Electron scattering on molecular nitrogen: common gas, uncommon cross sections

We discuss peculiar features of electron scattering on the N2 molecule and the N2+ ion, that are important for modeling plasmas, Earth’s and other planets’ atmospheres. These features are, among others: the resonant enhancement of the vibrational excitation in the region of the shape resonance around 2.4 eV, the resonant character of some of electronic excitation channels (and high values of these cross sections, both for triplet and singlet states), high cross section for the dissociation into neutrals, high cross sections for elastic scattering (and electronic transitions) on metastable states. For the N2+ ion we discuss both dissociation and the dissociative ionization, leading to the formation of atoms in excited states, and dissociative recombination which depends strongly on the initial vibrational state of the ion. We conclude that the theory became an indispensable completion of experiments, predicting many of partial cross sections and their physical features. We hope that the data presented will serve to improve models of nitrogen plasmas and atmospheres. Graphical abstract: [Figure not available: see fulltext.]

Cross Sections for Electron Collisions with NO, N2O, and NO2

Cross section data are compiled from the literature for electron collisions with oxides of nitrogen (NxOy) molecules: the species nitric oxide (NO), nitrous oxide (N2O), and nitrogen dioxide (NO2) are explicitly considered. Cross sections are collected and reviewed for total scattering, elastic scattering, momentum transfer, excitations of rotational, vibrational, and electronic states, dissociation, ionization, and dissociative attachment. For each of these processes, the recommended values of the cross sections are presented. The literature has been surveyed up to the end of 2017. These results are supplemented by a reanalysis of the swarm measurements for NO and newly calculated cross sections for rotational excitation of N2O and for rotational excitation and electronic excitation of NO2

Cross Sections for Electron Collisions with Acetylene

Cross section data are compiled from the literature for electron collisions with the acetylene (HCCH) molecule. Cross sections are collected and reviewed for total scattering, elastic scattering, momentum transfer, excitations of rotational and vibrational states, dissociation, ionization, and dissociative attachment. The data derived from swarm experiments are also considered. For each of these processes, the recommended values of the cross sections are presented. The literature has been surveyed through early 2016

Spectroscopic Studies of Intramolecular Proton Transfer in 2-(4-Fluorophenylamino)-5-(2,4-Dihydroxybenzeno)-1,3,4-Thiadiazole

Spectroscopic studies of the biologically active compound 2-(4-fluorophenylamino)-5-(2,4-dihydroxybenzeno)-1,3,4-thiadiazole (FABT), have been performed. Absorption studies in the UV-Vis region for FABT in polar solvents, like water or ethanol, exhibit the domination of the enol form over its keto counterpart, with a broad absorption band centered around 340 nm. In non-polar solvents such as n-heptane or heavier alkanes the 340 nm absorption band disappears and an increase of the band related to the keto form (approximately 270 nm) is observed. Fluorescence spectra (with 270 nm and 340 nm excitation energies used) show a similar dependence: for FABT in 2-propanol a peak at about 400 nm dominates over that at 330 nm while in n-heptane this relation is reversed. The solvent dependent equilibrium between the keto and enol forms is further confirmed by FTIR and Raman spectroscopies. As can be expected, this equilibrium also shows some temperature dependences. We note that the changes between the two tautomeric forms of FABT are not related to the permanent dipole moment of the solvent but rather to its dipole polarizability

Pre-cavities defect distribution in He implanted silicon studied by slow positron beam

Several techniques were applied to study distributions of point defects created after He implantation in Si at an energy of 20 keV. The evolution of the defect distributions as a function of isocronal thermal anneling was studied in the 150-900 degrees C temperature range. In particular Doppler-broadening measurements with a slow positron beam were performed to gain information on open volume defects precursors of the cavities that are formed in He implanted silicon after thermal treatment. Profiles of displaced Si atoms, He, and vacancies are presented for the meaningful thermal treatments, and discussed

He-implantation induced defects in Si studied by slow positron annihilation spectroscopy

Open volume defect profiles have been obtained by performing Doppler broadening measurements with a slow positron beam on p-type Si samples implanted near liquid nitrogen temperature with He ions at 20 keV and at 5 x 10(15) and 2 x 10(16) cm(-2) fluence. The evolution of the defect profiles was studied as a function of isothermal annealing at 250 degrees C. The fraction of released He was measured by thermal programmed desorption. The defects could be identified as a coexistence of monovacancies stabilized by He-related defects and divacancies. The number of defects decreases for annealing time of a few minutes, then increases at longer annealing times. The mean depth of the defect profiles in the as-implanted samples was found to be very near the surface. After annealing, the mean depth increases to less than one half of the projected He range. This complex dynamics has been interpreted as due to passivation of vacancies by He during the implantation process and the first annealing step when no appreciable He is lost, and to subsequent depassivation during He desorption. (C) 1999 American Institute of Physics. [S0021-8979(99)02103-9]

Evolution of defect profiles in He-implanted silicon studied by slow positrons

He implanted in silicon forms clusters and then out-diffuses from the surface of the crystal when the implanted silicon samples are annealed from 200 to 700 degrees C, Evaporated He leaves empty voids in the Si crystal. The dimension of the voids and the dynamics of the void formation are strictly related to the He dose. A variable-energy positron beam has been used for monitoring the formation and the evolution of the void distribution in p-type (100) Si (1.7-2.4 Omega cm), created by implantation at 77 K of He ions ( 5x10(15) ions/cm(2)) at 20 keV. The samples were treated at 250 degrees C for different annealing times. Thermal desorption (TD) measurements of He are also presented

Structural evolution in Ar+ implanted Si-rich silicon oxide

Silicon-rich silicon oxide films were deposited by plasma-enhanced chemical vapor deposition. Energy was released into the film by ion bombardment, with the aim of promoting formation of Si nanoclusters and reordering the oxide matrix. The effect of the initial stoichiometry, as well as the evolution of the oxide films due to the ion bombardment and to subsequent thermal treatments, has been studied by depth-resolved positron annihilation Doppler spectroscopy, Raman scattering and Fourier transform infrared spectroscopy. As-deposited films were found to contain an open volume fraction in the form of subnanometric cavities that are positively correlated with oxygen deficiency. No Si aggregates were observed. The ion bombardment was found to promote the formation of amorphous Si nanoclusters, together with a reduction of the open volume in the matrix and a substantial release of hydrogen. It also leaves electrically active sites in the oxide and produces gas-filled vacancy defects in the substrate, with the concentrations depending on the implantation temperature. Thermal treatment at 500degreesC removes charge defects in the oxide, but vacancy defects are not completely annealed even at 1100degreesC. In one case, heating at 1100degreesC produced cavities of about 0.6 nm in the oxide. Transformation of Si nanoclusters into nanocrystals is observed to occur from 800degreesC

Helium-implanted silicon: A study of bubble precursors

The interaction of helium atoms with the radiation damage imparted to (100) silicon single crystal by He+ implantation at 5 x 10(15) cm(-2), 20 keV, and liquid-nitrogen temperature is investigated by means of various complementary techniques during and after thermal treatments. Thermal programmed desorption was used to study the dissociation kinetics of helium from the defects and to plan suitable heat treatments for the other techniques. The helium profiles were determined by 8 MeV N-15(2+) elastic recoil detection, quantitative data on damage were obtained by channeling Rutherford backscattering spectrometry, double crystal x-ray diffraction, and positron annihilation spectroscopy. Isothermal treatments at 250 degrees C produce first helium redistribution and trapping in vacancy-like defects, rather than helium desorption from traps. The process is thermally activated with an effective activation energy, dispersed in a band from 1.1 to about 1.7 eV. For higher temperature treatments (2 h at 500 degrees C) the traps are almost emptied and at 700 degrees C all vacancy-like defects are annealed out. No bubbles or voids are observed by transmission electron microscopy, either in the as-implanted or in annealed samples. (C) 1999 American Institute of Physics. [S0021-8979(99)00903-2]