Table of contents

Inclusion entrapment in a crystal is one of the most important characteristics for the crystal growth technique from solution. Diamond single crystals grown from the Fe-C system at high temperature-high pressure usually contain inclusions related to the molten catalyst and the medium (pyrophyllite) for transmitting pressure. During the growth of the diamond, the inclusions are trapped by the growth front or are formed through reaction between the contaminants trapped in the diamond. In the present article, the inclusions related to the catalyst and pyrophyllite were systemically examined by transmission electron microscopy. The chemical composition and crystal structure of the inclusions were, for the first time, determined by selected area electron diffraction pattern combined with energy dispersive x-ray spectrometry. It was shown that the inclusions are mainly composed of orthorhombic Fe₃C, orthorhombic FeSi₂, hexagonal SiO₂ and face-centred cubic SiC.

Luminance-current characteristics of organic electroluminescent devices based on the europium complex of europium(dibenzoylmethanato)₃ (bathophenanthroline) (Eu(DBM)₃bath) have been investigated. Transient measurements were carried out to study the decay process of excited Eu³⁺ ions. A comparison of experimental data and theoretical calculation shows that biexcitonic quenching among the excited Eu³⁺ ions is an important channel in their decay process, and this quenching process is a primary cause for our observation of a rapid decrease in quantum efficiency with increasing current density. Extending the recombination zone is found to be beneficial to reducing this defective effect. The mechanism of the quenching process is also discussed.

The present Special issue collects original and review articles that were a matter of discussion at the workshop `All about metastables' organised as a subsection of the XVth ESCAMPIG conference held in Miskol-Lillafured in Hungary on 26-30 August 2000.

The Europhysics Sectional Conference on Atomic and Molecular Physics of Ionised Gases (ESCAMPIG) is an important biennial meeting for scientists involved in fundamental and applied sciences based on ionised gases. The conference is organised under the sponsorship of the European Physical Society. The ESCAMPIG is a meeting rich with discussion opportunities. Besides lectures, hot topics and poster sessions, it is a custom of the ESCAMPIG to arrange during the meeting short workshops or round tables focused on topics of large scientific impact on this community. Such discussions are mainly based on the participation of scientists attending the conference.

The International Scientific Committee of XVth ESCAMPIG envisaged a renewed interest in the topics concerning long-lived species, and particularly those concerning the atoms and molecules in electronically excited metastable states (metastable species). The importance of such species is largely known in different ionised gas environments like laboratory gas-discharges for lamps, lasers and panel display technologies, as well as in natural environments like the upper atmosphere. The energy transfers operated by such species are of primary importance in many kinds of afterglows containing nitrogen, oxygen and noble gases. Actually, most of the details (density temperature or internal state distributions, etc.) of long-lived species can be detected by many extremely sensitive techniques, most of them based on lasers. This knowledge therefore offers the possibility of approaching with more confidence the microscopic kinetic modelling of discharge and post-discharge systems containing long-lived species.

Most of the matter discussed at the workshop `All about metastables' is addressed by the articles in this special issue. The issue focuses on some recent achievements in the topics concerning the diagnostics and modelling of elementary kinetics and energy transfers of electronically excited metastables and ground state atoms and vibrationally excited molecules in N₂, O₂, N₂-O₂ and N₂-H₂O systems. It also highlights the role of long-lived species in high-pressure discharge and in supersonic flow generated by N₂ plasma jets as well as in oxygen-iodine lasers. Two reviews will help the reader to focus on the main aspects of the gas phase and the gas-surface kinetics modelling of the long-lived species in the glow and afterglow conditions.

Finally, despite the appealing title, the workshop highlighted a restricted number of topics on metastables and more generally long-lived species. The issue of noble-gas metastables was not touched upon. For this reason we think the reader will appreciate also finding in this special issue two regular articles on topics not covered by the workshop. We hope this special issue contributes to an overview of the many links of the metastable issue in the physics and chemistry of partially ionised gases.

Dr S De Benedictis, Guest Editor

Centro di Studio per la Chimica dei Plasma CNR, c/o Dipartimento di Chimica Universita' di Bari, 4, Via Orabona, 70126 Bari, Italy

June 2001

The role played by the N₂(A³Σ_u⁺) metastable on the overall kinetics of N₂ and N₂-O₂ stationary discharges is illustrated by using a kinetic model based on the self-consistent solutions to the Boltzmann equation coupled to the rate balance equations for the vibrationally and electronically excited molecules, atoms and charged particles, in which the sustaining electric field is self-consistently determined. It is shown that together with the vibrational distribution of N₂(X¹Σ_g⁺,v) molecules, the metastable state N₂(A³Σ_u⁺) plays a central role in the whole problem, since some important aspects of these discharges, such as ionization, gas phase chemistry and gas heating are associated with different processes involving the N₂(A³Σ_u⁺) state.

The memory effect, the phenomenon that some active species survive very long afterglow periods and affect subsequent breakdown, was observed more than 40 years ago. The effects have been observed even over periods of several hours. Attempts to explain the memory effect in nitrogen were mostly based on hypothetical metastables and on the A³Σ state. However, such explanations had to neglect some quenching processes which are known to be very effective under the conditions of the experiments. The explanation based on atoms remaining from the previous discharge and recombining on the cathode to produce initial electrons was shown to be fully consistent with all the experimental data for nitrogen including a wide range of pressures and the addition of oxygen impurities. The memory effect was also shown to be sensitive to the work function of the cathode material. Thus, an attempt was made to use the memory effect as a diagnostic tool to establish the data on the dominant loss of nitrogen atoms from the discharge which is recombination on the walls of the tube. However, a possible role of higher vibrational levels has not been fully addressed, mainly due to the shortage of data. On the other hand, the memory effect which was observed for rare gases cannot be explained on the basis of the standard data unless the presence of molecular impurities is invoked. Another open issue would be the role of charges accumulated on the glass surfaces and whether those may be released to the gas phase. The aim of this paper is to summarize the achievements of the model based on atom recombination and to point out how the breakdown model associated with the memory effect may be completed and how it may be applied in practical discharges.

The enhancement of N₂(B³Π_g) and N₂⁺(B²Σ_u⁺) states, responsible by the emission bands for the 1⁺ and 1^- systems of N₂, in the short-lived afterglow of a flowing N₂ microwave discharge at ω/(2π) = 433 MHz is investigated by modelling. It is shown that pronounced maxima for both emissions are obtained downstream from the discharge by assuming the formation of N₂(A³Σ_u⁺) and N₂(a'¹Σ_u^-) states by collisions of N₂(X¹Σ_g⁺,v) molecules in highly vibrational levels with N(⁴S) atoms. Other channels that can also produce the emissions are investigated as well, but the common factor in all of them is the role played by the vibrationally excited N₂(X,v) molecules in levels {>}35. During the relaxation process a pumping-up effect for the vibrational quanta at the higher vth levels occurs as a result of near-resonant V-V energy-exchange collisions, that produces a rise in the observed peaks for the N₂ electronic states in the flowing afterglow.

To gain an understanding of the processes responsible for the formation of the well-known short-lived afterglow (SLA) or pink afterglow of nitrogen, different diagnostic techniques are implemented in the afterglow of a 440 Pa microwave nitrogen discharge in a 3.8 cm diameter flow tube. Using the intracavity laser absorption spectroscopy technique, we measure the space-dependent absolute density of N₂(A ³Σ_u⁺; v = 0) metastable molecules, as well as their rotational temperature, which in fact corresponds to the gas temperature, T_g. The density of N₂(A ³Σ_u⁺) molecules is about 5×10¹⁷ m^-3 at the end of the discharge zone (z = 4 cm). It then continuously decays by almost two orders of magnitude to reach a minimum around z = 12 cm before monotonically increasing to a secondary maximum of 5×10¹⁶ m^-3 located around z = 19 cm. It then slowly decays at longer distances. The space-dependent N₂(B ³Π_g) fluorescence intensity evolves in exactly the same way. A simple kinetic model is developed and we conclude that metastable molecules are locally formed in the SLA and not carried to this region by the gas flow. Contributions to the creation of N₂(A ³Σ_u⁺) molecules from N-N atom recombination, as well as from high vibrational levels of the ground-state N₂(X, v) molecules are analysed. To account for the high density of N₂(A ³Σ_u⁺) molecules, despite the (2-3) ×10²¹ m^-3 density of N atoms in the afterglow, we propose that N(²P) metastable atoms, resulting from the efficient quenching of N₂(A ³Σ_u⁺) molecules by N(⁴S) atoms, react immediately with vibrationally excited N₂(X ¹Σ_g⁺; v^''⩾10) molecules to again form N₂(A ³Σ_u⁺) molecules. The absolute electron density in the SLA was also measured by microwave interferometry. Its axial dependence also shows a pronounced minimum at z = 12 cm before reaching a maximum of 6×10¹⁵ m^-3 at z = 19 cm. The possible processes for this local ionization, in the absence of any electric field, could be binary collisions of the electronically excited molecules, and/or of the vibrationally excited ground-state molecules in v^''⩾30 levels.

A laser-induced fluorescence technique, in combination with optical emission spectroscopy, was applied to investigate the spatial distribution of the number densities of neutral species in an asymmetric low-pressure 27.12 MHz discharge in pure nitrogen. The primary targets of this investigation were the three nitrogen triplet states C ³Π_u, B ³Π_g and the metastable A ³Σ_u⁺ each in their lowest vibrational levels v = 0,1,2 and additionally v = 8 of A ³Σ_u⁺ which can be populated very efficiently in the plasma sheath. The absolute number densities of all these states were determined in a Rayleigh scattering experiment, the results of which have been published elsewhere (Krames B, Glenewinkel-Meyer T and Meichsner J 2001 J. Appl. Phys.89 3115). The spatial profiles as a function of the discharge parameters were studied by varying the gas pressure between 10 and 100 Pa and the effective rf voltage up to 260 V. These profiles of the different molecular states vary markedly in the plasma bulk as well as in the sheath near the powered electrode. A model, which includes the production, diffusion and quenching rates of contributing species and also fluorescence cascade processes, explains these varieties. A comparison with the experimental data at 50 Pa confirms the results of the model calculations.

In this paper we demonstrate that the excitation of OH(A²Σ⁺) in a pulsed N₂-H₂O RF discharge is produced mainly by the reaction N₂(A³Σ_u⁺,v) + OH(X²Σ⁺) →N₂(X¹Σ⁺) + OH(A²Σ⁺,v'). We then obtain in the post-discharge the quasi-nascent rotational distribution of OH(A²Σ⁺,v = 0) produced by such an energy transfer reaction, showing the probable presence of three main de-excitation paths for N₂(A) in the process. Finally, we propose to use the knowledge gained in this study to design a N₂-H₂O pulsed RF discharge as a calibrated source of OH.

The high sensitivity cavity ringdown spectroscopy (CRDS) absorption technique is used to measure the absolute density of the ground state nitrogen molecules in the v'' = 18 vibrational level in a dc glow discharge. The experimental conditions are 2.3 Torr nitrogen pressure, 100 mA current and a discharge tube of 3.6 cm diameter and 80 cm length. The excimer-pumped dye laser is tuned on the Lyman-Birge-Hopfield, N₂ (a ¹Π_g; 8-X ¹Σ_g; 18) band at 227 nm and absorption rates on spectrally-resolved rotational lines are obtained. The gas temperature deduced from the rotational distribution is 500 K. We measure a total density of (9±3.5)×10¹⁹ molecules m^-3 in the v'' = 18 vibrational level, equivalent to 0.2% of the total nitrogen density. So high density suggests that the v'' = 18 level should be located in the Treanor plateau of the vibrational distribution function. We also analyse the validity domain of the CRDS measurements, regarding the optical saturation due to the high intensity of the laser radiation inside the CRDS cavity and depletion of the absorbing species. If, for our measurements, the linear absorption conditions are fulfilled, we show that very often the optical saturation becomes dominant, inducing a non-single exponential decay of the ringdown signal and leading to an underestimated absorber density.

The role of atomic and molecular electronically excited states on the whole kinetics of an high-enthalpy nozzle flow has been examined by using a self-consistent model which couples Euler equations with appropriate master equations and with the Boltzmann equation for the electron energy distribution function (eedf). The results show that in high-enthalpy flows metastable atomic nitrogen can form structures in the eedf through superelastic collisions, partially smoothed by electron-electron Coulomb collisions.

In dielectric barrier discharges in helium-nitrogen mixtures of 1 bar the rotational temperature of the first negative system (transitions N⁺₂(B→X)) depends on the nitrogen partial pressure p_N2 and increases up to 600 K at high p_N2 while that of the second positive system (transitions N₂(C→B)) is equal to 310±10 K for all discharge conditions. This difference comes from two different classes of excitation processes: `fast' reactions mainly of He metastables and ions during or immediately after the active microdischarge, i.e. in a hot environment, and `slow' reactions of the metastable N₂(A³Σ⁺_u) state. Chemical reactions in general determine the effective lifetimes of the metastables. The long-living nitrogen metastables diffuse far away from the microdischarge area and react in the (cool) ambient gas. For the various discharge conditions we evaluated, from absolutely measured line intensities, the concentrations of N₂(A³Σ⁺_u), NO, and OH (formed as a consequence of surface reactions) as functions of the average discharge power and estimated the electron density in the discharge. The variation of the measured rotational temperature of the first negative system with p_N2 can be attributed to changes of the gas heating and, consequently, the gas dynamics in the microdischarges.

The properties and kinetics of singlet delta oxygen (SDO) metastables in discharge plasma, which are important for the development of the discharge excited oxygen-iodine laser, are discussed. A short description is given of available experimental activity in the development of SDO generators. Specific requirements to the SDO discharge generator are reported. The capability of different kinds of discharge to meet these requirements is evaluated. Issues are discussed which are important for the study of SDO kinetics in more detail, including processes involving individual electronic-vibrational states.

A model of discharges in oxygen is developed which takes into consideration the role that the long-lived metastable state O₂ (a¹Δ_g) can play in their dynamics. This enables the electron temperature (T_e) versus pressure×plasma dimension (pL) characteristic to be found over the range 1 mTorr cm to 1 Torr cm. The conclusion is reached that only at the very low pressure end and high electron densities is oxygen a recombination-dominated gas and that generally at higher pressures it is detachment dominated. In between, both recombination and detachment are important to describe the loss of negative ions. The paper brings together two separate approaches to discharges in oxygen and demonstrates that each has its own place, while at the same time making it clear that oxygen is a special case rather than an exemplar for electronegative gases in general.

The metastable Xe(6 ³P₂) and resonance Xe(6 ³P₁) particle number densities have been studied in the positive column of a pulsed glow discharge in a mixture composed of He at pressure p_He = 2.5 Torr and Xe at p_xe = 0.05 Torr. Time-resolved measurements were performed in the active discharge phase and in the afterglow with a laser absorption technique. The non-stationary balance equation system for populations of excited states, electron density and temperature has been solved to describe the decay of the Xe(6 ³P₂) state density during 1000 µs and that of Xe(6 ³P₁) state during 40 µs in the afterglow. If only cooling of electrons in elastic collisions is considered then a fast decrease of the electron temperature to that of the neutral gas within 10 µs and a slow decay of the excited atoms, which does not correspond to the experimentally observed decay curves, are obtained. The population and decay of excited states are to a large extend influenced by the intermixing process due to electron collisions, with a rate coefficient exponentially depending on the electron temperature in the range from 300 to 1000 K. Additional heating of slow electrons in collisions with fast ones appearing in chemo-ionization and superelastic collisions maintains the electron temperature at a level of about 1000 K over hundreds of microseconds. This leads to a faster decay of the metastable atoms. The temporal course of electron temperature in the afterglow was analysed taking the heating mechanism of slow electrons into account. The experimental curves of the decay of metastable and resonance atoms at discharge currents of 20-200 mA were interpreted.

Electrons backscattered from a scanning tunnelling microscope operating in the field emission mode have been collected to produce images of a rough Si(111) surface. We have obtained a spatial resolution of about 40 nm in such images. Comparison between backscattered electron images and topographic images reveals that edge enhancement and shadowing are important contrast mechanisms.

We report the observation of a low-temperature secondary growth orientation in SrRuO₃ thin films grown on LaAlO₃ substrates by pulsed laser deposition. This growth phase is present in crystalline films grown at substrate temperatures below ~600 °C and occurs in combination with the standard (110) oriented growth observed in films grown at higher temperatures. This secondary growth orientation does not occur in films of CaRuO₃ grown on LaAlO₃ at the same temperatures, presumably due to lattice mismatch considerations. We report for the first time the effects of the change in growth temperature from 200 to 800 °C on the orientation, lattice parameter, mosaicity and resistivity of both SrRuO₃ and CaRuO₃ films. We also discuss the dependence of the ferromagnetic transition temperature in the SrRuO₃ thin films on the growth temperature, as determined from the temperature dependence of the resistivity.

Polycrystalline CdS_xTe_1-x thin films deposited using a novel procedure based on the close spaced sublimation (CSS) method were characterized through thermoelectric power (α) measurements. The results revealed that these types of compounds present mixed conductivity and that their net conductivity becomes n-type for S concentrations greater than 50% (x>0.5) and p-type when the S content is less than 50%.

The results were interpreted with the help of theoretical calculations of α versus T, carried out using a model which includes scattering processes inside the grain and in the grain boundaries as well as dimensional effects. Comparing experimental values of α versus T with the theoretical calculation, it was found that the electrical transport in the CdS_xTe_1-x thin films is mainly affected by the interaction of electrons and holes with acoustic phonons and by scattering processes in the intergrain regions. The surface and bulk thermal conductivity also influence the electrical transport of the CdS_xTe_1-x films.

A large magnetic entropy change (6.5-8 J kg^-1 K^-1) has been discovered in (Pr_1-xNd_x)_0.5Sr_0.5MnO₃ (x = 0, 0.3, 0.5, 0.7, 1) under a low magnetic field of 10 kOe at the charge-ordered state transition temperature. It exceeds the magnetic entropy change (3.1 J kg^-1 K^-1) in Gd at 293 K by a factor of two under the same conditions. The physical mechanism is related to the drastic change of magnetization with temperature at the field-induced magnetic, electron and structural phase transition temperature (from antiferromagnetic charge-ordered state to ferromagnetic charge-disordered state).

A new polarization insensitive neutron waveguide is proposed and realized experimentally for the simultaneous guiding of two neutron polarizations. The basic idea is the combination of thin film and anti-resonant reflecting waveguides into a single structure. With a special transmitted beam technique two guided modes pertaining to a single polarization are observed in good agreement with theoretical predictions.

We investigated the sequential generation characteristics of partial discharge (PD) pulses in SF₆ gas under the application of a positive dc voltage, in order to discuss the PD mechanism based on space charge behaviour and corona stabilization effect. Highly time-resolved measurement of PD current pulses enabled us to discuss the relationship between the PD charge q and the time interval Δt of sequential PD pulses. Experimental results revealed that q-Δt characteristics exhibited negative and positive correlation at small and large Δt regions, respectively. Numerical calculations of ion diffusion and ion drift verified the time-dependent characteristics of the space charge behaviour and the resultant corona stabilization effect.

The interaction of a 1 ps laser pulse of moderate contrast ratio (~10⁴) with a Cu target is investigated at laser intensities I of up to 3×10¹⁶ W cm^-2 with the use of corpuscular, x-ray and optical diagnostics. Intensity-dependent characteristics of ion and x-ray emission as well as reflectivity, electron temperature and average charge state of plasma produced by the laser pulse are presented and discussed. It is found that (a) the specular reflectivity dependence on intensity for a moderate-contrast ultrashort (⩽1 ps) pulse is qualitatively and quantitatively different from that for a high-contrast ultrashort pulse; (b) the scaling laws for parameters of thermal ions are essentially different from those for fast ions; (c) the soft x-ray yield scales as I^α with α≈2-2.5 in the intensity range 10¹⁴-3×10¹⁶ W cm^-2; and (d) electron temperature scales as I^0.44 at 10¹⁵<I<3×10¹⁶ W cm^-2.

The carbon gun is probably the most common plasma source used in plasma opening switches. Nevertheless, it either produces a contaminated plasma, as the flashover surface erodes, or requires regular treatment with graphite paint. The novel form of the plasma gun described in this paper overcomes the disadvantages of existing designs and produces a cleaner plasma. Experimental results illustrate the performance of a prototype system.

An extended-field (EF), two dimensional (2D) model formulation is proposed for inductively coupled plasma. By extending the calculating domain of the electromagnetic (EM) field outside of the plasma discharge region, the boundary conditions of vector potential used by the standard (ST) 2D model are replaced by simpler far field boundary conditions. The extended model converges faster than the standard formulation and gives rise to consistent solutions throughout the computational domain. Vector potential equations are solved with corresponding continuity, momentum, and energy transfer equations using the commercial code `FLUENT'. The computational domain for vector potential equations are extended well beyond the induction coil region, while for all the other equations, computations are limited to the discharge region inside the plasma confinement tube. The computational results are compared with those obtained using the ST 2D model. The difference between the results of the two models is noted mostly in the entrance regions of the flow, and close to the induction coil. To validate the EF model, a load with constant electric conductivity is placed centrally in the coil region and the calculated radial profile of the axial magnetic field is compared with existing analytical solutions. The results are in good agreement within an uncertainty of 1%.

By using a 3 electron-group model to describe the deviation from a Maxwellian electron energy distribution, a collisional radiative model describing a low temperature Ar-Hg plasma is greatly improved. Previously, the ionisation mechanisms of such plasmas, commonly used in fluorescent lamps, could not be satisfactory modelled. Where using a Maxwellian electron energy distribution showed the production of argon ions to be dominating over the production of mercury ions, the 3 temperature approximation yields a mercury ionization rate which is 30 times larger than the argon ionization rate.

The cross section and the current density at the two ends of a positive column, measured as functions of pressure, were found to be different in glow discharges operated between copper electrodes in air, nitrogen and helium in the pressure range of 100-760 Torr and at an electrode distance of 7 mm. Taking into consideration the occurrence of the dissociative recombination of positive molecular ions, the observed difference is explained by the different pressure dependences and magnitudes of the cathode and anode falls, which determine the current densities at these places.

CF₄ radio-frequency (rf) plasmas are often driven in time-modulation modes to obtain higher etching performance. For dynamic and stable control of CF₄ plasmas, we investigated the transient behaviour of the plasmas after step up or down of the amplitude of the power source voltage V_s(t). Using a one-dimensional fluid model, we simulated a CF₄ capacitively coupled plasma that transfers from a periodic steady state to another at 0.2 Torr in a 20 mm parallel-plate gap. In addition, we compared such plasma responses to the step down to 0 V at the phases at which V_s(t) takes zero and peak values. Fundamental properties of transient CF₄ plasmas were discussed.

Optical emission spectroscopy in the range 200-800 nm was applied for investigation of the copper plasma produced by a metal vapour vacuum arc plasma source. The experiments were conducted for the cases when the plasma was guided by straight and Ω-shaped curved solenoids as well as without solenoids, and also for different vacuum conditions. It was found that, besides singly- and doubly-charged ions, a relatively high concentration of excited neutral copper atoms was present in the plasma. The relative fraction of excited atoms was much higher in the region close to the cathode surface than in the plasma column inside the solenoid. The concentration of excited neutral, singly- and doubly-ionized atoms increased proportionally when the arc current was increased to 400 A. Some weak lines were attributed to more highly ionized copper species and impurities in the cathode material.

We present a new technique for imaging electric fields that takes advantage of the large field-dependent coupling of the Rydberg states of Hg. Specifically we demonstrate an optical scheme that takes advantage of the field-induced mixing of the 6s(² S)nd D and 6s(² S)(n + 1)p P states of Hg. With the appropriate laser probe of this mixing, 579.2 nm light is produced that is directly related to the strength of the local electric field. This allows strobe images to be made of the electric field surrounding an object on a 50 ps timescale. We demonstrate the utility of this technique by probing the electric field surrounding pads on a digital integrated circuit package.