Metastable Atoms Research Articles

Linear and nonlinear coherence transfer of resonant nuclear-spin-dressed effect via metastability-exchange collisions

We present a systematic analysis to explain the observed magnetic resonance spectrum with three or five peaks spaced at Rabi frequency when the metastable 3He atoms are probed with a transverse circularly or linearly polarized 1083 nm laser beam, respectively. The resonance signals are generated in the ground state of 3He atoms, dressed by a resonant magnetic field, and transferred to the metastable state by collisions. The triplet spectrum is thought to be the result of the linear coupling between the metastable state and the ground state, while the quintuplet is thought to be that of the nonlinear coupling, and indicates that higher-order atomic polarization is created during the atomic collisions. More features of the magnetic resonance signals are investigated.

Damage to porous SiCOH low-k dielectrics by O, N and F atoms at lowered temperatures

Information on the degradation of porous organosilicate glasses (OSGs) by active radicals and atoms is of high importance for their integration as low-k dielectrics in the next generation of ultra large scale integration (ULSI) production. The films’ degradation is caused by depletion of coverage of the pore surface methyl. OSG samples with differing porosities and pore sizes were treated by O, N, and F atoms at lowered temperatures (down to −45 °С) downstream of O2, N2, and SF6 inductively coupled plasma discharges, respectively. It has been shown that lowering the temperature reduces film degradation. In the case of O atoms, this reduction is insignificant, while the effect is much more noticeable for F atoms. In addition, an accumulation of F atoms forms a fluorocarbon layer during F atom treatment. The accumulated fluorine can interact with surface Si atoms, giving rise to film etching. The film degradation under O and F atoms increases with pore size and porosity, due to deeper atom penetration. In the case of N atoms, even a small temperature reduction essentially decreases OSG degradation, while –CH3 modification is less clear in the cases of O and F atoms. Density functional theory and ab initio molecular dynamics simulations of reaction mechanisms show that atom reactions with surface Si–CH3 groups are the initial stages of OSG degradation. Subsequent mechanisms include branched reaction pathways with the formation, modification, and destruction of different surface groups. In the cases of O and F atoms, these reactions lead to the formation of HxCО and CFx bonds. For the direct reaction of N(4S) atoms with –СН3 groups, significant activation energy is required. Under downstream plasma conditions, –СН3 groups are destroyed in reactions with metastable particles, and first of all, with metastable atoms N(2D), N(2P) and molecules , which are also produced inside the pores in the surface recombination of nitrogen atoms.

Detecting Magnetic Fields in Exoplanets with Spectropolarimetry of the Helium Line at 1083 nm

Abstract The magnetic fields of the solar system’s planets provide valuable insights into their interiors and can have dramatic consequences for the evolution of their atmospheres and interaction with the solar wind. However, we have little direct knowledge of magnetic fields in exoplanets. Here we present a method for detecting magnetic fields in the atmospheres of close-in exoplanets based on spectropolarimetric transit observations at the wavelength of the helium line at 1083 nm. This methodology has been successfully applied for exploring magnetic fields in solar coronal filaments. Strong absorption signatures (transit depths on the order of a few percent) in the 1083 nm line have recently been observed for several close-in exoplanets. We show that in the conditions in these escaping atmospheres, metastable helium atoms should be optically pumped by the starlight and, for field strengths more than a few × 10−4 G, should align with the magnetic field. This results in linearly polarized absorption at 1083 nm that traces the field direction (the Hanle effect), which we explore by both analytic computation and the Hazel numerical code. The linear polarization ranges from ∼10−3 in optimistic cases down to a few × 10−5 for particularly unfavorable cases, with very weak dependence on field strength. The line-of-sight component of the field results in a slight circular polarization (the Zeeman effect), also reaching . We discuss the detectability of these signals with current (SPIRou) and future (Extremely Large Telescope) high-resolution infrared spectropolarimeters, and we briefly comment on possible sources of astrophysical contamination.

Two-dimensional simulation and experiments of helium discharge between parallel-plate electrodes in short gaps at atmospheric pressure

ABSTRACT Helium gas is used as the coolant in high temperature gas-cooled reactor while its insulation property is not good and brings big challenges to insulation design work. In this paper, breakdown voltages of two parallel-plate electrodes in millimeter-scale gaps at atmospheric pressure are calculated numerically with a two-dimensional hydrodynamic model. The Finite-Difference Flux-Corrected Transport method (FD-FCT) is used to avoid spurious oscillation caused by convection and diffusion restrictions. The simulation results are in good agreement with our experimental work. Different discharge characteristics are presented for a breakdown or a non-breakdown voltage. Importantly, when a breakdown is applied, ground-state ionization will make up the most in early stages while later accumulation of metastable atoms and molecules will prompt fractions of penning ionization to increase rapidly. It is predicted that penning ionization will eventually become comparable with ground-state ionization, especially near the anode, as the current density keeps increasing to order of 1 A. Some other simulation results are presented such as time evolutions of the distribution of electrons, ions, field strength and axial potential, etc. Attainment of these breakdown voltages and characteristics of helium discharge is going to help with insulation design work of electric equipment in reactor engineering.

Two-body collisions in the time-of-flight dynamics of lattice Bose superfluids

We investigate two-body collisions occurring during the time-of-flight expansion of interacting three-dimensional lattice Bose superfluids. The number of collisions is extracted from the observed s-wave scattering halos located between the diffraction peaks of the superfluids. These faint halos can be monitored thanks to the large dynamical range in densities associated with detecting individual metastable Helium atoms. We monitor the number of collisions as a function of the total atom number and of the amplitude of the lattice, in a regime where the number of trapped atoms per lattice site is large. In addition, we introduce a classical model of collisions that quantitatively describes the experiment without adjustable parameters. Finally, the present work validates quantitatively the assumption of a ballistic expansion when investigating the Bose-Hubbard Hamiltonian with a unity occupation of the lattice.

Double Electric Layer of a Helium Discharge in Modulation Mode

The positive column of a helium gas discharge in modulation mode has been investigated. It has been revealed that the contribution of the stepwise ionization to population of the 31D2 level is low due to small population of metastable levels at weak discharge currents (20–100 mA). With increasing discharge current, the metastable atom population increases and hence, the modulation depth of spectral line intensities of metastable levels also increases. At high currents (>100 mA), the modulation depth of spectral line intensities decreases because of saturation of metastable level population.

Transversely optically pumped Ar:He laser with a pulsed-periodic discharge.

Optically pumped rare gas lasers have the potential for scaling to high-power cw systems with good beam quality. Metastable atoms of heavier rare gases that are the lasing species are produced in an electric discharge at near atmospheric pressure. The key problem for this class of lasers at present is the development of a suitable discharge system. In this paper, we present the results of optimization of a pulsed discharge system with the goal of minimizing cathode sputtering and peak discharge current. The first demonstration of a transversely pumped system and measurements of the optical pumping threshold for the Ar:He laser are also presented.

A combination of plasma diagnostics and Raman spectroscopy to examine plasma-graphene interactions in low-pressure argon radiofrequency plasmas

Graphene films were exposed to low-pressure capacitively coupled (E-mode) and inductively coupled (H-mode) argon radio frequency plasmas to investigate damage formation by very-low-energy ion irradiation. In the H-mode, plasma parameters were assessed by a Langmuir probe and plasma sampling mass spectrometry to determine the conditions of fixed ion fluence but with different average ion energies. The populations of argon metastable and resonant argon atoms were also measured by optical absorption spectroscopy to determine their contribution to the total energy flux during plasma treatment. In the H-mode, in which plasma-graphene interactions are dominated by ion irradiation effects, Raman spectroscopy reveals a significant rise in the D/G ratio and full width at half maximum of the G peak as well as the onset of graphene amorphization, even at very low ion energies (between 7 and 13 eV). In the E-mode characterized by comparable ion energy but much lower ion density, significant damage is also observed, a feature ascribed to the additional energy flux linked to the de-excitation of metastable argon species on the graphene surface.

Temporal electron collisions and adjunct ionization mechanisms in the positive column of a flowing N2 DC discharge

We studied a flowing nitrogen DC discharge by Langmuir double probes and optical emission spectroscopy (OES). We measured the reduced electric field (E/N) and electron density (ne) with the double probes. The gas (Tg) and N2(X1Σ+g) vibrational temperature (θ) were measured by OES from the nitrogen first and second positive systems transitions. We constructed a kinetic numerical model accounting for N2(X1Σ+g, 0 ≤ v ≤ 45), N2(A3Σ+u), N2(B3Πg), N2(a1Πg), N2(a′1Σ−u), N2(w1Δu), N2(a″1Σ+g), N2(W3Δu), N2(E3Σ+g), N2(C3Πu), N2+(X2Σ+g), N2+(B2Σ+u), N+, N3+, N4+, N(4S), N(2D), and N(2P) states. The species densities are calculated as functions of the discharge gas residence time in pure nitrogen for our experimental conditions. The kinetic model assumes as input parameters the experimental discharge gas pressure (p), tube radius (R), electron density (ne), gas temperature (Tg), reduced electric field (E/N). In this Letter, the kinetic model is applied to calculations of the fractional contribution of each ionization mechanism to the total ionization generated in the discharge. We compute 24 different ionization mechanisms to the five ions species. We have observed that ionization is generated by different physical-chemical reactions depending on the discharge gas residence time. It changes from dominant electron impact ionization process in the earlier times of the discharge to adjunct ionization mechanisms depending on the pooling of N2(a′1Σ−u) and N2(A3Σ+u) metastable molecules and associative ionization of N2(a′1Σ−u), N(2D) and N(2P) metastable atoms and molecules. Moreover, we have observed the occurrence of approximate charge neutrality for our studied experimental condition: discharge current of 30 mA, gas flow rate of 1 Slm−1, and gas pressure of 230 Pa. Also, the kinetic model fits quite well the measured N2(X1Σ+g) vibrational temperature at the end of the positive column.

On the Selectivity of Population of the Neon Excited Levels in the Decaying He–Ne Plasma

The results of a spectroscopic study of the afterglow of a pulsed discharge in a He–Ne mixture are presented, showing the unique selective population of the upper level of the 2p54p configuration (3p1 in Paschen’s notation) of a neon atom at a helium pressure of tens of Torr. The main measurements were conducted under conditions of competition between the excitation transfer from metastable He atoms (21S0) and the dissociative recombination of HeNe+ ions with electrons as sources of excited atoms. Helium pressure PHe ≈ 0.08–20 Torr, neon PNe ≈ 0.0005–0.003 Torr, the electron density [e] < 1011 cm–3. The results of the experiment indicate the existence of a mechanism that forms with an increase of helium pressure such a distribution of populations over 2p54p levels, in which more than 60% of the radiation flux of 2p53s ← 2p54p transitions is concentrated in one spectral line of 352.0 nm.

Scalable microplasma array for argon metastable lasing medium

Optically pumped rare gas laser systems require a lasing medium of high density metastable atoms. A scalable two-dimensional array of quarter-wave resonators produces high microwave electric fields suitable for generating a dense gas of argon 1s5 states. This resonator array is immersed within an evacuated waveguide in order to create a volume of plasma in a relatively wide discharge gap (13 mm) at argon-helium pressures up to 100 Torr using 30 W of continuous microwave power at 2.3 GHz. Spatial density profiles of argon 1s5 states are measured using diode laser absorption spectroscopy with 0.5 mm resolution. The region of argon 1s5 states is mapped in the discharge and shown to have an estimated density in excess of 1×1018m−3 within a volume of 11.6×3.2×0.8mm3.

Sub-Kelvin Stereodynamics of the Ne(^{3}P_{2})+N_{2} Reaction.

We present an experimental study of the low-energy stereodynamics of the Ne(^{3}P_{2})+N_{2} reaction. Supersonic expansions of the two reactants are superposed in a merged beam experiment, where individual velocity control of the two beams allows us to reach average relative velocities of zero, yielding minimum collision energies around 60mK. We combine the merged beam technique with the orientation of the metastable neon atoms and measure the branching between two reaction channels, Penning ionization and associative ionization, as a function of neon orientation and collision energy, covering the range 0.06-700K. We find that we lose the ability to orient Ne below ≈100 K due to dynamic reorientation. Associative ionization products Ne-N_{2}^{+} predissociate with a probability of 30%-60% and that associative ionization is entirely due to reactions of the Ω=2 state, where the singly occupied p orbital of the Ne^{*} is oriented along the interatomic axis.

Influence of N2, O2, and H2 admixtures on the electron power balance and neutral gas heating in microwave Ar plasmas at atmospheric pressure

A combination of optical emission and absorption spectroscopy of argon 2p –1s transitions (Paschen notation) combined with collisional-radiative (CR) modelling of argon 2p states was used to characterize microwave argon plasmas at atmospheric pressure in presence of N2, O2, and H2 admixtures. In particular, the neutral gas temperature (obtained from the broadening of argon 2p 2–1s2 and 2p 3–1s2 emission lines), the number density of argon 1s5 atoms (obtained from absorption spectroscopy of the argon 2p 9–1s5 transition using a tunable laser diode), the electron temperature (obtained from the comparison between measured and simulated argon 2p -to-1s relative line emission intensities), and the electron density (obtained from the Stark broadening of the Hβ line and argon relative line emission intensities) were recorded as a function of the axial distance along the microwave plasma column. The results show that, for a given position in the plasma and a higher amount of admixture in the nominally pure argon plasma, the neutral gas temperature increases and the electron number density decreases, while the electron temperature and the population of argon metastable atoms first decreases and then increases at higher concentrations. With such information, a detailed analysis of the electron power balance was performed. It is found that less than 1% of the admixture in the argon plasma already absorbed more than 80% of the microwave power. Part of this energy is used for neutral gas heating, mostly through electron-impact excitation of rotational levels.

Cathode spot formation possibly explained by cathode electron emission from impact of excited state atoms

A theory is given for electron emission from electric arcs with non-thermionic cathodes, for example the iron cathodes used in arc welding, proposing that the principal process for electron emission is from the impact of excited atomic states at the cathode. For plasmas in contact with a metal cathode, a sheath of positive ions is formed over the cathode surface because electrons move away from the cathode. A sheath thickness of only 0.1 µm causes secondary electrons produced at the cathode surface to gain ~12 V. The electron transit through this thin sheath is collision-less. These 12 V electrons produce excited states of atoms to a distance of approximately ~10 µm from the cathode. Diffusion and direct transfer of these excited atoms back to the cathode in the almost collision free region next to the cathode is calculated to be in times of ~10−8 s. This time is less than the radiative lifetimes of almost all atomic transitions except for some resonance transitions where there would be an enhancement of lifetimes due to imprisonment of radiation. If these transfer times are less than the radiative lifetimes, secondary emission coefficients of electrons from the cathode are likely to be approximately 1, the measured coefficients for metastable atoms. The energies of these excited atoms are ~12 V or more, much larger than the usual work function of ~4.5 V required to eject electrons. If such secondary electrons, together with secondary electrons produced from the impact of positive ions and photons, replace or exceed the original number of electrons, amplification is possible in successive avalanches to cause electron densities to grow to supply the total arc current. The mechanism would then explain why observed cathode sheath voltages are independent of arc current for currents varying from 10 to 20 000 A for various low melting point electrodes and arc gases. Electron emission from cold cathodes, when temperatures are too low for thermionic emission, would also be explained. Solutions of the densities of electrons, positive ions and excited state atoms as a function of time and distance from the cathode are presented which give a physical explanation for the formation of cathode spots.

NON-THERMAL PLASMA AT ATMOSPHERIC PRESSURE AND ITS OPPORTUNITIES FOR APPLICATIONS

The subject of this review is the low-temperature (or "cold") weakly-ionized but strongly non-equilibrium plasma created at atmospheric pressure in gaseous mixtures or directly in atmospheric air. Cold plasma is rather new, but very perspective object. The strong non-equilibrium of the weakly-ionized plasma leads to that energetic electrons despite their small quantity very effectively excite and dissociate the neutral particles which are contained in surrounding gas, for example, of a molecule of oxygen and water. The pointed above property of cold plasma is valuable from the practical point of view because it allows creating in plasma-forming gas rather intensive ultra-violet radiation and high concentration of physically and biochemically reactive species (metastable atoms and molecules, radicals, ozone, and others) with rather small specific energy consumption. Now the usage of cold plasma at atmospheric pressure gives the opportunity to solve many practical problems which were earlier seeming unsolvable. It is possible even to claim that the approaches based on the use of cold plasma in dense gases define modern progress in many fields of science, biomedicine and, in particular, in the field of chemical technology. The review of modern experimental methods of creation of the cold plasma at atmospheric pressure is given. Physical and chemical features of cold plasma in dense gases have been considered. Special attention is paid to the kinetics of the charged particles in non-equilibrium plasma and the vibrationally excited molecules as well. Additionally, the kinetics of the electronic excited and metastable states is taken into account because they also influence a biochemical activity of low-temperature plasma. A lot of places is given to concrete examples of the modern practical use of such plasma in ecology for the destruction of the low-concentrated harmful organic and inorganic impurities in the exhausted airflows at atmospheric pressure.

Spatio-temporal distribution of absolute densities of argon metastable 1s5 state in the diffuse area of an atmospheric pressure nanosecond pulsed argon microplasma jet propagating into ambient air

Atmospheric microplasma jets (MPJs) sustained in rare gases have gained increased attention due to their potential to generate highly reactive species. In this paper, we present space- and time-resolved argon metastable densities, Ar(1s5), measured in an atmospheric pressure MPJ operated in Ar and propagating into ambient air using tunable diode laser absorption spectroscopy (1s5 → 2p9 optical transition). The MPJ was produced using a dielectric barrier discharge energized by short duration (230 ns) high-voltage positive pulses (4.2–6.2 kV) at a repetition frequency of 20 kHz. The spectral absorption line profile was recorded and allowed measurements of the absolute metastable Ar(1s5) density integrated in the line-of-sight of the laser beam under various operating conditions of the MPJ. The results reveal a sensitive dependence of the Ar(1s5) density on spatial coordinates, i.e., distance from the exit of the capillary tube of the discharge and from the axis of the argon jet. The highest Ar(1s5) densities of about 3 × 1013 cm−3 were measured at the axis of the argon jet at longitudinal distances between 4 and 5.5 mm downstream from the nozzle of the tube. The temporal distribution of the Ar(1s5) density, which presents three maxima, is thoroughly discussed in this paper. The spatial distribution of the effective Ar(1s5) lifetime (&amp;lt;250 ns) is also reported, giving some insight into the surrounding environment of the argon metastable atoms. The determined spatiotemporal distributions of the Ar(1s5) density can be useful for the optimization of argon MPJs for different applications like surface or biomedical processes.

Emission and absorption diagnostics of a diffuse dielectric barrier discharge with multiple current peaks in helium at atmospheric pressure

Dielectric barrier discharge (DBD) is a simple means to sustain low-temperature diffuse discharges at atmospheric pressure. As atmospheric pressure plasma processing requires DBDs to work under various operating conditions, even diffuse discharges are often observed to exhibit multiple current peaks. In this work, multiple current peaks helium DBD is obtained by adding small concentration of argon in a nominally pure helium discharge. As the concentration of argon increases, multiple current peaks emerge while the discharge remains diffuse. Absorption spectroscopy is used to record the evolution of the He(23S) metastable atom. In a nominally pure helium discharge He(23S) metastable atom can reach up to 6 × 1011 cm−3. Even if quenching of the He(23S) metastable atom by argon is very effective, He(23S) density is found to still rise above 1010 cm−3 in a multiple current peaks discharge with 1100 ppm of argon. Optical emission spectroscopy of He n = 3 levels is also used to gain insights about the electron temperature (Te) through the means of a collisional-radiative model. For the multiple current peaks discharge with 1100 ppm of argon concentration, Te reaches about 1.0 eV during the primary peak but only about 0.3 eV during the secondary peaks.

Synaptic learning behavior of a TiO2 nanowire memristor

TiO2 nanowire memristors were fabricated by dielectrophoresis. The responding current of the memristor continuously increases and decreases with the consecutive positive and negative sweep voltage, which is similar to the nonlinear transmission characteristics of biological synapses. Spike-rate-dependent plasticity and learning behaviors of TiO2 memristor were studied by applying programmed pulses. The pulses with higher amplitude, bigger width and smaller interval cause a larger excitatory postsynaptic current. The number of relearning pulses is decreased with the learning experience, and a deepening memory will be consolidated by the repeated learning process. A mechanism based on the oxygen vacancy migration is proposed for the learning behavior. Excess oxygen vacancies are generated during the learning process and the conducting pathways are formed by the vacancy drift under the applied voltage. After removing the voltage at the forgetting process, back diffusion and electron trapping of the oxygen vacancies dominate the relaxation time, and the metastable atoms are formed with the involvement of the oxygen atoms. However, weak chemical bonding among the metastable atoms leads to the migration of the regenerated oxygen vacancies again, contributing to the enhanced current in the relearning process.

Time-dependent simulations of a CW pumped, pulsed DC discharge Ar metastable laser system.

Optically pumped rare gas lasers have the potential for scaling to output powers above the kW level. In these devices, electrical discharges through He/Rg mixtures (Rg = Ne, Ar, Kr and Xe) are used to generate metastable Rg atoms in the 1s5 state. Optical pumping to the 2p9 level, followed by collisional relaxation to 2p10, is then used to produce lasing on the 2p10-1s5 transition. Several computational models have been developed to analyze CW systems using steady-state approximations for the discharge excitation, optical pumping and lasing processes. However, recent experiments show that repetitively pulsed discharges have advantages for producing larger volume, high-pressure discharges. Here we present dynamic simulations of a CW laser that uses pulsed-discharge production of Ar metastables. Time-dependent equations are solved for both the discharge and lasing process. Two models are investigated. The first considers the conditions within the lasing medium to be spatially uniform (zero-dimensional model). The second allows for spatial variations along the lasing axis (one-dimensional model). The models were evaluated by simulating the performance characteristics of an experimentally demonstrated system that provides time-averaged output energies in the range of 3-4 W. Time-dependent species densities, laser power and longitudinal spatial distributions are presented.

Enhancement of O-atom density through collisions with Ne by laser-produced plasma in Ne–O2 gas mixtures and possible energy transfer mechanism

Laser-produced plasma in Ne and O2 gases and Ne–O2 gas mixture with different O2 fractions have been investigated. The plasma were produced by focusing a laser beam of 5 ns pulse duration from a Nd:YAG laser ([Formula: see text] = 1064 nm) through a lens of 5 cm focal length into the chamber filled with different gas species at different pressures. Possible enhancement in O-atom densities through collisions with metastable Ne atoms by the laser-created plasma in Ne–O2 gas mixtures and possible energy transfer mechanism was studied. Study of energy transfer through collisions was carried out by comparisons of the line intensities of the emitted spectra from laser-created plasma in gas mixtures and pure gases with the technique of optical emission spectroscopy (OES). Strong evidence of energy transfer from Ne to O was recorded. From the result, it was suggested that the high-lying metastable levels of Ne were responsible for enhancement of O-atoms density in gas mixture and Ne also play important role in energy transfer mechanism through collision. Also, behaviors of the line intensities under different laser energies were studied. Behaviors of line intensities and line widths were the main parameters measured. Stark broadening of lines in Ne with different laser energies was recorded. The electron temperature T[Formula: see text] and number density N[Formula: see text] were found to be 1.01 eV and 9.44 × 10[Formula: see text] cm[Formula: see text] in Ne at different laser energy/pulse with total pressure of 1000 mbar.