Abstract: In this letter, we investigate the effect of various levels of nitrogen impurity on the electrical performance of an atmospheric pressure dielectric barrier discharge in helium. We illustrate the different current profiles that are obtained, which exhibit one or more discharge pulses per half cycle and evaluate their performance in ionizing the discharge and dissipating the power. It is shown that flat and broad current profiles perform the best in ionizing the discharge and use the least amount of power per generated charged particle.

Abstract: In this theoretical study, the electron emission from a copper surface under ultrashort pulsed laser irradiation is investigated using a one-dimensional particle in cell model. Thermionic emission as well as multiphoton photoelectron emission were taken into account. The emitted electrons create a negative space charge above the target; consequently the generated electric field reduces the electron emission by several orders of magnitude. The simulations indicate that the space charge effect should be considered when investigating electron emission related phenomena in materials under ultrashort pulsed laser irradiation of metals.

Abstract: A reactive ReaxFF force field is developed for studying the influence of intrinsic point defects on the chemistry with TiO2 condensed phases. The force field parameters are optimized to ab initio data for the equations of state, relative phase stabilities for titanium and titanium dioxide, potential energy differences for (TiO2)n-clusters (n = 116). Also data for intrinsic point defects in anatase were added. These data contain formation energies for interstitial titanium and oxygen vacancies, diffusion barriers of the oxygen vacancies and molecular oxygen adsorption on a reduced anatase (101) surface. Employing the resulting force field, we study the influence of concentration of oxygen vacancies and expansion or compression of an anatase surface on the diffusion of the oxygen vacancies. Also the barrier for oxygen diffusion in the subsurface region is evaluated using this force field. This diffusion barrier of 27.7 kcal/mol indicates that the lateral redistribution of oxygen vacancies on the surface and in the subsurface will be dominated by their diffusion in the subsurface, since both this barrier as well as the barriers for diffusion from the surface to the subsurface and vice versa (17.07 kcal/mol and 21.91 kcal/mol, respectively, as calculated with DFT), are significantly lower than for diffusion on the surface (61.12 kcal/mol as calculated with DFT).

Abstract: We describe the plasma chemistry in a helium flowing atmospheric pressure afterglow (FAPA) used for analytical spectrometry, by means of a quasione-dimensional (1D) plasma chemical kinetics model. We study the effect of typical impurities present in the feed gas, as well as the afterglow in ambient humid air. The model provides the species density profiles in the discharge and afterglow regions and the chemical pathways. We demonstrate that H, N, and O atoms are formed in the discharge region, while the dominant reactive neutral species in the afterglow are O3 and NO. He* and He2* are responsible for Penning ionization of O2, N2, H2O, H2, and N, and especially O and H atoms. Besides, He2+ also contributes to ionization of N2, O2, H2O, and O through charge transfer reactions. From the pool of ions created in the discharge, NO+ and (H2O)3H+ are the dominant ions in the afterglow. Moreover, negatively charged clusters, such as NO3H2O− and NO2H2O−, are formed and their pathway is discussed as well. Our model predictions are in line with earlier observations in the literature about the important reagent ions and provide a comprehensive overview of the underlying pathways. The model explains in detail why helium provides a high analytical sensitivity because of high reagent ion formation by both Penning ionization and charge transfer. Such insights are very valuable for improving the analytical performance of this (and other) ambient desorption/ionization source(s).

Abstract: The unique capabilities of microsecond dwell time (DT) single-particle inductively coupled plasma mass spectrometry (spICPMS) were utilized to characterize the cloud of ions generated from the introduction of suspensions of gold nanoparticles (AuNPs) into the plasma. A set of narrowly distributed particles with diameters ranging from 15.4 to 100.1 nm was synthesized and characterized according to established protocols. Statistically significant numbers of the short transient spICPMS events were evaluated by using 50 μs DT for their summed intensity, maximum intensity, and duration, of which all three were found to depend on the particle diameter. The summed intensity increases from 10 to 1661 counts and the maximum intensity from 6 to 309 counts for AuNPs with diameters from 15.4 to 83.2 nm. The event duration rises from 322 to 1007 μs upon increasing AuNP diameter. These numbers represent a comprehensive set of key data points of the ion clouds generated in ICPMS from AuNPs. The extension of event duration is of high interest to appoint the maximum possible particle number concentration at which separation of consecutive events in spICPMS can still be achieved. Moreover, the combined evaluation of all above-mentioned ion cloud characteristics can explain the regularly observed prolonged single-particle events. The transport and ionization behavior of AuNPs in the ICP was also computationally modeled to gain insight into the size-dependent signal generation. The simulated data reveals that the plasma temperature, and therefore the point of ionization of the particles, is the same for all diameters. However, the maximum number density of Au+, as well as the extent of the ion cloud, depends on the particle diameter, in agreement with the experimental data, and it provides an adequate explanation for the observed ion cloud characteristics.