Exceptional electrostatic phenomenon in ultrathin nanorods: the terminal σ‑hole

An in-depth understanding of the physicochemical properties of nanorods during the initial growth process has a profound impact on the rational design of high-performance nanorods catalysts. Herein, we conducted a systematic DFT study on the transition metal Co, Ni and alloyed nanoclusters/rods systems to simulate an atomic process from the initial nanoclusters growth to nanorods/wires. We found that the highly active sites of nanorods depend on an interesting electrostatic phenomenon. The surface electrostatic potential analysis shows that all nanoclusters and nanorods structures have formed σ-hole. Unlike nanoclusters, the σ-hole only appears at terminal sites in nanorods, called terminal σ-hole. The elemental composition in nanorods has a certain influence on the maximal surface electrostatic potential (VS,max) i.e., terminal σ-hole. Interestingly, we found that the terminal σ-hole formed in nanorods is generally higher in magnitude than smaller nanoclusters. First-principle calculations show that terminal σ-hole is closely related to the physicochemical activities of nanorods. For example, the work function of the directions forming terminal σ-hole is smaller than other directions. More interestingly, we found that in almost all nanorods, compared with other atoms, the d-orbital of the atoms forming terminal σ‑hole shifts close to the Fermi level and exhibits a shallower d-band center, showing higher chemical activity. In short, it is the first time that we discovered terminal σ-hole in nanorods, explained the theoretical basis of terminal σ-hole in nanorod systems, and provided theoretical guidance for the rational design of high-performance nanorods catalysts.

Detecting dark photons from atomic rearrangement in the galaxy

We study a new dark sector signature for an atomic process of “rearrangement” in the galaxy. In this process, a hydrogen-like atomic dark matter state together with its anti-particle can rearrange to form a highly-excited bound state. This bound state will then de-excite into the ground state emitting a large number of dark photons that can be measured in experiments on Earth through their kinetic mixing with the photon. We find that for DM masses in the GeV range, the dark photons have enough energy to pass the thresholds of neutrino observatories such as Borexino and Super-Kamiokande that can probe for our scenario even when our atomic states constitute a small fraction of the total DM abundance. We study the corresponding bounds on the parameters of our model from current data as well as the prospects for future detectors.

Modelling ion populations in astrophysical plasmas: carbon in the solar transition region

The aim of this work is to improve the modelling of ion populations in higher density, lower temperature astrophysical plasmas, of the type commonly found in lower solar and stellar atmospheres. Ion population models for these regions frequently employ the coronal approximation, which assumes conditions more suitable to the upper solar atmosphere, where high temperatures and lower densities prevail. The assumed conditions include all ions being in the ground state and steady-state equilibrium, where there is sufficient time for ionisation and recombination to take place. Using the coronal approximation for modelling the solar transition region gives theoretical lines intensities for the Li-like and Na-like isoelectronic sequences which are often factors of two to five times lower than observed. The works of Burgess & Summers (1969, ApJ, 157, 1007) and Nussbaumer & Storey (1975, A&A, 44, 321) show the important part ions in excited levels play when included in the modelling. As density increases metastable levels become populated and ionisation rates increase, whereas dielectronic recombination through highly excited levels is suppressed. Photo-ionisation is also shown by Nussbaumer & Storey to have an effect on the charge-state distribution of carbon in these regions. Their models, however, use approximations for the atomic rates to determine the ion balance. Presented here is the first stage in updating these earlier models of carbon by using rates from up-to-date atomic calculations and more recent photo-ionising radiances. Where atomic rates were not readily available, in the case of electron impact direct ionisation and excitation–auto-ionisation, new calculations were made using the Flexible Atomic Code and Autostructure, and compared to theoretical and experimental studies. The effects each atomic process has on the ion populations as density changes is illustrated, and final results from the modelling are compared to the earlier works. Lastly, the new results for ion populations were used to predict line intensities for the solar transition region in the quiet Sun. In comparison to coronal approximation modelling the new results show significantly improved agreement with observations.

Distorted Wave Theories Applied to Double Ionization by Ion Impact: Simulation of Higher-Order Processes

One of the goals in studies of double ionization (DI) of simple atoms by electron or ion impact is to elucidate and assess the different mechanisms that lead to this atomic process. In this work we present an attempt to model the mechanisms beyond the first order in DI of helium by highly charged projectiles. To this end we employ the continuum distorted wave-eikonal initial state (CDW-EIS) formalism joint with a Monte Carlo event generator (MCEG). The MCEG allows us to generate theoretical event files that represent the counterpart of the data obtained from a kinematically complete experiment. Starting from these event files, a new data analysis tool used to contrast theory and experiment in DI, the four-body Dalitz plots, is easily produced. The higher order mechanisms are simulated by considering DI as a sequential process: a single ionization of a helium atom as a first step and another single ionization of a single-charged helium ion as a second step. Some of the features in the experimental data are very well reproduced by these simulations.