Plasma Recombination Research Articles

A revised formula for 3-body recombination that cannot exceed the unitarity limit

We discuss the recent developments in theory and experiment that bear on our understanding of three-body recombination at ultracold temperatures. Some of these predictions include the fourth-power dependence of the three-body rate on the two-body scattering lenght a, the appearance of quantum mechanical shape resonance and Feshbach resonances, and the existence of destructive interference minima at large positive scattering lenghts. At very large scattering lenghts, however, the a 4 dependence of the recombination rate eventually will exceed the unitarity limit. We propose in this paper a simple way to correct this problem, permitting an approximate extension of previous predictions to slightly higher energies above the three-body fragmentation threshold. A preliminary comparison is carried out with a very recent experiment by Grimm and coworkers, showing reasonable agreement.

Light emission from Al/HfO2/silicon diodes

The metal–insulator–silicon light-emitting diode (MIS LED) using a high-dielectric-constant material (HfO2) is studied. The external quantum efficiency for light emission at room temperature from the MIS LED was observed to be 2.0×10−6, as compared to 0.5×10−6 for the metal–oxide–silicon (MOS) LED. The large hole concentration at the Si/HfO2 interface created by the high dielectric constant of HfO2 may be responsible for the enhancement. The emission line shape of the MIS LED can be fitted by the electron-hole plasma recombination model, similar to the MOS LED. The Al/HfO2/silicon LED with a high interface trap density has a continuous spectrum below the Si gap beside the electron-hole plasma emission, probably due to the radiative recombination between the electrons and holes via the interface states.

Existence and Stability of Cold Rydberg Gases and Plasmas

Recently, the spontaneous evolution of an ultracold gas of Rydberg atoms into a plasma has been demonstrated experimentally, as well as the reverse process of plasma recombination into atomic Rydberg states. Here, we discuss the existence and stability properties of the gas and plasma phases over a wide range of initial-state parameters.

Long-lived plasmoids as initiators of combustion of gas mixtures

A slipping surface discharge excited along a multielectrode metal-dielectric system in a chemically active (combustible) CH4-O2 mixture has been studied experimentally. Results obtained with the help of high-speed hotography, shadowgraphy, optical spectroscopy, and piezoelectric measurements are presented. It is hown that the slipping surface discharge is a source of high-temperature, thermally equilibrium metal plasmoids hose lifetime is much longer than the plasma recombination time (long-lived plasmoids). The experimental investigation of the evolution of plasmoids introduced into a combustible gaseous medium allows one to conclude that the medium significantly increases the lifetime of plasmoids and that plasmoids, in turn, play an important role in the initiation of gas-mixture combustion.

Stimulated emission from ZnO–SiO 2–Si thin film nanoresonators obtained by magnetron sputtering method

Investigations of the spontaneous and stimulated emission spectra by optical pumping of ZnO layers deposited on silicon oxide were carried out. The stimulated emission pumped under ultraviolet 337 nm N 2 laser excitation was observed at 397 nm at room temperature from ZnO–SiO 2–Si type thin film structures. The threshold pumped power for the electron-hole plasma recombination laser process is of the order of 35 MW/cm 2.

Energy loss rate for guiding-center antihydrogen atoms

Collisional drag between a bound positron and a background positron plasma is considered as a mechanism for guiding-center antihydrogen atoms to relax to deeply bound states. Contrary to previous assessment, an adiabatic cutoff to the drag is predicted at deep binding, when the bound positron’s E×B drift speed vd exceeds the plasma positron thermal speed. In this regime, small-impact parameter collisions neglected in the drag calculation become the dominant 3-body recombination mechanism. At shallow binding, when ξ=vd/v̄≪1, the atom’s energy loss rate due to drag scales like ξ3/2 log2 ξ. When ξ≫1 the adiabatic cutoff takes over and the rate scales as ξ7/6 exp(−32(2ξ)2/3). The adiabatic cutoff implies that collisional drag can only assist positron–antiproton recombination up to a finite binding energy.

Hydrogen subordinate line emission at the epoch of cosmological recombination

The balance equations for the quasi-stationary recombination of hydrogen plasma in a black-body radiation field are solved. The deviations of the excited level populations from equilibrium are computed and the rates of uncompensated line transitions determined. The expressions obtained are stable for computations of arbitrarily small deviations from equilibrium. The average number of photons emitted in hydrogen lines per irreversible recombination is computed for plasma parameters corresponding to the epoch of cosmological recombination.

Effect of uniaxial stress on photoluminescence in GaN and stimulated emission inInxGa1−xN/GaNmultiple quantum wells

Photoluminescence and stimulated emission (SE) in a wurtzite GaN bulk crystal and ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ multiple quantum wells (MQW's) are investigated under uniaxial stress applied perpendicularly to the c axis. The strain in GaN induces a decrease in the photoluminescence intensity of B excitons relative to A excitons due to an increase in the energy splitting between the two states. In ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ MQW's, SE and optical gain in the localized states are observed at 6 K under low excitation power below the Mott density of excitons. The uniaxial stress induces a low-energy shift of the gain peak and a decrease in the threshold carrier density of SE. In the excitation power above the Mott density, the SE arises from an electron-hole plasma recombination. The gain value at 0.43 GPa is 1.34 times as large as that without stress at 6 K, which is comparable with a theoretical estimation. The observed effects of strain are ascribed to a decrease in the density of states at the valance-band maximum.

Plasma formation from ultracold Rydberg gases

Recent experiments have demonstrated the spontaneous evolution of a gas of ultracold Rydberg atoms into an expanding ultracold plasma, as well as the reverse process of plasma recombination into highly excited atomic states. Treating the evolution of the plasma on the basis of kinetic equations, while ionization/excitation and recombination are incorporated using rate equations, we have investigated theoretically the Rydberg-to-plasma transition. Including the influence of spatial correlations on the plasma dynamics in an approximate way we find that ionic correlations change the results only quantitatively but not qualitatively.

Effects of the radial plasma-wall interaction on the Hall thruster discharge

The interaction of the plasma discharge with the ceramic walls of a Hall thruster leads to plasma recombination, energy losses, and extra electron collisionality. These three phenomena are included in a one-dimensional axial model of the discharge through source terms obtained from an auxiliary model of the radial dynamics. Spatial solutions are presented for different discharge voltages and wall materials, and agree satisfactorily with experimental data. The parameters related to wall effects are investigated extensively. The energy balance among Joule heating, wall-losses cooling, and heat conduction shapes the temperature profile; three different profile types are identified depending on the wall material and the discharge voltage. For long chambers, the main source of energy losses is the plasma interaction with the walls, even for zero secondary electron emission. By contrast, wall collisionality due to primary/secondary exchanges of electrons is negligible always. The current utilization is related directly to the total energy losses. The propellant utilization is set by the balance between gas ionization and wall recombination in the acceleration region. The rate of wall recombination suggested by the axial solution is much lower than the values given by radial models based on a Maxwellian electron distribution function.

Pump–probe studies of EUV and X-ray emission dynamics of laser-irradiated noble gas droplets

The interaction of high intensity 100-ps laser pulses with micron-sized noble gas (argon and krypton) droplets is experimentally investigated via a series of pump–probe experiments monitoring the delay-dependent X-ray and extreme ultraviolet (EUV) emission, and by imaging frequency-doubled probe light scattered from the interaction region. An understanding of the time scales for this interaction is important for optimization of EUV sources for next-generation lithography that utilizes laser-produced plasmas (LPP). Depending on the spectral region of interest, the type of emission, and the droplet characteristics, the effective emission lifetime was found to extend from a few hundred picoseconds to as long as several nanoseconds, in agreement with the expected plasma expansion, EUV excitation, and recombination emission time scales.

Acoustically pumped stimulated emission in GaAs/AlGaAs quantum wells

We report on an extremely narrow linewidth of a two-dimensional electron-gas photoluminescence in GaAs/AlGaAs quantum wells pumped by ultrasonic vibrations. Below the threshold pumping amplitude, the observed emission line is weak, broad, and undergoes a redshift consistent with the evolution of an electron-hole plasma recombination in the oscillating piezoelectric fields accompanying the sample vibrations. Above the threshold, a narrow emission line arises, which sharpens up to similar to0.1-meV full width at half maximum, and gains intensity with increasing pumping amplitude. Weighing different alternatives, it is suggested that the data are best explained within the framework of stimulated emission originating from the acoustically pumped injection of enhanced electron and hole densities. (Less)

Probing excitation/ionization processes in millisecond-pulsed glow discharges in argon through the addition of nitrogen

The addition of N 2 to a millisecond-pulsed glow discharge (PGD) allows diagnostic measurements of the PGD but is found to drastically influence the transient signals arising from the argon and sputtered analyte atoms. Penning excitation between metastable argon atoms and ground state nitrogen molecules and charge transfer between argon ions and the added nitrogen reduce the degree of ionization of sputtered atoms during the power-on, plateau, period by a factor of ∼10 (at 1% N 2 by vol.). The added nitrogen affects sputtered atom emission signals less at this time because electron excitation dominates the excitation of these species. Upon power termination, afterpeak, the added nitrogen prevents plasma recombination in two major mechanisms: (i) the nitrogen reduces the number of argon ions available for recombination in the afterpeak; and (ii) vibrationally excited states of nitrogen slow the thermalization of electrons thereby decreasing recombination efficiency. The argon ion population contributes significantly to the afterpeak increase in the number of metastable argon atoms. These atoms are essential for the afterpeak ionization of sputtered atoms. Judicious selection of the nitrogen partial pressure can tune the delay time of afterpeak ionization/recombination by up to 200 μs. This could be particularly beneficial for time-resolved optical or mass spectrometric analyses.

Three-body problem in Fermi gases with short-range interparticle interaction

We discuss 3-body processes in ultracold two-component Fermi gases with short-range intercomponent interaction characterized by a large and positive scattering length $a$. It is found that in most cases the probability of 3-body recombination is a universal function of the mass ratio and $a$, and is independent of short-range physics. We also calculate the scattering length corresponding to the atom-dimer interaction.

Experimental investigations on hydrocarbon-enhanced MAR processes in low temperature plasma in divertor simulator MAP-II

Plasma recombination phenomena were observed in the interactions of hydrogen or hydrocarbon gasses with low temperature helium plasma stream along the longitudinal magnetic field, which simulated the divertor region. The reduction of the ion saturation current implies the existence of molecular activated/assisted recombination (MAR) processes. The recombination efficiency is much stronger in the methane and ethane than in the hydrogen injection case, namely, He≪H 2≪CH 4<C 2H 6. In order to clarify the dominant processes in the hydrocarbon-enhanced MAR phenomena, spectroscopy on the A 2Δ–X 2Π band of CH radical was conducted. In the electron temperature lower than about 6 eV, the dominant dissociation and emission processes of the CH radicals have qualitatively been revealed to be related to their charge exchange with bulk ions, which can be followed by the dissociative recombination with electrons.

Enhancement of Low-Energy Electron Ion Recombination in a Magnetic Field

Electron-ion recombination in cold magnetized plasmas shows a dramatic enhancement of the radiative recombination rate for bare highly charged ions relative to what standard radiative recombination rates predict. To understand the mechanism of this enhancement we investigate the classical chaotic dynamics of an electron in the combined Coulomb field of the ion and the magnetic field in the electron cooler. An increased flux of electrons visiting the vicinity of the target ion leads to an enhancement of the recombination process.

Correlation of Surface Potential, Free Carrier Concentration and Light Emission in ELO GaN Growth Domains

We have carried out spatially resolved micro-Raman spectroscopy, cathodoluminescence microscopy and scanning capacitance microscopy in order to obtain a comprehensive understanding about the properties of different domains formed in epitaxial laterally-overgrown GaN. For this purpose a spherical pit was fabricated into the sample by mechanical grinding and polishing, penetrating through to the buffer layer at its center. We found areas showing sharp excitonic luminescence corresponding to local free-carrier concentrations n below 10 17 cm -3 as well as domains exhibiting broad luminescence originating from recombination of a doping plasma with n reaching 10 19 cm -3 . Simultaneously, we observed in the scanning microscopy investigations a substructure which could be explained by the existence of internal space charge regions.

Emission Spectroscopy of Hydrogen Molecules in Technical and Divertor Plasmas

The paper gives an overview of the diagnostics of hydrogen molecules in technical plasmas (MW and RF discharges) and in divertor plasmas of fusion experiments (ASDEX Upgrade (Tokamak at the Max-Planck-Institut fur Plasmaphysik in Garching near Munich, Germany). The Fulcher transition in the visible spectral range was chosen for analysis since this is the most prominent band in the spectrum of molecular hydrogen. Examples for diagnostics of molecular densities will be given, and the problems arising in the interpretation of spectra will be discussed. In divertor plasmas the diagnostics of molecular fluxes will be introduced and the contribution of molecules to the plasma recombination will be discussed. Results for vibrational populations in the ground state and the correlation to the upper Fulcher state will be given, providing an electron temperature diagnostic. Finally, the influence of surfaces (high-grade steel and graphite) on vibrational populations and on reflection coefficients of atoms will be shown. Special attention is given on a comparison of the isotopes hydrogen and deuterium.

Collapsing dynamics of attractive Bose–Einstein condensates

The self-similar collapse of 3D and quasi-2D atom condensates with negative scattering length is examined. 3D condensates are shown to blow up following the scenario of weak collapse, for which 3-body recombination weakly dissipates the atoms. In contrast, 2D condensates undergo a strong collapse, that absorbs a significant amount of particles.

Collective correlation and screening effects on radiative recombination in classical nonideal plasmas

Collective correlation and plasma screening effects on the K-shell radiative recombination of the free electron with the ion in classical nonideal plasmas are investigated. The radiative recombination cross section is obtained by the principle of detailed balance with the photoionization cross section of the hydrogenic ion in nonideal plasmas. It is found that the collective and plasma screening effects significantly reduce the radiative recombination cross section in nonideal plasmas. It is also found that the collective and screening effects on the recombination cross section is almost independent of the projectile energy.