Penning Ionization Processes Research Articles

Spin-dipole-induced lifetime of the least-boundΣg+5state ofHe(2S13)+He(2S13)

The properties of the least-bound vibrational level (v=14) of the quintet sigma state formed during the ultracold collision of two spin-polarized metastable helium atoms are crucial to studies of photoassociation spectroscopy of metastable helium. We report a calculation of the autoionization lifetime of this state induced by spin-dipole coupling of the quintet sigma state to the singlet sigma state from which Penning and associative ionization processes are highly probable. We find a lifetime of about 150 microseconds, significantly larger than the recent experimental estimates of (4-5) microseconds.

Ion formation in laser-irradiated cesium vapor

We study theoretically the formation of Cs + and Cs 2 + during cw laser radiation resonant with 6s-7p transition of Cs atomic vapor. This is done by numerically solving rate equations for the evolution of atomic state and electron populations. The results of calculations for the atomic and molecular ions density at different values of laser power clarified that the associative ionization and Penning ionization process play an important role for producing the Cs 2 + and Cs + , respectively, during the plasma formation. Also, the results showed that laser power of the order of 150 mW and 40–50 ns irradiation time are optimal in producing a fully ionized plasma.

Energy dependence of the Penning ionization electron spectrum of Ne* (3P2,0)+Kr

A crossed beam experiment is carried out to measure the energy of electrons emitted in Penning ionization processes by Ne*(3P2,0)–Kr collisions. The electron energy spectra have been measured at four different collision energies: 0.050, 0.140, 0.190, 0.460 eV. The analysis of the results allows the separation of spin orbit contributions both in the entrance and in the exit channels providing the related cross-section ratios. Some theoretical considerations have been made to clarify nature and role of interatomic potentials driving the collisions and some general features about the role of atomic fine structure in the Penning ionization processes.

속이 빈 원통형음극 방전의 전압-전류 곡선에서 음 저항 영역 관찰

We measured the optogalvanic signal and discharge voltage-current(V-I) curve under the two different discharge conditions with different buffer gases, Ar, and Ne. When the Gd was used as a cathode material at low discharge current less than 10mA, a significant change was observed in the current-voltage curve. Time resolved optogalvanic signal measurement were measured by the diode laser of which wavelengths correspond to metastable transition line of these gases (Ar, Ne). From these measurements, we found that the characteristics of the V-I curve strongly depend on the Penning ionization process.

Measurement of metastable He*(2S13) density in dielectric barrier discharges with two different configurations operating at around atmospheric pressure

We have measured the density of metastable He atoms in the lowest triplet state (2S13) with a diode-laser absorption spectroscopic technique in atmospheric pressure plasmas produced by dielectric barrier discharge schemes. Two different types of electrode configuration are employed: one is a conventional parallel-plate system and the other is a microdischarge integrated system with stacked metal-mesh electrodes covered by insulating films. We have analyzed the pressure-broadened spectral line corresponding to the 2S13→2PJ3 (J=0–2) transition to derive the broadening coefficient and to calibrate absolute densities. The measured density ranges from 1011 to 1012cm−3, but the values in the mesh-type system are larger than those in the parallel-plate system by about one order of magnitude. The density, however, depends strongly on the gas flow rate, showing the influence of quenching by the Penning-ionization process with impurities. Those behaviors are consistent with the variation of the electron density estimated by millimeter-wave transmittance measurement.

Penning ionization of N2O molecules by He*(2S3,1) and Ne*(P2,3) metastable atoms: A crossed beam study

The energetics of [Rg... N2O]* autoionizing collision complexes (where Rg=He or Ne) and their dynamical evolution have been studied in a crossed beam apparatus, respectively, by Penning ionization electron spectroscopy (PIES) and by mass spectrometry (MS) techniques in the thermal energy range. The PIES spectra, detected by an electron energy analyzer, were recorded for both complexes at four different collision energies. Such spectra allowed the determination of the energy shifts for Penning electron energy distributions, and the branching ratios for the population of different electronic states and for the vibrational population in the molecular nascent ions. For the [Ne...N2O]* collision complex it was found, by MS, that the autoionization leads to the formation of N2O+, NO+, O+, and NeN2O+ product ions whose total and partial cross sections were measured in the collision energy range between 0.03 and 0.2 eV. The results are analyzed exploiting current models for the Penning ionization process: the observed collision energy dependence in the PIES spectra as well as in the cross sections are correlated with the nature of the N2O molecule orbitals involved in the ionization and are discussed in term of the Rg-N2O interaction potentials, which are estimated by using a semiempirical method developed in our laboratory.

Diagnostics of microdischarge-integrated plasma sources for display and materials processing

Two different types of microdischarge-integrated plasma sources have been operated at around the atmospheric pressure range. The discharge characteristics were diagnosed by optical emission spectroscopy (OES), laser absorption spectroscopy (LAS) and microwave transmission (MT) techniques. The dynamic spatiotemporal behaviour of excited atoms was analysed using OES and LAS and the temporal behaviour of the electron density was estimated using the MT method. In Ar and Xe/Ne gases, waveforms of the MT signal followed the current waveform in the rise period and lasted longer according to the recombination losses. However, in He the waveform followed the density of metastable atoms, reflecting the production of a large amount of electrons by the Penning ionization process with impurities. The estimated peak electron density in those plasma sources is of the order of 1012 cm−3, and the metastable atom density can reach 1013 cm−3. Thus, it is suggested that these sources can be potentially applied to convenient material processing tools of large area operated stably at atmospheric pressure.

Collisional deexcitation of optically allowed excited atoms by axially symmetric molecules

The deexcitation process of an atom in an optically allowed excited state by a collision with an axially symmetric molecule is considered. In order to make a precise comparison with experimental data that have recently been obtained, we extend previous work for a Penning ionization process [T. Watanabe and K. Katsuura, J. Chem. Phys. 47, 800 (1967)]. Using the straight-line trajectory impact parameter method, the probability of deexcitation in the incident atom is described by a discrete-continuum excitation transfer mechanism. The effects of the ionization yield eta in molecular target and of the molecular anisotropic property of the optical transition dipole are considered. The cross-section formula sigma is presented by a similar formula for Penning ionization of the atomic target by introducing a stereo factor C(lambda) as sigma = C(lambda)[e(4)mu(2)mu(E, perpendicular)/(4piepsilon(0))(2) (2)hv](2/5). Here, v is the relative velocity of the colliding system and lambda is given by the ratio lambda =(mu(E||)/mu(E perpendicular)), where mu, mu(E perpendicular), mu(E||) are the transition dipole moments of an excited atom, A-->A(*), those of a molecule at energy E for the perpendicular component and the parallel component with respect to molecular axis. Applications to He(*)(2(1)P)+H(2) (or D(2)), Ne(*)[2p(5)((2)P(1/2))3 s (1)P(1)]+ H(2) (or D(2)) systems and systems of the same projectiles on C(6)H(6), (or C(6)D(6)) molecules are made. The results for hydrogen molecules are compared with the experimental data.

Design and construction of glow discharge-ion trap mass spectrometer (GD-ITMS) and its application for the analysis of volatile organic samples

In this study, we present a newly designed see-through type hollow cathode glow discharge (St-HCGD) cell developed for the analysis of volatile organic materials in an ion trap mass spectrometer. The cell was interfaced with a homemade ion trap mass spectrometer by adopting skimmer and sampler in an optimized dimensions based on the computer simulation done by SIMION software. The St-HCGD cell has a relatively small size (4×4×7 cm) with the diameter of the inner tube of 0.25′′. The anode and cathode were made of stainless steel-304 and helium was used as a buffer gas for discharge to enhance the Penning ionization process rather than sputtering process. Mass spectra of volatile organic samples such as benzene, toluene, cyclohexane were obtained by using the St-HCGD-ITMS.

Interaction of VUV photons with molecules

A survey is given of recent progress in experimental studies of spectroscopy and dynamics of molecules in the superexcited states which are produced in the interaction of vacuum ultraviolet photons with molecules. Molecules studied here ranged from simple diatomic and triatomic molecules to polyatomic molecules such as hydrocarbons, alcohols, ethers, and some Si-containing compounds in the gas phase. Most of the observed superexcited states are assigned to high Rydberg states which are vibrationally (or/and rotationally), doubly, or inner-core excited, and converge to each of ion states. Non-Rydberg superexcited states are also observed. Some remarks are also presented of superexcited states as reaction intermediates or collision complexes in electron-ion recombination, electron attachment, and Penning ionization processes.

Close-coupled calculation of field-free collisions of cold metastable helium atoms

A quantum-mechanical close-coupled calculation of field-free collisions of cold metastable helium atoms has been performed. Theoretical predictions of the rate coefficients for elastic processes and losses due to the Penning and associative ionization processes are reported for temperatures ranging from 1 $\ensuremath{\mu}\mathrm{K}$ to 1 K. The sensitivity of the rate constants to uncertainties in the molecular potentials and autoionization widths has been investigated. The total ionization loss rates were found to be sensitive to the short-range form of the ${}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ molecular potential and the autoionization widths, while the total elastic rate constants exhibit greater sensitivity to the short-range ${}^{5}{\ensuremath{\Sigma}}_{g}^{+}$ molecular potential.

Close-coupled calculation of collisions of magnetostatically trapped metastable helium atoms

A quantum-mechanical close-coupled calculation for collisions of spin-polarized triplet helium atoms in a magnetostatic trap is presented in which the Penning and associative ionization processes are represented by a complex optical potential. The rate coefficients for the spin relaxation and relaxation-induced Penning ionization processes for a range of magnetic fields and temperatures, spanning the cold and ultracold temperature regimes, have been calculated and the sensitivity of these results to uncertainties in the molecular potentials and autoionization width investigated. The rate constants for both elastic and inelastic processes were found to be particularly sensitive to the short-range ${}^{5}{\ensuremath{\Sigma}}_{g}^{+}$ molecular potential and, to a lesser extent, the rate constants for relaxation-induced Penning ionization displayed sensitivity to the short-range form of the ${}^{1}{\ensuremath{\Sigma}}_{g}^{+}$ molecular potential and the autoionization width.

Two-dimensional multifluid modeling of the He-Xe discharge in an AC plasma display panel

A numerical analysis of the discharge plasma which forms in an AC plasma display panel cell has been made using time-dependent, two-dimensional multifluid equations to understand the discharge physics of He-Xe discharge. The time dependent distributions of the plasma parameters such as the electron temperature, electron density, various ion densities, and excited state species densities, etc., are obtained during the sustain period in a surface type AC plasma display panel (PDP) cell. Because of its importance in the AC PDP operation, the behavior of the wail charges accumulated on the dielectric surface is also examined. It has been found that a DC bias voltage applied to the address electrode controls the distribution as well as the peak values of the plasma parameters. The reaction analysis showed that the dominant reaction process for Xe ion production is the direct electron impact ionization processes and not the Penning ionization process. The cells which have a smaller electrode gap and larger sustain electrode width could accumulate more wall charges and consequently require less voltage for discharge sustainment.

Effects of He on Cu film formation by rf sputtering

Cu films have been deposited at room temperature by the rf sputtering using a gas mixture of Ar and He in order to investigate the effects of He gas on film formation by sputtering. The plasma diagnosis was performed as a preliminary experiment by the single probe method and the optical emission spectroscopoy analysis for the He+Ar sputtering plasma. From the results, it was found that the electron density increased by mixing He gas with the sputtering Ar gas, suggesting that Ar + ions increased. This indicates that Ar + ions are effectively generated by collision with He m atoms in neutral excited metastable states with high energies through the Penning ionization process, so that the deposition rate of Cu films increased. For Cu films deposited using the He+Ar gas, the preferential growth of a (111) plane was observed. The film surface of Cu films deposited using the He+Ar gas were smoother than those deposited using the pure Ar gas. The films deposited using the He+Ar were conductive even at thickness less than 5nm and have lower resistivities than thoes deposited using the pure Ar gas. These results suggest that the migration of Cu atoms on film surfaces is enhanced by an increase in an amount of Ar + ions which bombard the Cu film surface.

Oxidation of Zn films by irradiation with an excited metastable He beam

The oxidation of Zn films was attemmed by irradiation with an excited metastable atom beam of noble gas (He, Ne, Ar or Kr) in an oxygen atmosphere of the order of 10 −3 Pa. The excited metastable atoms were generated in a glass tube by an RF discharge and were ejected through a nozzle to a Zn film surface, to which a negative bias voltage was applied. The presence of the excited metastable atoms on the surface was confirmed by detection of the electron current ejected from a Cu plate which was irradiated with the excited metastable atom beam. For Ar and Kr, Zn films were hardly oxidized. On the other hand, transparent ZnO films were obtained for He and Ne. Judging from the fact that the energies of the excited metastable states of He and Ne are higher than the ionization potential of O 2 molecules, the oxidation of Zn films is attributed to the generation of O 2 + molecular ions on the film surface through the Penning ionization process by the excited metastable atom beam.

The interaction of metastable helium atoms with alkaline earth atoms: He * (2 3 S ,2 1 S )+Mg, Ca, Sr and Ba

We have carried out experimental and theoretical studies of Penning ionization processes occurring in thermal energy collisions of state-selected metastable He*(23S) and He*(21S) atoms with ground state alkaline earth atoms X(X=Mg, Ca, Sr, Ba). Penning ionization electron energy spectra for these eight systems, measured with a crossed-beam set-up perpendicular to the collision velocity at energy resolutions 40–70 meV, are reported; relative populations of the different ionic X+ (ml) states are presented and well depths D*e for the He*+X entrance channel potentials with uncertainties around 25 meV are derived from the electron spectra as follows: He*(23S)+Mg/Ca/Sr/Ba: 130/250/240/260 meV; He*(21S) +Mg/Ca/Sr/Ba: 300/570/550/670 meV. The spectra show substantial differences for the three ionic states X+(2S), X+(2P) and X+(2D) and reveal that transitions to a repulsive potential — attributed to He+X+(2P)2 Σ formation — are mainly involved for the X+(2P) channel. Ab initio calculations of potential curves, autoionization widths, electron energy spectra and ionization cross sections are reported for the systems He*(23S)+Ca and He*(21S)+Ca. The respective well depths De* are calculated to be 243(15) meV and 544(15) meV; the ionization cross sections at the experimental mean energy of 72 meV amount to 101 A2 and 201 A2, respectively. Very good overall agreement with the experimental electron spectra is observed.

Further Study of a Rate Model for Inductively Coupled Plasma Analyte Spectra Using a Monte Carlo Technique

Based on Lovett's model, an attempt has been made to evaluate the effect of a number of processes possibly responsible for the analyte spectra emitted from an analytical ICP (inductively coupled plasma) with a Monte Carlo technique. The simulation results show that under conventional ICP operating conditions, absorption, stimulated emission, atom collisional ionization, atom recombination, autoionization, autoionization recombination, and dielectronic recombination made very little or no contribution to the level population. The Penning ionization process has an important effect on the ionization and excitation of Mn only at number density of metastable Ar greater than 1011/cm−3. Electron collisional processes, radiative recombination, and radiative decay are mainly responsible for Mn ionization and excitation. Using the above-mentioned key processes, together with the spatial distribution of the number density of evaporated analyte particles in the ICP obtained previously, the spatial distribution of the number density of excited or ionized Mn under different conditions is simulated. The simulation program is valid and the mechanisms evaluated are reasonable.

Chemical influence of inert gas on the thin film stress in plasma-enhanced chemical vapor deposited a-SiN: H films

The growth of amorphous hydrogenated silicon nitride (a-SiN:H) films by plasma enhanced chemical vapor deposition (PECVD) of SiH4−NH3−N2 reactive gas mixtures has been studied. Films were deposited at low temperature (T < 250 °C) in a commercial PECVD system commonly used to grow a-SiN: H for semiconductor integrated circuit passivation. It has been observed that the stress of the a-SiN: H film can be controlled through dilution of the film precursors with an inert gas. Experiments indicate that the influence of the inert gas on the process extends from growth kinetics and plasma chemistry to hydrogen bonding, elemental composition, and biaxial elastic modulus. The stress in films deposited without dilution is tensile. When argon is added to the plasma, Si–Hx plasma chemistry and film hydrogen bond density change producing a reduction in the amount of tensile stress. Dilution with helium can be used to shift the film stress from tensile to compressive with minimum change in growth rate. The observed helium/film stress relationship is associated with helium-based Penning ionization processes, which create metastable reactive gas species. In turn, the metastables influence nitrogen and hydrogen incorporation into the film. Nitrogen incorporation produces volume expansion of the film, increasing the compressive character of the film stress. This effect is similar to that observed when the RF power is varied or when low or multifrequency plasma excitation is used during PECVD growth of a-SiN: H.

Relative sensitivity factors in direct current glow discharge mass spectrometry using Kr and Xe gas ? Estimation of the role of Penning ionization

Relative sensitivity factors (RSF) (Fe=1) were determined for steel, aluminium and copper standard samples by direct current glow discharge mass spectrometry using Kr and Xe as discharge gas. In general, the RSF values in Kr and Xe were higher than in Ar; however for S and P in Kr gas and P, As and Se in Xe gas they were considerably lower. This decrease is related to the relative importance of the Penning ionization process in overall ionization of the sputtered species. The results showed the percentage of Penning ionization to account for 75.6–82.0% for Se, As and P and 64.2% for S.

Preparation of Cu–O Films by Sputtering Using He Gas

Cu–O films have been deposited by rf sputtering using a mixture of Ar, He and O2 as a sputtering gas. When He was mixed with the sputtering gas, highly oxidized Cu–O films were obtained with low O2 content of the sputtering gas, compared with those deposited without He. Activated particles in the plasma were investigated by optical emission spectroscopy analysis. This enhancement of oxidation of Cu–O films is related to an increase in O2+ and O* generated through the Penning ionization process by Hem in neutral excited metastable states.