Table of contents

This volume of Journal of Physics: Conference Series contains a selection of papers presented at the 27th Summer School and International Symposium on the Physics of Ionized Gases – SPIG 2014, as General Invited Lectures, Topical Invited Lectures, Progress Reports and associated Workshop Lectures. The conference was held in Belgrade, Serbia, from 26-29 August 2014 at the Serbian Academy of Sciences and Arts. It was organized by the Institute of Physics Belgrade, University of Belgrade and Serbian Academy of Sciences and Arts, under the auspices of the Ministry of Education, Science and Technological Development, Republic of Serbia.

A rare virtue of a SPIG conference is that it covers a wide range of topics, bringing together leading scientists worldwide to present and discuss state-of-the art research and the most recent applications, thus stimulating a modern approach of interdisciplinary science. The Invited lectures and Contributed papers are related to the following research fields:

1. Atomic Collision Processes (Electron and Photon Interactions with Atomic Particles, Heavy Particle Collisions, Swarms and Transport Phenomena)

2. Particle and Laser Beam Interactions with Solids (Atomic Collisions in Solids, Sputtering and Deposition, Laser and Plasma Interaction with Surfaces)

3. Low Temperature Plasmas (Plasma Spectroscopy and other Diagnostic Methods, Gas Discharges, Plasma Applications and Devices)

4. General Plasmas (Fusion Plasmas, Astrophysical Plasmas and Collective Phenomena)

Additionally, the 27th SPIG encompassed three workshops that are closely related to the scope of the conference:

• The Workshop on Dissociative Electron Attachment (DEA) – Chaired by Prof. Nigel J Mason, OBE, The Open University, United Kingdom

• The Workshop on X-ray Interaction with Biomolecules in Gas Phase (XiBiGP), Chaired by Dr. Christophe Nicolas, Synchrotron SOLEIL, France

• The 3rd International Workshop on Non-Equilibrium Processes (NonEqProc) – Chaired by Prof. Zoran Lj. Petrović, Institute of Physics Belgrade, University of Belgrade, Serbia

The Editors would like to thank the members of the Scientific and Advisory Committees of SPIG conference for their efforts in proposing the program of the conference and to the referees that have reviewed submitted papers, as well as the chairmen of the associated workshops for their efforts and help in organizing them and a selection of excellent invited talks. We particularly acknowledge the efforts of all the members of the Local Organizing Committee in the organization of the Conference. We are grateful to all sponsors of the conference: SOLEIL synchrotron, RoentDek Handels GmbH, Klett Publishing House Ltd, Springer (EPJD and EPJ TI), IOP Publishing (IOP Conference Series), DEA club, Austrian Cultural Forum Belgrade, Institut français de Serbie and Collegium Hungaricum Belgrade.

Holding on to a long tradition is never easy and the only way to achieve that is to have a large number of people who appreciate the conference, so we would like to thank all the invited speakers and participants for taking part in the 27th SPIG conference.

Editors of the issue:

Dr Dragana Marić (Instutute of Physics Belgrade, University of Belgrade

Dr Aleksandar R. Milosavljević (Instutute of Physics Belgrade, University of Belgrade)

Prof Zoran Mijatović (Faculty of Sciences, University of Novi Sad)

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

2014 is the centenary of the first announcement of the Franck-Hertz experiment [1], now regarded as one of the pivotal experiments of modern physics. The Franck-Hertz experiment is widely regarded as an experiment that provided validation of the Bohr theory of atomic structure, itself only published in 2013, however it should also be viewed as the first quantitative experiment in electron scattering and the birth of scientific study of atomic and molecular phenomena by collisions. Today we recognize that electron-atom and electron- molecule collisions are prevalent across nature, describing disparate phenomena whilst the exploitation of such collisions underpins many of the technologies upon which modern society relies. The centenary of the Franck-Hertz experiment is thus a suitable opportunity to review both our current knowledge of electron interactions and to consider the directions of future research. In this article I therefore aim to both review our current state of knowledge and look forward, proposing that recent advances are providing something of a renaissance to the field and are vital for emerging technologies as well as answering some of the greatest scientific challenges of the 21st century.

Molecular imaging through magnetic resonance spectroscopy (MRS) can provide information about key metabolites. Conventional applications of MRS are hampered by data analysis via the fast Fourier transform (FFT). Most MRS studies for cancer detection have relied upon estimations of a mere handful or even a single composite metabolite, e.g. total choline. These have yielded incremental improvements in diagnostic accuracy. In vitro studies reveal richer metabolic information for identifying cancer, particularly in closely-overlapping resonances. Among these are phosphocholine, a marker of malignant transformation. The FFT cannot assess these congested spectral components. This can be done by the fast Pade transform (FPT), an advanced, high-resolution, quantification-equipped method, applied to MRS time signals as encoded from patients with breast cancers and other cancers, with benign pathology and with normal tissue, as illustrated herein for the latter. With realistic noise levels, the FPT accurately computes the metabolite concentrations, including phosphocholine, which completely underlies phosphoethanolamine. In sharp contrast, the FFT produces a rough envelope spectrum with only a few shortened, broadened peaks, and key metabolites altogether absent. The FPT clearly separates true metabolites from spurious resonances. The efficiency and high resolution of the FPT translates into shortened examination time of the patient. These capabilities strongly suggest that by applying the FPT to time signals encoded in vivo from breast cancer and other malignancies, MRS will fulfill its potential to become a clinically- reliable, cost-effective method for early cancer detection.

The need for ions in radiotherapy stems from the most favorable localization of the largest energy deposition, precisely at the tumor site with small energy losses away from the target. Such a dose conformity to the target is due to heavy masses of ions that scatter predominantly in the forward direction and lose maximal energy mainly near the end of their path in the vicinity of the Bragg peak. The heavy masses of nuclei preclude noticeable multiple scattering of the primary ion beam. This occurrence is responsible for only about 30% of ion efficiency in killing tumor cells. However, ionization of targets by fast ions yields electrons that might be of sufficient energy to produce further radiation damage. These δ-electrons, alongside radicals produced by ion-water collisions, can accomplish the remaining 70% of tumor cell eradication. Electrons achieve this chiefly through multiple scattering due to their small mass. Therefore, energy depositions by both heavy (nuclei) and light (electrons) particles as well as highly reactive radicals need to be simultaneously transported in Monte Carlo simulations. This threefold transport of particles is yet to be developed for the existing Monte Carlo codes. Critical to accomplishing this key goal is the availability of accurate cross section databases. To this end, the leading continuum distorted wave methodologies are poised to play a pivotal role in predicting energy losses of ions in tissue as discussed in this work.

Single-electron capture cross sections in collisions between fast bare projectiles and heliumlike atomic systems are investigated by means of the four-body boundary-corrected first Born (CB1-4B) approximation. The prior and post transition amplitudes for single charge exchange encompassing symmetric and asymmetric collisions are derived in terms of twodimensional real integrals in the case of the prior form and five-dimensional quadratures for the post form. The dielectronic interaction V₁₂ = 1/r₁₂ = 1/|r₁ - r₂| explicitly appears in the complete perturbation potential V_f of the post transition probability amplitude T⁺_if. An illustrative computation is performed involving state-selective and total single capture cross sections for the p - He (prior and post form) and He²⁺, Li³⁺Be⁴⁺B⁵⁺C⁶⁺ - He (prior form) collisions at intermediate and high impact energies. We have also studied differential cross sections in prior and post form for single electron transfer from helium by protons. The role of dynamic correlations is examined as a function of increased projectile energy. Detailed comparisons with the measurements are carried out and the obtained theoretical cross sections are in reasonable agreement with the available experimental data.

The results of an experimental investigation of electrons colliding with a set of biomolecules that are assumed to be analogues of the building blocks of DNA (furan, 3- hydroxytetrahydrofuran and pyrimidine) and proteins (formamide, N-methylformamide) are presented. Absolute differential cross sections at medium incident electron energies 40 eV- 300 eV are presented and compared for these different targets. The experimental results are also compared with available calculations, based on the corrected form of independent atom model and show good agreement over the energy range studied.

Experimental study of indium atom using electron and optical spectroscopy is presented in this paper. Both experimental techniques including experimental setups are described. Differential and integrated cross sections on elastic and inelastic electron scattering by indium atom are measured using electron spectrometer. The measurements are performed at incident electron energies of E0 = 10, 20, 40, 60, 80 and 100 eV within the large scattering angles ranging from 10° to 150° in steps of 10°. The experimental results are presented and comparison with the values predicted by calculated optical potentials method is conducted, showing good agreement. The differential cross sections (DCSs) for electron-impact excitation of the resonant state 6s 2S1/2 of Indium atom are measured at small and large angles. The forward scattering function method has been used for normalizing the generalized oscillator strengths (GOS) to determine optical oscillator strength and obtaining the absolute DCS values. Optical spectrum of In I and In II lines has been acquired by a streak camera. The experimental results regarding indium lines obtained by time resolved laser induced breakdown spectroscopy (LIBS) could be useful for obtaining the important plasma parameters such as temperature, electron density as well as plasma-expansion velocity and plasma starting times.

LXCat is a web-based, community-wide project on the curation of data needed in the modelling of low-temperature plasmas. LXCat is organized in databases, contributed by members of the community around the world and indicated by the contributor's chosen title. This paper presents the status of the data available on the IST-LISBON database with LXCat. IST-LISBON contains up-to-date electron-neutral collisional data (together with the measured swarm parameters used to validate these data) resulting from the research effort of the Group of Gas Discharges and Gaseous Electronics with Instituto de Plasmas e Fusao Nuclear, Instituto Superior Tecnico, Lisbon, Portugal. Presently, the IST-LISBON database includes complete and consistent sets of electron scattering cross sections for argon, helium, nitrogen, oxygen, hydrogen and methane.

We study the electron transport in gas mixtures used by Resistive Plate Chambers (RPCs) in high energy physics experiments at CERN. Calculations are performed using a multi term theory for solving the Boltzmann equation. We identify the effects induced by non-conservative nature of electron attachment, including attachment heating of electrons and negative differential conductivity (NDC). NDC was observed only in the bulk component of drift velocity. Using our Monte Carlo technique, we calculate the spatially resolved transport properties in order to investigate the origin of these effects. We also present our microscopic approach to modeling of RPCs which is based on Monte Carlo method. Calculated results for a timing RPC show good agreement with an analytical model and experimental data. Different cross section sets for electron scattering in C₂H₂F₄ are used for comparison and analysis.

We study the wake effect in a supported graphene layer induced by external charged particles moving parallel to it by using the dynamic polarization function of graphene within the random phase approximation for its π electrons described as Dirac's fermions. We explore the effects of a substrate assuming that graphene is supported by an insulating substrate, such as SiO₂, and a strongly polar substrate, such as SiC, under the gating conditions. Strong effects are observed in the wake pattern in the induced density of charge carriers in supported graphene due to finite size of the graphene-substrate gap, as well as due to strong coupling effects, and plasmon damping of graphene's π electrons. We find that the excitation of surface phonons in the substrate may exert quite strong influences on the wake effect in the total electrostatic potential in the graphene plane at low particle speeds.

We experimentally investigated some of the initial reactions in a liquid induced by electron or positive-ion irradiation from an atmospheric-pressure dc glow discharge in contact with the liquid. We used an H-shaped glass reactor to observe the effects of electron irradiation and positive-ion irradiation on the liquid-phase reaction separately and simultaneously. Aqueous solutions of NaCl, AgNO₃, HAuCl₄, and FeCl₂ are used as the electrolyte. Solutions of AgNO₃ and HAuCl₄ are used for the generation of Ag and Au nanoparticles, respectively. Solution of FeCl₂ is used for the generation of ferromagnetic particles. Experimental results showed that electron irradiation of the liquid surface generates OH⁻ in water and that positive-ion irradiation of the liquid surface generates H⁺ in water even without the dissolution of gas-phase nitrogen oxide. A possible reaction process is qualitatively discussed. We also showed that the control of reductive and oxidative environment in the liquid is possible not only by the gas composition for the plasma generation but also by the liquid composition.

In the present study, a nitrogen atmospheric pressure plasma jet (APPJ) was used for irradiation of oral cancer cells. Since cancer cells are very susceptible to plasma treatment, they can be used as a tool for detection of APPJ-effective areas, which extended much further than the visible part of the APPJ. An immunofluorescence assay was used for DNA damage identification, visualization and quantification. Thus, the effective damage area and damage level were determined and plotted as 3D images.

The low-temperature low-pressure hydrogen based plasmas were used to study the influence of processes and discharge conditions on corrosion removal. The capacitive coupled RF discharge in the continuous or pulsed regime was used at operating pressure of 100-200 Pa. Plasma treatment was monitored by optical emission spectroscopy. To be able to study influence of various process parameters, the model corroded samples with and without sandy incrustation were prepared. The SEM-EDX analyzes were carried out to verify corrosion removal efficiency. Experimental conditions were optimized for the selected most frequent materials of original metallic archaeological objects (iron, bronze, copper, and brass). Chlorides removal is based on hydrogen ion reactions while oxides are removed mainly by neutral species interactions. A special focus was kept for the samples temperature because it was necessary to avoid any metallographic changes in the material structure. The application of higher power pulsed regime with low duty cycle seems be the best treatment regime. The low pressure hydrogen plasma is not applicable for objects with a very broken structure or for nonmetallic objects due to the non-uniform heat stress. Due to this fact, the new developed plasmas generated in liquids were applied on selected original archaeological glass materials.

Plasma during Electrolytic Oxidation (PEO) of magnesium-aluminium alloys is studied in this work by means of Optical Emission Spectroscopy (OES). Spectral line shapes of the H_β, Al II 704.21 nm and Mg II 448.11 nm line are analyzed to measure plasma electron number density N_e. From the H_β line profile, two PEO processes characterized by relatively low electron number densities N_e ≈ 10¹⁵ cm⁻³ and N_e ≈ 2 × 10¹⁶ cm⁻³ were discovered while the shape and shift of Al II and Mg II lines revealed the third process characterized by large electron density N_e = (1-2) × 10¹⁷ cm⁻³. Low N_e processes, related with breakdown in gas bubbles and on oxide surface, are not influenced by anode material or electrolyte composition. The ejection of evaporated anode material through oxide layer is designated here as third PEO process.

Using the Boltzmann plot technique, electron temperature of 4000 K and 33000 K is determined from relative intensities of Mg I and O II lines, respectively. Several difficulties in the analysis of spectral line shapes are met during this study and the ways to overcome some of the obstacles are demonstrated.

The procedure for diagnostics of laser induced plasma (LIP) by optical emission spectroscopy technique is described. LIP was generated by focusing Nd:YAG laser radiation (1.064 nm, 50 mJ, 15 ns pulse duration) on the surface of pellet containing among other elements lithium. Details of the experimental setup and experimental data processing are presented. High speed plasma photography was used to study plasma evolution and decay. From those images optimum time for plasma diagnostics is located. The electron number density, N_e, is determined by fitting profiles of Li I lines while electron temperature, T_e, was determined from relative intensities of Li I lines using Boltzmann plot (BP) technique. All spectral line recordings were tested for the presence of self-absorption and then if optically thin, Abel inverted and used for plasma diagnostic purposes.

The electrical breakdown and discharges in air are of great significance for science and technology due to wide applications. Besides a breakdown itself, the pre-breakdown phenomena and relaxation after the breakdown have a significant role. In this report the analysis of DC glow discharge and its relaxation processes is presented. Statistical analysis of time delay distributions is presented and the statistical model based on the mixture distribution is applied to data measured with different electrodes from which electron yield is calculated. The occurrence of mixture distributions is physically explained based on analysis of the cathode surface. Also, by analyzing the memory curve measured in synthetic air the early and late relaxation of DC discharge are discussed. The processes responsible for the memory effect are identified and corresponding rate coefficients are determined. In order to confirm that the correct particles are identified, the 2D numerical model for relaxation is developed.

The experimental and theoretical analyses of different regimes of argon DC glow discharge are reported. The experiments were carried out on the argon gas tube with a plane- parallel electrode system made from OFHC (oxygen-free high thermal conductivity) copper. Modelling of the static breakdown voltages was performed by simple fluid model. The applicability of fluid models for modelling of I - U (current-voltage) characteristics at different values of pd (pressure times inter-electrode distance) is tested. The formative time delays are determined from experiment and compared to modeled values obtained by [1D] and [2D] fluid models. The memory curve _d (τ) (the dependence of the mean value of breakdown time delay on the relaxation time) is presented and the main processes responsible for the memory effect were determined by applying the analytical and numerical models.

Recent results from LHD experiments are presented together with the progress of the related fusion technology supporting the experiment. An ultra-long pulse discharge with central electron and ion temperatures of 2 keV and electron density of 1.2 x 10¹⁹m⁻³ was successfully maintained for 48 minutes. Such a high performance plasma was heated and sustained by electron cyclotron resonance heating (ECH) with newly developed gyrotrons and ion cyclotron range of frequency (ICRF). Due to the continuous heating with the relatively high power of 1.2 MW, the total heating energy during the long pulse discharge reached 3.36 GJ, which is the world's record. The highest central ion temperature of 8.1 keV was also recorded in the last experimental campaign. In these experiments, it is essential to reduce the edge electron density, i.e., edge neutrals. Dedicated wall conditioning was performed to evacuate neutrals in the vessel wall before the main discharge with the high heating power. This process realizes the quite low recycling for particles, which results in the reduction of the edge plasma density.

Last year (2013) was more or less the 50th anniversary of the discovery of quasars. It is an interesting time to review what we know (and don't know) about them both empirically and theoretically. These compact sources involving line emitting plasma show extraordinary luminosities extending to one thousand times that of our Milky Way in emitting volumes of a few solar system diameters (log L_boi= 44.0 - 48.0 erg s⁻¹: D=1-3 light months ~ 10³ - 10⁴ gravitational radii). The advent of 8-10 meter class telescopes enables us to study them spectroscopically in ever greater detail.

In 2000 we introduced a 4D Eigenvector 1 parameters space involving optical, UV and X- ray measures designed to serve as a 4D equivalent of the 2D H-R diagram so important for depicting the diversity of stellar types and evolutionary states. This diagram has revealed a principal sequence of quasars distinguished by Eddington ratio (proportional to the accretion rate per unit mass). Thus while stellar differences are primarily driven by the mass of a star, quasar differences are apparently driven by the ratio of luminosity-to-mass.

Out of this work has emerged the concept of two quasars populations A and B separated at Eddington ratio around 0.2 which maximizes quasar multispectral differences. The mysterious 8% of quasars that are radio-loud belong to population B which are the lowest accretors with the largest black hole masses. Finally we consider the most extreme population A quasars which are the highest accretors and in some cases are among the youngest quasars. We describe how these sources might be exploited as standard candles for cosmology.

Electron impact excitation from the ground state of bismuth atoms has been studied. A beam of electrons was scattered from a beam of atoms and the intensity of scattered electrons was measured for scattering angles up to 150° and incident electron energies of 40 and 60 eV. Obtained intensities were used for the calculation of relative differential cross sections (DCS). In addition, we recorded the energy loss-spectra at different incident electron energies and scattering angles. These spectra were analysed in order to identify the energy levels of bismuth atom below and above (autoionisation) the first ionization limit in electron spectroscopy. The presence of bismuth was confirmed in spectra of the chemically peculiar (CP) magnetic Ap 73 Dra and HR 465 and nonmagnetic Hg-Mn HR 7775 and χ Lupi stars. The obtained results for relative DCS and identified autoionised energy levels of bismuth were analysed and compared with previous experimental and theoretical data. The connection between our investigations of bismuth and astrophysical measurements are discussed.

We investigate the physical conditions and kinematics of broad absorption line region clouds of Si IV in 21 HiBAL Quasars. We use the Danezis et al. method [1], [2], [3] in order to fit and analyze the broad absorption troughs of Si IV resonance lines in the UV region of the electromagnetic spectrum. We find that the BAL flow is not smooth but instead plasma clouds are formed in it. BAL troughs present multicomponent structure which indicates the existence of more than one absorbing cloud in the line of sight, where every absorbing cloud produces a Si IV doublet. We show that the blending of these doublets produces the apparent broad absorption troughs we observe. One of our main achievements is that we managed to decompose and deblend each complex absorption trough to the individual doublets that it consists of. Apart from that, we succeeded in deblending the resonance lines of every doublet. By achieving accurate fits to the BAL troughs we calculated some physical and kinematical parameters that describe the plasma clouds in the line of sight. These parameters are: the radial outflow velocities of the clouds, the random velocities of ions inside each plasma cloud, the apparent optical depth in the center of every absorption component, the FWHM and the equivalent width. As a final step we correlate these physical parameters in order to draw useful conclusions.

The dynamics of excited atom interactions with other atoms, which often lead to associative ionization, is largely governed by stochastic diffusion of the valence electron through Rydberg states prior to the ionization. Such processes are associated with random changes of the energy state of the highly excited electron, and they are likely to influence the nuclear dynamics, especially at subthermal collision energies. Possibilities of manipulation of the chaotic dynamics of Rydberg states require a detailed exploration. For an electron in a given Rydberg state moving in a microwave field, which can be generated via interaction with another atom or molecule, there exists critical field strength, above which motion of the electron in the energy space is chaotic. Recently a way to block the dynamic chaos regime was shown, if a given Rydberg state is located somewhat above the middle between the two other states with the orbital quantum number differing by one, whereby level shifts can be controlled by employing Stark/Zeeman shifts in external DC electric/magnetic fields. The stochastic effects in collisions involving Rydberg particles, in which the initial and final reaction channels are connected via intermediate highly excited collision complexes with multiple crossings of energy levels, can be treated using the dynamic chaos approach (Chirikov criterion, Standard and Keppler mapping of time evolution of the Rydberg electron, solution of the Fokker-Plank- and Langevin-type of equations, etc.). Such approach to obtaining dynamics characteristics is a natural choice, since the treatment of Rydberg electron dynamics as a kind of diffusion process allowing one to bypass the multi-level-crossing problem, which can hardly be solved by conventional quantum chemistry methods.

Here, the processes of photo-association and absorption charge-exchange in He+H+ collisions together with the process of molecular ion HeH+ photo-dissociation are considered as factors of influence on the opacity of the atmospheres of helium-rich white dwarfs in the far UV region. It is shown that they should be taken into account even in the cases of the atmospheres of white dwarfs with H:He =10⁻⁵. Also, it is established that in the cases of white dwarfs with H:He ≳ 10⁻⁴, especially when H:He ≈ 10⁻³, these processes have to be included ab initio in the corresponding models of their atmospheres.