Late Afterglow Research Articles

Kinetics of fluorine atoms in high-density carbon–tetrafluoride plasmas

Reaction processes of fluorine (F) atoms in high-density carbon–tetrafluoride (CF4) plasmas were investigated using vacuum ultraviolet absorption spectroscopy. A scaling law nF∝(nenCF4)0.5–0.7 was found experimentally, where nF is the F atom density and ne and nCF4 stand for the electron and parent gas (CF4) densities, respectively. The lifetime measurement in the afterglow showed that the decay curve of the F atom density was composed of two components: a rapid decay in the initial afterglow and an exponential decrease in the late afterglow. The decay time constant in the initial afterglow τ1 satisfied the scaling law τ1∝(nenCF4)−(0.3–0.4), which is a consistent relationship with the scaling law for the F atom density. The two scaling laws and the lifetimes of CFx radicals suggest that the major loss process of F atoms in the initial afterglow is the reaction with CFx radicals (probably, x=3) on the wall surface. The loss process in the late afterglow was simple diffusion to the wall surface. The surface loss probability of F atoms on the chamber wall was evaluated from the decay time constant in the late afterglow, and was on the order of 10−3.

Surface recombination in breakdown time delay experiments: Effect of different cathode materials

The late afterglow in nitrogen with iron electrode is studied by the breakdown time delay method, i.e., by measuring the breakdown time delay td as a function of the afterglow time τ. It is proposed that the cause of the secondary electrons initiating the breakdown is the energy of the surface recombination of nitrogen atoms on the iron electrode. The gas-phase and macrokinetic diffusive models are used to describe the experimental breakdown time delay data. By fitting the theoretical curve to the experimental data: (1) it has been confirmed that the recombination on the molybdenum glass is of the second order and the value of the surface recombination coefficient is determined at 4 mbar; (2) it has been shown that the surface recombination on the iron electrode is of the second order, and the effective recombination coefficients are determined; (3) the analytical form of the recombination coefficient as a function of the adsorption characteristics of surfaces and the pressure of the parent gas has been derived. In addition, the orders of surface recombination on the molybdenum-, aluminum-, and gold-plated electrode were determined by the same method.

Influence of Impurities on Surface Recombination of Nitrogen Atoms in Late Afterglow

The influence of impurity contents (water vapour and oxygen) on surface recombination of nitrogen atoms on the glass walls and the copper electrode surface is studied. The decay of nitrogen atom number density in late afterglow has been detected by the breakdown time delay method and the memory effect was found for nitrogen with large abundance of impurities (technical purity gas). The dominant reaction on the glass walls covered with water vapour was found to be of the second order. The surface recombination coefficient has been increased by about two orders of magnitude compared to the pure gas. Also, the increase of the secondary electron yield by about one order of magnitude occurs caused by chemisorbed oxygen on the electrode surface.

Active species in N2 flowing post-discharges

The afterglow in D.C. and H.F. N2 flowing discharges has been analysed by emission spectroscopy. Production of excited species has been studied in conditions of late Δt =(1 - 5) ×10-1 s and early Δt =(0.5 - 5) ×10-2 s afterglows. The late afterglow is characterized by the N2 1st positive emission with a strong enhancement of the N2(B, $v'~=~11$) level compared to the other N2(B, v') levels which is a signature of N atom density. By NO titration, we have determined that N/N2 dissociation degrees are respectively 4.1% and 2.7% in a 60 W H.F. and in a 50 mA D.C. post-discharge at 4.5 Torr. The early afterglow is characterized by a strong 1st positive emission which has been analysed from the N2(A) + N2(X, v>5) excitation transfer reaction. By introducing a little CH2(10-4-10-3) or H2(10-2-10-1) into the N2 D.C. and H.F. postdischarges, N2(A) quenching rates have been obtained indicating that the N2(A) is only excited on the two first vibrational states in the early afterglows.

Kinetics of activated nitrogen states in late afterglow by the time-delay method

A long-time decay of activated nitrogen is studied by the time-delay method. The kinetics in late nitrogen afterglow is determined by nitrogen atom recombination. Surface-catalysed excitation of nitrogen atom recombination at the cathode gives a metastable induced electron yield in the inter-electrode space (the Auger de-excitation process) that determines the time delay. The probability of N2(A3 Sigma u+) metastable state formation by recombination at the electrode surface is determined, as are the contribution to electron yield from this mechanism and the contribution from the gas phase metastable species. On the basis of the above-cited mechanism, the time-delay method is found to be very efficient for detection of activated nitrogen.

Explanation of memory curve for nitrogen by surface-catalysed excitation

A theory and numerical model are proposed that explain the dependence of time delay on afterglow period, td=f( tau ) (the memory curve), for the Cu-N2 system by surface-catalysed excitation at nitrogen atom recombination on the cathode. The kinetics in the late nitrogen afterglow are determined by nitrogen atom recombination. It is shown that the nitrogen atom recombination on the container walls (glass) is of second order in N (the recombination on copper is of first order). The probability of the metastable formation of N2(A3 Sigma u+) by recombination at the electrode surface is determined. The electron yield in the interelectrode space caused by metastable secondary emission (the Auger de-excitation process) determines the time delay. On the basis of this mechanism, the time-delay method is very efficient in detecting nitrogen atoms.

Role of multistage collisions of the second kind in the mechanism of pumping of a helium–strontium recombination laser

A study was made of multistage charge transfer and of a multistage Penning process in the pumping of levels of an ionic He–Sr laser (λ = 430.5 nm). An analysis was made of the results of an experimental study of the kinetics of the populations of metastable strontium and helium levels, and of electrical and optical filling (emptying) of these states, and also of the results of a mathematical modeling of an He–Sr laser. These results demonstrated that multistage charge transfer played a minor role and that recombination pumping predominated. The Penning process could make a significant contribution (up to 30%) to the formation of doubly charged strontium ions recombining in the late afterglow.

Collisional transfer between the6s′[12]0,1and6p[12]1xenon levels

The destruction rates of the xenon $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0}$ metastable and $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{1}$ resonance atoms by collisions with ground-state xenon atoms have been measured by studying the temporal dependence of the number densities of the $6{s}^{\ensuremath{'}}$ and $6p$ atoms. The studies have been made both during the earlier afterglow of a xenon pulsed discharge and after the selective production of the $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0,1}$ atoms by a laser excitation in the late afterglow. An original optical pumping technique, using a tunable pulsed dye laser, was employed to populate the $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0,1}$ levels by excitation of the $6s{[\frac{3}{2}]}_{2}$ long-lived metastable atoms to $6{p}^{\ensuremath{'}}$ levels. The pressure dependence of the decay frequencies of these $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0,1}$ atoms was analyzed in the xenon pressure range of 0.04 to 1 torr and a kinetic model describing the evolution of the densities of these atoms after their production was established. The rate coefficients for the relaxation of the $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0}$ metastable and $6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{1}$ resonance atoms by two-body collisions involving a ground-state xenon atom are (8.45\ifmmode\pm\else\textpm\fi{}0.85) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}12}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ and (6.65 \ifmmode\pm\else\textpm\fi{} 1.00) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}11}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\ensuremath{-}1}$, respectively. Results show that, in fact, at least 75% of this relaxation is a transfer of population to the $6p{[\frac{1}{2}]}_{1}$ level following ${\mathrm{Xe}}^{*}(6{s}^{\ensuremath{'}}{[\frac{1}{2}]}_{0,1})+\mathrm{Xe}\ensuremath{\rightleftarrows}{\mathrm{Xe}}^{*}(6p{[\frac{1}{2}]}_{1})+\mathrm{Xe}\ensuremath{-}\ensuremath{\Delta}E$. These collisional transfer processes have been explained by means of Mulliken's xenon molecular potential curves.

XeF* (C1/2) radiative lifetime measurement

By monitoring the spontaneous emission from electron-beam-excited xenon difluoride (XeF2) plasmas, the radiative lifetime of XeF* [C (1/2)] has been determined. Excited XeF was formed directly through electron-impact dissociative excitation of XeF2. The time behavior of the subsequent 351-nm fluorescence displayed two distinct exponential decay regions. The first was characterized by a pressure-independent decay constant of 16.5±5 ns, the radiative lifetime of XeF* [C (1/2)]. The time constant of the second region was linearly dependent on XeF2 pressure, indicating that C-state XeF molecules are, in the late afterglow, formed by a two-body collision process exhibiting a large rate constant (∼3×10−10 cm3 sec−1).

Resonance cone and Bernstein wave interference patterns in a magnetoplasma

General dispersion relations for a hot magnetoplasma are used to explain the interference observed between electrostatic waves and the electromagnetic field in an afterglow plasma. Observations are made near both oblique resonance cones (connected with whistler and Z‐propagation modes) and in the frequency region corresponding to “Bernstein” waves. Independent measurements of the lower (whistler) and upper (Z‐mode) resonance cones, for a particular case, lead to the same isotropic temperature in the late afterglow, namely 540 K. Independent measurements of the lower resonance cone and of Bernstein waves in the early afterglow also lead to consistent isotropic temperatures. As the real and imaginary components of the wave number increase, a point of inflection is reached on the various dispersion curves for homogeneous plane waves beyond which these curves can no longer be used to explain the observed interference fringes. For homogeneous waves with a wave number smaller than that corresponding to the inflection point, reasonable values of Landau damping are obtained which agree roughly with observed damping.

Decay of Krypton1s2and1s3Excited Species in the Late Afterglow

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Determination of temperatures of high current spark channels †

Abstract This paper reviews the experimental methods of determination of temperatures of recovering high current, spark channels in different, stages of the afterglow. Host of the experiments described were performed at atmospheric pressure or pressures sufficiently high that thermal equilibrium among all species is achieved so that the electron, ion and gas temperatures are considered equal. In the early afterglow (within approximately 50 μs after spark initiation) when luminosity of the spark is sufficient, spectroscopic methods are employed to determine channel temperatures. In the intermediate stage of the afterglow (50 μs-500 μs) when luminosity is not sufficient for optical techniques to be employed, measurement, of electrical conductivity or sound or shock-wave velocity can be used to derive the gas temperature. In the late afterglow (1 ms after spark initiation) when the gas has recovered to a neutral state, measurements of reignition voltage together with Poschen's law arc used to derive gas temp...

The application of Langmuir probes to the measurement of very low electron temperatures

A summary is given of some important results obtained from the attempts to measure low electron temperatures ( T e < 1000°K) in afterglow plasmas of pure rare gas and rare gasoxygen mixtures using cylindrical Langmuir probes. The afterglow plasma is a uniquely suitable medium in which to assess the accuracy of probe measurements by virtue of the fact that T e in the late afterglow is, under appropriate conditions, expected to equal the gas temperature. The results of such measurements indicate that erroneously high estimates of T e will inevitably be obtained when observable ‘hysteresis’ is associated with the probe current-voltage characteristics and that the absence of hysteresis is a necessary but not sufficient condition for the accurate measurement of T e Accurate measurements of T e were only obtained after thorough cleaning of the probe and the reference electrode, and in the presence of atomic and molecular oxygen ‘electron temperatures’ were always erroneously higher than the gas temperature (≈300°K) by about 200–300°K. It is suggested that the ionospheric implications of these laboratory results are such as to place considerable doubt on the accuracy of ionospheric in situ probe measurements of electron temperatures particularly those low electron temperatures occurring in the lower ionosphere where atomic and molecular oxygen is abundant. The body of evidence indicates the necessity to obtain the current-voltage characteristic for voltage sweeps of opposite senses in order that the accuracy of the data may be assessed. However, if on-board processing of probe data obtained from rocket and satellite-borne probes is essential because of telemetry constraints, the accuracy of the data so obtained is likely to remain in doubt.

Cumulative Ionization in Optically Pumped Helium Discharges: A Source of Polarized Electrons

Polarization analysis of electrons extracted from a weak helium discharge in which the $2^{3}S_{1}$ metastable atoms are spin oriented by optical pumping indicates that the reaction $\mathrm{He}(2^{3}S_{1})+\mathrm{He}(2^{3}S_{1})\ensuremath{\rightarrow}\mathrm{He}(1^{1}S_{0})+{\mathrm{He}}^{+}+{e}^{\ensuremath{-}}$ is the predominant source of ionization in the discharge. The reaction conserves spin angular momentum and therefore produces polarized electrons. The process has been exploited to produce continuous beams of electrons with polarization as high as 10%. For a source-gas pressure of 0.035 torr, a continuous 4-\ensuremath{\mu}A beam with 8% spin polarization has been realized. Electrons extracted from the late afterglow of a pulsed discharge have 17% polarization at an optimum pressure of 0.1 torr.

Production and loss of N2 + ions during the decay period of plasmas produced in helium-nitrogen mixtures

The time dependence of the number density of N2+ ions during the decay period of plasmas produced in helium containing 0.05, 0.17 and 0.5 percent nitrogen was studied in the pressure range from about 0.3 to 7 Torr by means of mass spectrometer techniques. During the early part of the afterglow period the time dependence of N2+ is controlled by ambipolar diffusion loss towards the plasma container walls. The product of the ambipolar diffusion coefficientDa and the reduced pressurep0 wasDap0=900± 50 cm2 Torr/sec. The production of N2+ by collisions between metastable nitrogen molecules determines the temporal behavior of the N2+ density during the late afterglow for extremely pure discharge conditions. From the data it follows that the metastable molecules involved are de-excited by collisions with ground state helium atoms with a rate constant of 3.4 × 10−15 cm3 sec−1, while the radiative lifetime of these metastable molecules is at least 20 msec. The surface catalytic efficiency for de-excitation upon striking the molybdenum covered plasma container walls was estimated to be smaller than 10−3. Energy and radiative lifetime requirements suggest that N2+ is produced during the plasma decay period by the process N2 (a′1 Σu−)+N2 (a′1σu−→N2+ (X2Σu+)+N2(X)+e.

Electron Loss Processes in Air Afterglows

A modified microwave cavity technique was used to measure the decay rates of electron densities in air afterglows. Measurements of low-duty cycle or single discharges which were not affected by the discharge products of preceding events were obtained by monitoring the shifts of the resonant frequencies. Results of measurements in late afterglows show that one loss mechanism can be described by an effective ambipolar diffusion coefficient representing participation of two positive ions in the process; the ions being most likely O2+ and NO+. The other loss mechanism is due to two-body attachment of electrons, the likely attaching specie being NO2. The measurements on two different x-band cavities in the pressure range 0.1-10.0 Torr for ambipolar diffusion yielded: Da,eff p = 122 ± 18cm2/sec Torr and for attachment: hav1 = (4.9 ± 0.15) × 103sec−1Torr−1.

Mobility and diffusion cross section of atomic xenon ion in xenon

An x-band microwave cavity method has been employed to study the decay of electron density in low gas pressures of very pure xenon. The main electron loss process in the late afterglow is via ambipolar diffusion affected by diffusion cooling of electrons. Extrapolation to zero pressure yields a value of (054 ± 003) cm2 v-1 s-1 for the mobility of Xe+ diffusing in xenon at 295 °K, in good agreement with previous work. A value of 246 × 10-14 cm2 is deduced for the diffusion cross section, in reasonable agreement with a theoretically calculated value.

Investigation of the helium afterglow I. Mass spectrometric observations

Mass spectrometric observations of helium afterglows by ion sampling from the walls of a large discharge vessel are described. It is shown that in very pure cataphoretically-cleaned helium in the pressure range 0?3 to 2 torr the only important processes occurring in the late afterglow are ambipolar diffusion of atomic and molecular ions and electrons, and conversion of atomic to molecular ions by three-body collisions with neutral helium atoms. The atomic and molecular ion mobilities corresponding to the observed diffusion rates are respectively 11?2 ? 0?4 cm2 v-1 s-1 and 15?9 ? 0?4 cm2 v-1 s-1, in acceptable agreement with several previous mobility measurements, and the value obtained for the three-body conversion coefficient, 85 ? 3 torr-2 s-1 (pressure reduced to 0 ?C) is in good agreement with a recent similar measurement by other workers in a higher pressure range. The wall temperature of the discharge vessel during these measurements was 295 ?K. Measurements carried out in helium samples which were not subjected to cataphoretic purification demonstrated the serious effect of small amounts of impurity on the decay rate of the molecular helium ion.

Microwave Heating by a Standing Wave in the Helium Afterglow

Spatial variations of electron temperature in the late helium afterglow in the pressure range 0.55–68 Torr are measured experimentally. The variations are due to the presence of an applied standing wave at microwave frequencies. A normalized temperature profile is deduced from variations in the afterglow light quenching during a microwave pulse. Large temperature variations occur at higher pressures and smaller variations are seen at lower pressures. The observations are in good agreement with theory.

Spectral Emission of the Helium Afterglow

The spectral emission from a dc pulsed helium discharge for pressures from 5 to 30 Torr has been investigated during the early afterglow using a spectrometer with gated photomultiplier and from the early afterglow through the late afterglow using interference filters with gated photomultiplier. The afterglow spectrum was predominately that of molecular light which was attributed to the collisional-radiative recombination of molecular ions. The time-decay of molecular light at late times indicated ambipolar diffusion control by an ion with a Dap of 710±10 cm2/sec, which corresponds to a μ0 of 16.4±0.2 cm2/V·sec, in good agreement with values reported for the helium molecular ion employing other techniques.