Resonant Charge Transfer Collisions Research Articles

Chamber with Inverted Electrode Geometry for Measuring and Control of Ion Flux-Energy Distribution Functions

Measurements of ion flux-energy distribution functions at the high sheath potential of the driven electrode in a classical low-pressure asymmetric capacitively coupled plasma are technically difficult as the diagnostic device needs to float with the applied radio frequency voltage. Otherwise, the ion sampling is disturbed by the varying electric field between the grounded device and the driven electrode. To circumvent such distortions, a low-pressure plasma chamber with inverted electrode geometry, where the larger electrode is driven and the smaller electrode is grounded, has been constructed and characterized. Measurements of the ion flux-energy distribution functions with an energy-selective mass spectrometer at the high sheath potential of the grounded electrode are presented for a variety of conditions and ions. The potential for suppressing low-energy ions from resonant charge transfer collisions in the sheath by the dilution of the working gas is demonstrated. Additionally, the setup is supplemented by an inductively coupled plasma that controls the plasma density and consequently the ion flux to the substrate while the radio frequency bias controls the ion energy. At high ion energies, metal ions are detected as a consequence of the ionization of sputtered electrode material. The proposed setup opens a way to study precisely the effects of ion treatment for a variety of substrates such as catalysts, polymers, or thin films.

State-specific transport properties of partially ionized flows of electronically excited atomic gases

State-to-state approach for theoretical study of transport properties in atomic gases with excited electronic degrees of freedom of both neutral and ionized species is developed. The dependence of atomic radius on the electronic configuration of excited atoms is taken into account in the transport algorithm. Different cutoff criteria for increasing atomic radius are discussed and the limits of applicability for these criteria are evaluated. The validity of a Slater-like model for the calculation of state-resolved transport coefficients in neutral and ionized atomic gases is shown. For ionized flows, a method of evaluation for effective cross-sections of resonant charge-transfer collisions is suggested. Accurate kinetic theory algorithms for modelling the state-specific transport properties are applied for the prediction of transport coefficients in shock heated flows. Based on the numerical observations, different distributions over electronic states behind the shock front are considered. For the Boltzmann-like distributions at temperatures greater than 14,000K, an important effect of electronic excitation on the partial thermal conductivity and viscosity coefficients is found for both neutral and ionized atomic gases: increasing radius of excited atoms causes a strong decrease in these transport coefficients. Similarly, the presence of electronically excited states with increased atomic radii leads to reduced diffusion coefficients. Nevertheless the overall impact of increasing effective cross-sections on the transport properties just behind the shock front under hypersonic reentry conditions is found to be minor since the populations of high-lying electronic energy levels behind the shock waves are low.

Momentum transfer theory of ion transport under the influence of resonant charge transfer collisions: the case of argon and neon ions in parent gases

Transport properties of ion swarms in presence of Resonant Charge Transfer (RCT) collisions are studied using Momentum Transfer Theory (MTT). It was shown that, not surprisingly, RCT collisions may be represented as a special case of elastic scattering. Using the developed MTT we tested a previously available anisotropic set of cross-sections for Ar+Ar + collisions by making the comparisons with the available data for the transverse diffusion coefficient. We also developed an anisotropic set of Ne+Ne + integral cross-sections based on the available data for mobility, longitudinal and transverse diffusion. Anisotropic sets of cross-sections are needed for Monte Carlo simulations of ion transport and plasma models.

Emission Characteristics of a Low-Pressure Laser-Induced Plasma: Selective Excitation of Ionic Emission Lines of Copper

Under reduced pressures, the plasmas produced by irradiation of a Q-switched Nd:YAG laser have favorable features as an excitation source for solid samples in atomic emission spectrometry, such as relatively low fluctuation of the emission intensity and a high signal-to-background ratio. For the analytical application of low-pressure laser-induced plasma spectroscopy (LP-LIPS), the spectra should be examined regarding the type of emission lines and their relative intensities. Further, information on the excitation mechanism in the plasma could be useful in determining optimum operation conditions of the experimental parameters. Spectrum patterns of LP-LIP are principally determined by the type of the filled gas. Especially, particular ionic lines of Cu are selectively excited when Ar or Ne are employed as the filled gas. This feature seems to be due to the (quasi-) resonance charge transfer collisions between a ground state ion of the Ar or Ne gas and a ground state atom of Cu. The plasma plume expanding after the laser irradiation also becomes an excitation source for sample atoms introduced by laser ablation.

Analytical performance of high-voltage neon plasma in glow discharge optical emission spectrometry

In a high-voltage Ne glow discharge plasma (Ne-GDP), calibration factors as well as the limit of determination were compared between atomic resonance lines and singly-ionized lines of copper and aluminium in optical emission spectrometry. These elements have intense ionic lines which are excited by resonance charge-transfer collisions of Ne ions. The ionic lines gave better detection sensitivity in the Ne-GDP, whereas the atomic resonance lines were commonly employed as analytical lines in the other plasma sources such as Ar-GDP and ICP. The limit of determination was 1.3 × 10–3 mass % for the Cu II 248.58 nm line and 1.0 × 10–3 mass % Al for the Al II 358.66 nm line at a discharge parameter of 1.60 kV/36 mA.

Non-Boltzmann distribution among energy levels of singly-ionized vanadium in dc glow discharge and rf inductively coupled discharge plasmas

The Boltzmann plots of ionic vanadium lines are investigated using an Ar glow discharge plasma as well as an inductively coupled plasma. Departures from a linear plot are observed with both of the plasmas, meaning that LTE conditions are not realized. It results from increased population of the 3d3(a 4F)4d 5H levels to which the V II 264.433 and 265.565 nm lines are assigned, which could be explained from resonance charge-transfer collisions between vanadium atom and argon ion.

Criterion for determining the analytical emission lines in glow discharge optical emission spectrometry.

Two major factors for selecting analytical lines, the sensitivity and the self-absorption effect, were compared between atomic lines and singly-ionized lines of copper as well as vanadium in glow discharge optical emission spectrometry. They have intense ionic lines which are excited through resonance charge-transfer collisions. It was found that the ionic lines are more sensitive compared to the atomic resonance lines; in addition, the ionic lines are essentially free from self-absorption, while the atomic lines suffer from self-absorption to a certain extent. The Cu II 224.70 nm, V II 309.31 nm, and V II 266.32 nm lines in the argon plasma, and the Cu II 270.32 nm line in the neon plasma are recommended as analytical lines of these elements.

Diagnostic on N 2 plasma with an energy resolved quadrupole mass spectrometer at the powered electrode in a reactive ion etching system: ion energy distribution of N 2+ and N +

An energy-resolved quadrupole mass spectrometer (E-QMS) was assembled underneath the powered electrode of a diode reactive ion etcher. The plasma ions reach the QMS through an orifice in the powered electrode with a diameter of 100 μm. The ion energy distributions (IEDs) of N 2 + and N + from N 2 plasmas in the pressure range 0.1–2 Pa for d.c. bias potentials between 50 V and 350 V and a frequency of 13.56 MHz were investigated. The IEDs of N 2 + and N + always show a saddle-shaped peak at an energy corresponding to the total potential drop across the sheath given by U DC + U P1, where U DC is the d.c. bias potential and U P1 is the time averaged plasma potential. In the energy range from 0 eV to eU DC there are multiple peaks in the IEDs of N 2 + owing to symmetric resonant charge transfer collisions, whereas the N + shows only a broad distribution. Thus we conclude that the collision processes of N + are predominantely elastic. With increasing pressure, the intensity of the peaks in the IEDs of N 2 + below eU DC exceeds the intensity of the saddle-shaped peaks at U DC + U P1 owing to a higher probability for charge transfer collisions in the sheath. The intensity of the broad distribution in the IEDs of N + also increases with increasing pressure, but never exceeds the intensity of the saddle-shaped peak.

Integral alignment of Na(3p) in resonant charge transfer collisions

Experimental results for the integral alignment of Na(3p) in resonant charge transfer collisions with Na+ for collision energies in the range 90 to 2500 eV are presented. Among the three possible configurations 3p sigma , 3p pi + and 3p pi - the 3p sigma turns out to be most efficient for the charge transfer process at all investigated energies. At 90 eV its relative cross section is approximately 50% and decreases to approximately 33% at higher energies.

Study of the background currents in the radio frequency mass spectrometer

Large and variable background currents are observed in the normal operation of a five-stage radio frequency mass spectrometer. A hypothesis of resonance charge transfer collisions occurring in the mass analyzer can explain the cause of these currents. Positive ions of excess energy, generated by the partial neutralization of doubly charged ions, give rise to a positive background. Also produced in the charge transfer collisions are high energy neutral particles. Bombardment of the final grid of the mass analyzer by these neutral particles causes secondary electron emission. Detection of the secondary electrons results in a negative component of the background. By operating the instrument with an ion deflector and an offset detector, the background ions from the mass analyzer, as well as the secondary electron currents, were eliminated. However, the background problem was not entirely solved due to low energy background ions which were generated at the ion deflector. Alternative solutions to the background problem are suggested.

Ion-ion recombination in (X++Y-+X) systems at low gas densities. I. Symmetrical resonance charge transfer contribution

The study of Bates and Moffett (1966) for recombination in (X++X-+X) systems is generalised so as to cover arbitrary positive or negative ions. Results are provided for the contribution of symmetrical resonance charge transfer collisions to the rate of recombination in (X++Y-+X) or (X++Y-+Y) systems over a wide range of ion mass ratios. The Thomson and the bottleneck approaches within a detailed treatment of the energy change collision are discussed in depth. The role of continuum-bound and bound-bound collisional transitions in the present quasi-equilibrium treatment and in the Thomson (1924) and bottleneck (1962) approaches is thoroughly investigated and interesting insights emerge.

Spin Dependent Charge Transfer in Low-Energy Collisions between Helium Ions and Cesium Atoms

The experiment demonstrated that total spin angular momentum is conserved in the near resonant charge transfer collisions between oriented helium ions and oriented cesium atoms. The resulting spin dependence of the total charge transfer cross-sections is described using singlet and triplet channels based on the molecular potential energy curves of the collision pair. The experiment was performed with helium ions stored in a rf-quadrupole trap in interaction with an optically pumped beam of cesium atoms. Total spin dependent charge transfer cross-sections were measured in the energy range of a few eV.

Resonance absorption of N +2 first negative bands in irradiated air (U)

A prescription is given for calculating the resonance absorption of N + 2 first negative (0, v) band radiation in strongly irradiated air. This prescription is based on a band model of resonance absorption reported previously. The rotational lines are assumed to be Lorentzian. The effects of overlapping lines and unequal emission and absorption line widths on the resonance absorption cross sections are included. Emission lines are broadened by pressure deactivation of the N + 2 B-state; absorption lines are broadened in resonant charge transfer collisions of N + 2 X-state ions with neutral N 2. N + 2 X-state ions are produced in cascades through the excited A-state. Vibrational populations of the X-state ions are nearly equal for v = 0, 1, 2. Most ions are formed with sufficient excitation energy (>1 eV) to activate ion-molecule reactions with N 2. Roughly 10% of N + 2−N 2 orbiting collisions result in ion-molecule reactions. The resonance absorption cross sections and N + 2 ion densities are related to dose rate, and an example is given of N + 2 first negative radiation from a uniformly dosed column of air.

Calculations of ion-atom interactions relating to resonant charge-transfer collisions

The interaction between an atom and ion of the same element leads to gerade and ungerade states of the diatomic molecular ion. The energy splittings between the gerade and ungerade states determine the cross section for resonant charge transfer. Using the JWKB approach these energy splittings are derived from the asymptotic forms of the wave functions for the isolated atom and ion. Pseudopotential calculations of the splittings are reported for ${\mathrm{Li}}_{2}^{+}$, ${\mathrm{Na}}_{2}^{+}$, ${\mathrm{Rb}}_{2}^{+}$, and ${\mathrm{Cs}}_{2}^{+}$, and are used together with previous ab initio and model-potential calculations to test the JWKB method. The comparison shows that the method is sufficiently reliable to facilitate accurate calculations of the cross sections for resonant charge transfer at low energies.