectral radiation characteristics of OH(A) can be obtained by using this improved C-R model. Results of these calculations are then compared with other series of experimental data of exhaust plume and shock layer, the results show that both the original model and the improved model can be used to calculate the radiation characteristics in shock layer, however, due to the great error, the original model not apply to calculation of the OH(A) number density in exhaust plume, and the result of the improved model is in good agreement with the Atlas experimental data. As the vibrational-rotational level distribution number density has a close relationship with radiative spectral characteristics, a more focused OH(A) particle number density model will be more helpful to the hypersonic target detection and recognition.

Comparative Spectral Analysis of the Superluminous Supernova 2019neq

Abstract We present a spectroscopic analysis of the recently discovered fast-evolving Type I superluminous supernova (SLSN-I) SN 2019neq (at redshift z = 0.1059). We compare it to the well-studied slowly evolving SLSN-I SN 2010kd (z = 0.101). Our main goal is to search for spectroscopic differences between the two groups of SLSNe-I. Differences in the spectra may reveal different ejecta compositions and explosion mechanisms. Our investigation concentrates on optical spectra observed with the 10 m Hobby–Eberly Telescope Low Resolution Spectrograph-2 at McDonald Observatory during the photospheric phase. We apply the SYN++ code to model the spectra of SN 2019neq taken at −4 days, +5 days, and +29 days from maximum light. We examine the chemical evolution and ejecta composition of the SLSN by identifying the elements and ionization states in its spectra. We find that a spectral model consisting of O iii, Co iii, and Si iv gives a SYN++ fit that is comparable to the typical SLSN-I spectral model consisting of O ii, and conclude that the true identification of those lines, at least in the case of SN 2019neq, is ambiguous. Based on modeling the entire optical spectrum, we classify SN 2019neq as a fast-evolving SLSN-I having a photospheric velocity gradient of km s−1 day−1, which is among the highest velocity gradients observed for an SLSN-I. Inferring the velocity gradient from the proposed Fe ii λ5169 feature alone would result in km s−1 day−1, which is still within the observed range of fast-evolving SLSNe-I. In addition, we derive the number density of relevant ionization states for a variety of identified elements at the epoch of the three observations. Finally, we give constraints on the lower limit of the ejecta mass and find that both SLSNe have an ejecta mass at least one order of magnitude higher than normal SNe Ia, while the fast-evolving SN 2019neq has an ejecta mass a factor of two lower than the slowly evolving SN 2010kd. These mass estimates suggest the existence of a possible correlation between the evolution timescale and the ejected mass of SLSNe-I.

Ionisation fractions of sputtered titanium species at target and substrate region in HiPIMS

In this paper, the ground state number densities of titanium atom and ion, ionised density fraction and ionised flux fraction were determined in a high power impulse magnetron sputtering process. Three regions of interest within the deposition chamber were studied; the magnetised plasma region, middle region, and substrate region. Pulse length and applied power were kept constant and working pressure and duty cycle were varied. The effective branching fractions method was utilised to evaluate the number densities of sputtered species from a measured optical emission signal. A biasable quartz crystal microbalance system was used to evaluate the ionised flux fraction and deposition rate. The highest ionised density fraction of 90% was reached in the target region for 1.2% duty cycle and it was mostly independent on working pressure. Moving from the target, the ionised density fraction was observed to decrease. In the substrate region for a duty cycle below 2% and the whole studied working pressure range, the ionised density fraction always reached at least 50%.

Dopants and Traps in Nanocrystal-Based Semiconductor Thin Films: Origins and Measurement of Electronic Midgap States

Different models are proposed to explain the origin of electronic midgap states that limit charge transport in nanocrystal thin films. Here, we investigate midgap states using optoelectronic and electrochemical techniques, supported by theoretical calculations. We find that charge trapping in PbS NC thin films can be explained by (1) nanocrystal dimerization and (2) charging of doped nanocrystals. In these models, there are no states within the band gap of individual nanocrystals; the nanocrystals themselves act as trap states. These findings allow us to formulate strategies to improve transport by increasing free carrier densities while reducing the number density of trap states.

Influence of sputtered species ionisation on the hysteresis behaviour of reactive HiPIMS with oxygen admixture

In this paper, the hysteresis behaviour of a reactive magnetron sputtering process with oxygen admixture is studied by both experiment and model. The ground state number densities of titanium atoms and ions, and the deposition rates in high power impulse magnetron sputtering discharge were determined for a constant average applied power and pulse duration, while the repetition frequency and reactive gas supply were varied. The hysteresis curve reduced in width and shifted towards the lower oxygen supply with a decrease in the repetition frequency. These experimentally observed trends were well reproduced by a modified Berg model. The presented model utilised measured ionisation fraction of sputtered species and considered the back-attraction of the ionised sputtered species to the target. Significance of the back-attraction process was observed as large fraction of ionised sputtered particles was found to be lost back to the target. This drastically decreased the number of particles deposited onto the substrate, which reduced not only the deposition rate, but also the gettering of the reactive gas. Consequently, the simulated hysteresis curve was shifted and reduced in width as it was observed experimentally.

In-situ quantification of alkali metals from biomass combustion by optical emission spectroscopy

In this study, an in-situ method was developed to quantify the alkali metal emissions and its speciation from the combustion of biomass in a 150 kWth furnace. An optical emission spectrometer was used to detect the spontaneous atomic emissions of the potassium and sodium in the flame. A calibration is performed to obtain the radiation intensities of potassium and sodium, which are precursive values needed to quantify each alkali metals’ relative ground state number densities. The results show that the spontaneous emissions of the alkali metals appear as doublet lines in the spectra. The calibrated radiation intensities for sodium were converted to relative values via Lorentzian fitting. The relative excited state number densities for sodium and potassium were within the range of 0.536 to 0.791 cm−3 and 23.468 and 30.320 cm−3 respectively. This shows that potassium vapours and its compounds were higher compared to sodium. These number densities will be compared to the sodium and content in the ash for further discussion and validation.

Top-Down Fabrication Process of ZnO NWFETs

ZnO NWFETs were fabricated with and without Al2O3passivation. This was done by developing a new recipe for depositing the thin film of ZnO. By using a high donor concentration of 1.7 x 1018cm-3for the thin film, contact resistance values were lowered (passivated device had Rcon= 2.5 x 104Ω; unpassivated device had Rcon= 3.0 x 105Ω). By depositing Zn first instead of O2, steep subthreshold slopes were obtained. The passivated device had a subthreshold slope of 225 mV/decade and the unpassivated device had a slope of 125 mV/decade. Well-behaved electrical characteristics have been obtained and the passivated device shows field effect mobility of 10.9 cm2/Vs and the un-passivated device shows a value of 31.4 cm2/Vs. To verify the results, 3D simulation was also carried out which shows that the obtained values of sub-threshold slope translate into interface state number densities of-1.86 x 1013cm-2for the unpassivated device and 3.35 x 1014cm-2for the passivated device. The passivated device is suitable for biosensing applications.

Experimental investigation of plasma parameters in 13.56 MHz/2 MHz cylindrical inductively coupled plasma

Inductively coupled plasmais widely used in semiconductor and display process because of its desirable characteristics such as high plasma density, simple structure and independently controllable ion energy. The driving frequency is a significant parameter that generates and maintains the plasma. However, the effects of different driving frequencies on the radial distribution of the plasma parameters are hardly investigated. So a large area cylindrical inductively coupled plasma source driven separately by 2 MHz and 13.56 MHz is investigated. In order to perform a comprehensive investigation about the effect of driving frequency, the radially resolved measurements of electron density, electron temperature and density of metastable state atoms for the argon discharge are systematically analyzed by Langmuir double probe and optical emission spectroscopy at various power values and gas pressures. It is found that input power values at high frequency (13.56 MHz) and low frequency (2 MHz) have different effects on plasma parameters. When discharge is driven at high frequency, the electron density increases obviously with the increase of power. However, when discharge is driven at low frequency, the electron temperature increases evidently with the increase of power. This can be explained by calculating the skin depths in high and low frequency discharge. When the discharge is driven at high frequency, the induced electromagnetic field is higher than that at low frequency, and the single electron obtains more energy. It is easier to ionize, so the energy is mainly used to increase the electron density. When the discharge is driven at low frequency, the skin layer is thicker, the number of heated electrons is larger, and the average energy of electrons is increased, so the energy is mainly used to raise the electron temperature. At a gas pressure of 10 Pa, the electron density shows a ‘convex’ distribution and increases with the increase of input power for both the high-frequency and low-frequency discharge. While the distributions of electron temperature are obviously different. When the discharge is driven at high frequency, the electron temperature is relatively flat in the center of the chamber and slightly increases on the edge. When the discharge is driven at low frequency, the electron temperature gradually decreases along the radial position. This is due to the one-step ionization in the high-frequency discharge and the two-step ionization in the low-frequency discharge. In order to prove that the low-frequency discharge is dominated by two-step ionization, the spectral intensities of the argon plasma under the same discharge conditions are diagnosed by optical emission spectroscopy. The number density of metastable states is calculated by the branch ratio method. The results are consistent with the analyses. At a gas pressure of 100 Pa, the electron density increases and then decreases with the increase of radial distance, and the overall distribution shows a " saddle shape” for high frequency and also for low frequency discharge. Although the uniformity of electron density improves with the gas pressure, the uniformity at low frequency is better than that at high frequency. The reason can be attributed to the fact that the skin layer of low frequency is thicker and the heating area is wider.

Measuring the Electronic Structure of Nanocrystal Thin Films Using Energy-Resolved Electrochemical Impedance Spectroscopy.

Use of nanocrystal thin films as active layers in optoelectronic devices requires tailoring of their electronic band structure. Here, we demonstrate energy-resolved electrochemical impedance spectroscopy (ER-EIS) as a method to quantify the electronic structure in nanocrystal thin films. This technique is particularly well-suited for nanocrystal-based thin films as it allows for in situ assessment of electronic structure during solution-based deposition of the thin film. Using well-studied lead sulfide nanocrystals as an example, we show that ER-EIS can be used to probe the energy position and number density of defect or dopant states as well as the modification of energy levels in nanocrystal solids that results through the exchange of surface ligands. This work highlights that ER-EIS is a sensitive and fast method to measure the electronic structure of nanocrystal thin films and enables their optimization in optoelectronic devices.

Time-resolved study of the electron temperature and number density of argon metastable atoms in argon-based dielectric barrier discharges

A combination of optical emission spectroscopy and collisional-radiative modelling is used to determine the time-resolved electron temperature (assuming Maxwellian electron energy distribution function) and number density of Ar 1s states in atmospheric pressure Ar-based dielectric barrier discharges in presence of either NH3 or ethyl lactate. In both cases, Te values were higher early in the discharge cycle (around 0.8 eV), decreased down to about 0.35 eV with the rise of the discharge current, and then remained fairly constant during discharge extinction. The opposite behaviour was observed for Ar 1s states, with cycle-averaged values in the 1017 m−3 range. Based on these findings, a link was established between the discharge ionization kinetics (and thus the electron temperature) and the number density of Ar 1s state.

Superheavy thermal dark matter and primordial asymmetries

The early universe could feature multiple reheating events, leading to jumps in the visible sector entropy density that dilute both particle asymmetries and the number density of frozen-out states. In fact, late time entropy jumps are usually required in models of Affleck-Dine baryogenesis, which typically produces an initial particle-antiparticle asymmetry that is much too large. An important consequence of late time dilution, is that a smaller dark matter annihilation cross section is needed to obtain the observed dark matter relic density. For cosmologies with high scale baryogenesis, followed by radiation-dominated dark matter freeze-out, we show that the perturbative unitarity mass bound on thermal relic dark matter is relaxed to 1010 GeV. We proceed to study superheavy asym-metric dark matter models, made possible by a sizable entropy injection after dark matter freeze-out, and identify how the Affleck-Dine mechanism would generate the baryon and dark asymmetries.