Showing papers on "Ionization published in 1995"
TL;DR: In this paper, the interpenetrating network formed from a phase-segregated mixture of two semiconducting polymers is shown to provide both the spatially distributed interfaces necessary for efficient charge photo-generation, and the means for separately collecting the electrons and holes.
Abstract: THE photovoltaic effect involves the production of electrons and holes in a semiconductor device under illumination, and their subsequent collection at opposite electrodes. In many inorganic semiconductors, photon absorption produces free electrons and holes directly1. But in molecular semiconductors, absorption creates electrona¤-hole pairs (excitons) which are bound at room temperature2, so that charge collection requires their dissociation. Exciton dissociation is known to be efficient at interfaces between materials with different electron affinities and ionization potentials, where the electron is accepted by the material with larger electron affinity and the hole by the material with lower ionization potential3. A two-layer diode structure can thus be used, in which excitons generated in either layer diffuse towards the interface between the layers. However, the exciton diffusion range is typically at least a factor of 10 smaller than the optical absorption depth, thus limiting the efficiency of charge collection3. Here we show that the interpenetrating network formed from a phase-segregated mixture of two semiconducting polymers provides both the spatially distributed interfaces necessary for efficient charge photo-generation, and the means for separately collecting the electrons and holes. Devices using thin films of these polymer mixtures show promise for large-area photodetectors.
3,165 citations
TL;DR: The ionization rate of the H{sub 2}{sup +} molecular ion in intense, short-pulse laser fields is found to exhibit maxima at large {ital R}, exceeding the atom limit by an order of magnitude.
Abstract: We study the ionization of the ${\mathrm{H}}_{2}^{+}$ molecular ion in intense, short-pulse laser fields by numerically solving the three-dimensional time-dependent Schr\"odinger equation as a function of internuclear distance $R$. Anomalously high ionization for the molecular ion at large internuclear separations is observed for orientations parallel to the linearly polarized laser field. The ionization rate is found to exhibit maxima at large $R$, exceeding the atom limit by an order of magnitude. This is attributed to transitions between pairs of chargeresonant states which are strongly coupled by the field in diatomic molecular ions. The effect is shown to also occur in higher odd-charge diatomic molecular ions and can be attributed to field-induced nonadiabatic transitions between the charge-resonant states and electron tunneling suppression by the instantaneous Stark field of the laser.
570 citations
TL;DR: The rate of nonlinear ionization is strongly enhanced as a molecule is stretched beyond its equilibrium internuclear separation, reaching a peak rate that is many orders of magnitude greater than that at either small or large distances as discussed by the authors.
Abstract: The rate of nonlinear ionization is strongly enhanced as a molecule is stretched beyond its equilibrium internuclear separation, reaching a peak rate that is many orders of magnitude greater than that at either small or large distances. The enhancement results from nonadiabatic electron localization near the nuclei and its presence is insensitive to the laser frequency and intensity. Most intense-field dissociative ionization experiments are influenced by this effect.
533 citations
TL;DR: In this paper, an analytic, first-order model was developed to calculate irradiance thresholds for laser-induced breakdown in condensed media, including ocular and aqueous media.
Abstract: An analytic, first-order model has been developed to calculate irradiance thresholds for laser-induced breakdown (LIB) in condensed media, including ocular and aqueous media. The model is derived from the simple rate equation formalism of Shen for cascade breakdown in solids and from the theory of multiphoton ionization in condensed media developed by Keldysh. Analytic expressions have been obtained for the irradiance thresholds corresponding to multiphoton breakdown, to cascade breakdown, and to initiation of cascade breakdown by multiphoton ionization of seed electrons (multiphoton initiation threshold). The model has been incorporated into a computer code and code results compared to experimentally measured irradiance thresholds for breakdown of ocular and aqueous media by nanosecond, picosecond, and femtosecond laser pulses in the visible and near-infrared. The code and comparison of code results to experiment have been documented in part II.
306 citations
TL;DR: In this article, the dependence of the number of desorbed particles on laser fluence has been investigated for matrix-assisted laser desorption/ionization of analyte and matrix ions as well as for (photoionized) neutral matrix molecules using a homogeneous “flat-top” laser profile.
298 citations
Book•
02 May 1995
TL;DR: In this paper, theoretical and experimental methods for elementary atomic processes are described. But they do not consider the effect of ion-atom collisions on the charge exchange process.
Abstract: Introduction. Part I: Theoretical Methods. Relativistic Kinematics. Fields of Moving Charges. Relativistic Electron Motion. Ion-Atom Collisions. Part II: Elementary Atomic Processes. Excitation and Ionization. Ionization-Many Electrons. Charge Exchange. Radiative Electron Capture. Electron-Positron Pair Production. Part III: Experimental Methods. Charge-State Preparation. Target Arrangements. Cross Section Determination. Appendix. Bibliography. Index.
290 citations
TL;DR: In this paper, the authors used simple computational algorithms based on fundamental chemical structure theory to estimate ionization pKas of organic molecules strictly from molecular structure, where molecular structures are broken at each essential single bond into functional units with intrinsic properties.
Abstract: Ionization pKas for a large set of molecules were calculated using reactivity models developed in the computer program SPARC. SPARC uses relatively simple computational algorithms based on fundamental chemical structure theory to estimate ionization pKas of organic molecules strictly from molecular structure. Molecular structures are broken at each essential single bond into functional units with intrinsic properties. Reaction centers (acid or base) are identified and the impact of appended molecular structure on ionization pKa is quantified by perturbation theory. Resonance, electrostatic, solvation and H-bonding models have been developed and tested on 4338 pKas for 3685 compounds. The RMS deviation for the acids and the amino reaction center was 0.36 pKa units whereas that for the in-ring N and = N reaction centers was 0.41. Microscopic ionization constants, zwitterionic constants, isoelectric points, and molecular speciation as a function of pH can be calculated using the SPARC models.
279 citations
01 Jan 1995
TL;DR: In this article, the authors studied the properties of Fusion Edge Plasmas and the role of atomic and molecular processes, and the relationship between the VQN of H2 and D2 molecules on the Vibrational Quantum Number (VQN).
Abstract: Basic Properties of Fusion Edge Plasmas and Role of Atomic and Molecular Processes (R.K. Janev). Spectroscopic Processes and Data for Fusion Edge Plasmas (W.L. Wiese). Elastic and Excitation Electron Collisions with Atoms (S. Trajmar, I. Kanik). Electron Impact Ionization of Plasma Edge Atoms (T. Mark). Electron-Ion Recombination Processes in Plasmas (Y. Hahn). Excitation of Atomic Ions by Electron Impact (S.S. Tayal et al.). Ionization of Atomic Ions by Electron Impact (D. Defrance et al.). The Dependence of Electron Impact Excitation and Ionization Cross Sections of H2 and D2 Molecules on Vibrational Quantum Number (M. Capitelli, R. Celiberto). Electronmolecular Ion Collision Processes (J.B.A. Mitchell). Energy and Angular Distributions of Secondary Electrons Produced by Electronimpact Ionization (Y.K. Kim). Elastic and Related Cross Sections for Low Energy Collisions among Hydrogen and Helium Ions, Neutrals, and Isotopes (D.R. Schultz et al.). Rearrangement Processes Involving Hydrogen and Helium Atoms and Ions (F. Brouillard, X. Urbain). Reactive Ionmolecule Collisions Involving Hydrogen and Helium (F. Linder et al.). 3 additional articles. Index.
271 citations
TL;DR: Generally, good agreement with experiment is obtained with the available differential, integrated, ionization, and total cross sections, as well as with the electron-impact coherence parameters up to and including the 3 {sup 3}{ital D}-state excitation.
Abstract: We present the convergent close-coupling theory for the calculation of electron-helium scattering. We demonstrate its applicability at a range of projectile energies of 1.5 to 500 eV to scattering from the ground state to n\ensuremath{\le}3 states. Generally, good agreement with experiment is obtained with the available differential, integrated, ionization, and total cross sections, as well as with the electron-impact coherence parameters up to and including the 3 $^{3}$D-state excitation. This agreement is shown to be overall the best of the currently used electron-helium scattering theories. On occasion, some significant discrepancies with experiment are observed, particularly for the triplet-state excitations.
267 citations
TL;DR: In this paper, an active control of the distribution of products of a chemical reaction was demonstrated by using a method based on the principle of quantum mechanical interference, where hydrogen iodide (HI) molecules were simultaneously excited above their ionization threshold by two competing pathways.
Abstract: Active control of the distribution of products of a chemical reaction was demonstrated by using a method based on the principle of quantum mechanical interference. Hydrogen iodide (HI) molecules were simultaneously excited above their ionization threshold by two competing pathways. These paths were absorption of three ultraviolet photons of frequency ω 1 and one vacuum ultraviolet photon of frequency ω 3 = 3ω 1 . The HI + and I + signals were modulated as the phase between the lasers was varied, with the HI + signal lagging by 150° ± 15°. A mechanism consisting of autoionization and predissociation is proposed.
260 citations
TL;DR: It is shown that the time profile of the harmonics emitted by a single atom exposed to a strong electromagnetic field may be obtained through a wavelet or a Gabor analysis of the acceleration of the atomic dipole, shedding some light on the competition between the atomic excitation or ionization processes and photon emission.
Abstract: We show that the time profile of the harmonics emitted by a single atom exposed to a strong electromagnetic field may be obtained through a wavelet or a Gabor analysis of the acceleration of the atomic dipole. This analysis is extremely sensitive to the details of the dynamics and sheds some light on the competition between the atomic excitation or ionization processes and photon emission. For illustration we study the interaction of atomic hydrogen with an intense laser pulse.
TL;DR: In this article, a complete theoretical model is presented to simulate the electron seeding and the evolution of the plasma of electron-ion in the applied field, and the results of the theory verified by small scale experiments are used to simulate lightning in atmosphere, and helps to define the parameters of a laser system for lightning protection.
Abstract: Ultraviolet pulses of 200 fs duration and low energy (/spl ap/0.2 mJ) have a sufficiently high peak power to ionize oxygen and nitrogen by three- and four-photon ionization, respectively. It is shown that the resultant ionization channel induces a lightning like discharge at half of the natural self-breakdown voltage in nitrogen or air. The laser triggered discharging process is studied by monitoring the voltage between two planar electrodes. The effects of oxygen on the induced breakdown is investigated. A complete theoretical model is presented to simulate: (1) the electron seeding; and (2) the evolution of the plasma of electron-ion in the applied field. The results of the theory verified by small scale experiments-are used to simulate the process of laser triggered lightning in atmosphere, and helps to define the parameters of a laser system for lightning protection. >
TL;DR: These results are obtained with an apparatus incorporating a time-of-flight mass spectrometer with position-sensitive detection of the product ions, believed to yield more reliable results than those previously reported.
Abstract: Absolute partial cross sections from threshold to 1000 eV are reported for the production of ${\mathrm{Ar}}^{\mathit{n}+}$ (n=1--4) by electron-impact ionization of argon. The total cross sections, obtained from an appropriately weighted sum of the partial cross sections, are also reported. These results are obtained with an apparatus incorporating a time-of-flight mass spectrometer with position-sensitive detection of the product ions. The simple apparatus design embodies recent developments in pressure measurement and particle detection and is believed to yield more reliable results than those previously reported. For singly charged ions, the overall uncertainty in the absolute cross section values reported here is \ifmmode\pm\else\textpm\fi{}3.5%. Previous measurements of absolute partial and total cross sections are reviewed and compared with the present results.
TL;DR: In this article, the authors identify the "warm absorbers" seen in many AGN X-ray spectra with this gas, but argue that most of the UV absorption lines seen must be due to a small amount of more weakly ionized gas which is embedded in the main body of the reflecting gas.
Abstract: When an AGN is obscured, the warm reflecting gas nearby can be seen by a combination of bremsstrahlung, intrinsic line emission, and reflection of the nuclear continuum, both by electron scattering and by resonance line scattering. Resonance lines, due both to intrinsic emission and scattering, are particularly prominent in the soft X-ray band.
When our line of sight to the nucleus is not obscured, the dominant effect is absorption. In the soft X-ray band, ionization edges of highly ionized species and resonance lines contribute comparably to the opacity; in the ultraviolet, the gas is almost transparent except for a small number of resonance lines. We identify the "warm absorbers" seen in many AGN X-ray spectra with this gas, but argue that most of the UV absorption lines seen must be due to a small amount of more weakly ionized gas which is embedded in the main body of the warm, reflecting gas. Because the ionization equilibration timescales of some ions may be as long as the variability timescales in AGN, the ionic abundances indicated by the transmission spectra may not be well-described by ionization equilibrium.
TL;DR: It is demonstrated that contributions from the direct tunneling processes in the absence of rescattering are not sufficient to describe the observed ATI spectra, and the high-energy portion of the spectrum, including recently discovered rings, are due to rescattering processes.
Abstract: A generalized strong-field approximation is formulated to describe atoms interacting with intense laser fields. We apply it to determine angular distributions of electrons in above-threshold ionization (ATI). The theory treats the effects of an electron rescattering from its parent ion core in a systematic perturbation series. Probability amplitudes for ionization are interpreted in terms of quasiclassical electron trajectories. We demonstrate that contributions from the direct tunneling processes in the absence of rescattering are not sufficient to describe the observed ATI spectra. We show that the high-energy portion of the spectrum, including recently discovered rings (i.e., complex features in the angular distributions of outgoing electrons) are due to rescattering processes. We compare our quasiclassical results with exact numerical solutions.
TL;DR: The interaction of an intense laser pulse with large clusters present in pulsed gas jets is shown to produce novel plasmas with electron temperatures far in excess of that predicted by above-threshold ionization theory.
Abstract: The interaction of an intense laser pulse with large ({similar_to}100 A) clusters present in pulsed gas jets is shown to produce novel plasmas with electron temperatures far in excess of that predicted by above-threshold ionization theory. The enhanced absorption of the laser light by the dense clusters results in the production of high ion charge states via collisional ionization resulting in strong x-ray emission from the hot plasma. {copyright} {ital 1995} {ital The} {ital American} {ital Physical} {ital Society}.
IBM1
TL;DR: In this paper, the results of a Monte Carlo study of carrier multiplication in silicon bipolar and field-effect transistors and of electron injection into silicon dioxide are presented, and it is shown that quantization effects in inversion layers cause a shift of the threshold energy for impact ionization.
Abstract: Results of a Monte Carlo study of carrier multiplication in silicon bipolar and field‐effect transistors and of electron injection into silicon dioxide are presented. Qualitative and, in most cases, quantitative agreement is obtained only by accounting for the correct band structure, all relevant scattering processes (phonons, Coulomb, impact ionization), and the highly nonlocal properties of electron transport in small silicon devices. In addition, it is shown that quantization effects in inversion layers cause a shift of the threshold energy for impact ionization which is very significant for the calculation of the substrate current in field‐effect transistors. Conservation of parallel momentum, image‐force corrections, dynamic screening of the interparticle Coulomb interaction, and improvements to the WKB approximation are necessary to treat correctly the injection of electrons from silicon into silicon dioxide. The validity of models—analytic or Monte Carlo—which treat hot‐electron transport with oversimplified physical approximations is argued against. In a few words, there is no shortcut.
TL;DR: It is found that the charged droplet formation cannot be ascribed to the traditional models of friction electrification, electrical double layer, or statistical charging, and an explanation is proposed based on the ion concentration distribution in a small droplet.
Abstract: We have developed a sonic spray ionization method, in which a methanol and water solution is sprayed from a fused-silica capillary with gas flow coaxial to the capillary. Ions as well as charged droplets are produced under atmospheric pressure, and their intensities depend on the gas flow rate (gas velocity). Positive ions produced from dilute solutions of molecules regarded as neurotransmitters, such as catecholamine, by this ionization method have been analyzed with a quadrupole mass spectrometer. The protonated dopamine molecule is detected in the spray of the 10 nM solution, and the mass spectrum is compared with that obtained by the ion spray ionization method. A comparison between the mass-analyzed ion intensity and the ion current, which represents the sum of ions and charged droplets, shows that most ions are produced from the charged droplets after spraying. Furthermore, we found that the charged droplet formation cannot be ascribed to the traditional models of friction electrification, electrical double layer, or statistical charging. An explanation is proposed based on the ion concentration distribution in a small droplet.
TL;DR: In this paper, quasi-electrostatic (QE) fields that temporarily exist at high altitudes following the sudden removal (e.g., by a lightning discharge) of thundercloud charge at low altitudes are found to significantly heat mesospheric electrons and produce ionization and light.
Abstract: Quasi-electrostatic (QE) fields that temporarily exist at high altitudes following the sudden removal (e.g., by a lightning discharge) of thundercloud charge at low altitudes are found to significantly heat mesospheric electrons and produce ionization and light. The intensity, spatial extent, duration and spectra of optical emissions produced are consistent with the observed features of the Red Sprite type of upward discharges.
TL;DR: In this paper, the authors describe an instrument that measures the chemical composition of single aerosol particles, which are analyzed less than 0.5 ms after leaving ambient conditions and without touching any surfaces.
Abstract: We describe an instrument that measures the chemical composition of single aerosol particles. To facilitate detection of volatile species, particles are analyzed less than 0.5 ms after leaving ambient conditions and without touching any surfaces. Particles are introduced into a vacuum through a differentially pumped nozzle, then cross a He-Ne laser beam. The scattered light provides both size information and the trigger for an excimer laser that desorbs and ionizes molecules from the particle. Mass spectra with excellent signal to noise have been obtained from laboratory and ambient particles 0.3 to 16 μm diameter.
TL;DR: A series of peaks seen in the calculated proton energy spectra can be interpreted as occurring preferentially at (i) turning points of laser-induced vibrationally trapped states, and (ii) at the ionization maxima that occur at large internuclear distances of H{sub 2}{sup +}.
Abstract: The time-dependent Schr\"odinger equation for ${\mathrm{H}}_{2}^{+}$ in a strong laser field is solved numerically for a model that uses the exact three-body Hamiltonian with one-dimensional nuclear motion restricted to the direction of the laser electric field. The influence of ionization on possible stabilization against dissociation is investigated. Unexpectedly high ionization rates from high vibrational states, exceeding those of neutral atomic hydrogen, are found. The ionization rates as functions of the internuclear distance R were also calculated for fixed nuclei, and these exhibit two strong maxima at large R, which explain the full dynamical results. A series of peaks seen in the calculated proton energy spectra can therefore be interpreted as occurring preferentially at (i) turning points of laser-induced vibrationally trapped states, and (ii) at the ionization maxima that occur at large internuclear distances of ${\mathrm{H}}_{2}^{+}$.
TL;DR: In this article, the behavior of a low-dimensional quantal system whose classical, deterministic, Hamiltonian counterpart is non-integrable and undergoes a transition to chaos is investigated.
TL;DR: In this paper, the authors studied the evolution from multiphoton to tunneling, the energy and momentum of the photoelectrons, and the spectrum of emitted photons in the optical harmonic generation (OHG) process.
Abstract: The behavior of atoms submitted to intense electromagnetic fields is a subject of wide interest and active research. Much of the knowledge in this field is provided by studying the ionization dynamics, the energy and momentum of the photoelectrons, and the spectrum of emitted photons. Multiphoton ionization (MPI) is the process by which an atom is ionized by simultaneous absorption of several photons. The number of photons absorbed is in general even larger than the minimum required by energy conservation. The excess energy can be transferred to the photoelectron whose energy spectrum is composed of a series of lines separated by the photon energy. This process is known as above-threshold ionization (ATI). Alternately, the electron can be recaptured, emitting a series of energetic photons at odd harmonic frequencies of the driving field, dubbed optical harmonic generation (OHG). The particular topics studied in this paper are: Transient Resonances and Excited State Population Trapping; Evolution from Multiphoton to Tunneling; Electron Energy Distributions; Effects of Rescattering on the Photoelectron Energy and Momentum; {open_quotes}Direct{close_quotes} Channel in Multiphoton Double Ionization; {open_quotes}Nonsequential{close_quotes} Channel in Tunneling Double Ionization; and Nonsequential Rate.
TL;DR: In this paper, a simple model for ionization of sputtered metals by a high-density plasma is presented, and ion flux fractions of greater than 80% can be obtained by sputtering aluminum into a region of dense plasma (ne∼1012 cm−3).
Abstract: A simple model for ionization of sputtered metals by a high‐density plasma is presented. Experimentally, ion flux fractions of greater than 80% can be obtained by sputtering aluminum into a region of dense plasma (ne∼1012 cm−3). Such a process has important applications in the filling of high‐aspect‐ratio features encountered in microelectronics fabrication. Both electron‐impact and Penning ionization mechanisms are considered in this model. Under conditions of low electron density (ne≪1011 cm−3), Penning ionization is found to be the dominant ionization path. This is consistent with the accepted ionization mechanism for conventional diode sputtering. When high electron densities are generated, however, electron‐impact ionization plays a significant ionization role. Langmuir probe measurements of the inductively coupled plasma indicate that the electron density lies between 2×1011 and 2×1012 cm−3. The model, in combination with measured plasma density, is used to calculate ion fractions. Modeled and exper...
TL;DR: In this paper, it was shown that the rate of ion ejection is directly related to solvent evaporation, estimated as that for a clean surface of pure solvent using the Schottky hump from the image potential model (IPM).
Abstract: The phenomenon of ion evaporation from charged liquid surfaces is at the basis of electrospray ionization, a source of a stunning variety of gas phase ions. It is studied here by producing a monodisperse cloud of charged droplets and measuring the charge q and diameter dr of the residue particles left after complete evaporation of the solvent. When the droplets contain small monovalent dissolved ions, the electric field E on the surface of their solid residues is found to be independent of dr. One can thus argue that the source of small ions in electrospray ionization is field‐emission, and not other proposed mechanisms such as Dole’s charged residue model. A consequence of the observed independence of E on dr is that the rate of ion ejection is simply related to the rate of solvent evaporation, estimated here as that for a clean surface of pure solvent.The reduction G(E) brought about by the electric field E in the activation energy for ion evaporation has thus been inferred as a function of the measured field E in the range 1.5
TL;DR: In this article, a two-step model of Coulomb explosions of diatoms in intense laser fields is presented, in which the molecule loses several electrons when the atoms are at the equilibrium internuclear distance and then fast Coulomb explosion occurs until the products reach a critical distance Rc approximately 9 Bohr, at which several additional electrons are lost due to a recently discovered maxima of ionization rates occurring at R=Rc.
Abstract: A two-step model of Coulomb explosions of diatoms in intense laser fields is presented. In this model the molecule loses several electrons when the atoms are at the equilibrium internuclear distance and then fast Coulomb explosions occur until the products reach a critical distance Rc approximately=9 Bohr, at which several additional electrons are lost due to a recently discovered maxima of ionization rates occurring at R=Rc. Then the subsequent Coulomb explosion for the higher-charged ions takes place. The total combined Coulomb explosion energy agrees well with experimental results, showing striking regularities. The origin and intensity dependence of unexpectedly high ionization rates of dissociating nuclei at preferential, large internuclear distances R=Rc is also discussed and an analytic expression for Rc is derived.
TL;DR: In the terrestrial atmosphere (TA) and in interstellar clouds (ISC) by the action of solar/stellar UV and cosmic rays on the ambient neutral atmospheres, the precursor ions so formed develop largely via gas-phase ion chemistry from the simple ions characteristic of the ambient atmospheres to the observed, more complex ions as mentioned in this paper.
Abstract: Ions are formed in the terrestrial atmosphere (TA) and in interstellar clouds (ISC) by the action of solar/stellar UV and cosmic rays on the ambient neutral atmospheres. The precursor ions so formed develop largely via gas-phase ion chemistry from the simple ions characteristic of the ambient atmospheres to the observed, more complex ions. In the upper TA and in ISC, only bimolecular ion chemistry occurs. In the tenuous upper TA, the ion types remain simple, O+, O, and NO+ being dominant, and similarly in the tenuous diffuse ISC, ions such as H+, H22+, C+, and CH+ are dominant. At the high pressures of the lower TA, termolecular association reactions are important, and the initial ions, e.g., O and N formed by cosmic ray ionization are quickly converted to very complex cluster ions of the type H+(H2O)n (bases)m with bases including NH3 and CH3CN, and a parallel negative ion chemistry develops from the primary negative ions O− and O producing ions like NO(H2O)n(acid)m with acids HNO3 and H2SO4. In the dense ISC, the ion chemistry proceeds to produce polyatomic ions from the most important initial ions C+, H, and CH, and the process of radiative association, the bimolecular analog of termolecular association, is important in producing, for example, ions such as CH M, where M are the common observed interstellar molecules such as the carboxy, cyano, and amino molecules, which are formed via this ion chemistry. The major ion neutralization process in the upper TA and in ISC is positive ion–electron recombination, which limits the degree of ionization in these regions, whereas in the lower TA, where negative ions balance the positive ions, the charge neutralization process is ion–ion recombinarion. © 1996 John Wiley & Sons, Inc.
TL;DR: A parametric study of plasma power and central gas flow was carried out to study the transition from normal analytical conditions to cooler plasma conditions using an inductively coupled plasma mass spectrometer having a balanced load coil as mentioned in this paper.
Abstract: A parametric study of plasma power and central gas flow was carried out to study the transition from normal analytical conditions to cooler plasma conditions using an inductively coupled plasma mass spectrometer having a balanced load coil. ‘Cold plasma' conditions (low power and high central gas flow) permit the determination of K, Ca and Fe at trace levels. The effect of changing the position of the ground reference of the load coil was investigated. Trace element ionization is consistent with thermal ionization at low electron density. Ion–molecule chemistry (charge transfer) with NO+ or O2+ may be important at the cooler plasma temperature. Suppression of analyte signals by concomitant matrix elements is partially correlated with the ionization potentials of the matrix element. If the analyte ion signals are normalized to that for NO+, the suppression of signals appears to be independent of the matrix element, and a modest dependence on the ionization potential of the analyte element is apparent. For high concentrations of elements of low ionization potential, an additional or enhanced mechanism of ionization is evident. The onset for this enhanced ionization is sharply defined by a characteristic ionization potential near 6.0 eV. The sensitivity for trace elements does not appear to be affected by this enhanced ionization. The appearance of the enhanced ionization is made evident by a change in the ratio of the NO+ and O2+ signals. Use of cold plasma conditions for the determination of K, Ca and Fe in high-purity waters and acids is evident. It appears that the method may also be used for samples having moderate salt content if the analytical protocol includes measurement of the background ions NO+ and O2+.
TL;DR: Besides electron-impact excitation, fast-ion and atom- impact excitation are found to be the dominant production processes at the high voltages used here, while loss of the metastable atoms is caused predominantly by diffusion and also by electron quenching to the nearby resonant states.
Abstract: To calculate the behavior of metastable argon atoms in a direct-current glow discharge, a balance equation is constructed, taking into account all known production and loss processes of the metastable atoms. Density profiles and fluxes of the metastable atoms are computed. The relative importance of different production and loss processes determining the metastable density is calculated for the case of a molybdenum cathode in pure argon. Besides electron-impact excitation, fast-ion and atom-impact excitation are found to be the dominant production processes at the high voltages used here, while loss of the metastable atoms is caused predominantly by diffusion and also by electron quenching to the nearby resonant states. The role of metastable atoms in the total discharge is investigated. They are found to play a minor role in the ionization of argon atoms, but their part in the ionization of sputtered molybdenum cathode atoms appears to be rather important. Moreover, they seem to have a significant effect on the secondary electron emission at the cathode. The investigation also includes the influence of pressure, voltage, and current on the metastable densities and fluxes, on the relative importance of the different production and loss processes, and on the role of metastable atoms in the entire discharge.
TL;DR: In this article, a simplified collisional-radiative model has been constructed for the system of the ground state, electronically excited stable states, and the ionic state of molecular hydrogen in plasma.
Abstract: A simplified collisional‐radiative model has been constructed for the system of the ground state, electronically excited stable states, and the ionic state of molecular hydrogen in plasma. Effective rate coefficients have been calculated for production of electrons, molecular ions, protons, and hydrogen atoms from molecular hydrogen. The ratio of the effective ionization rate of molecular hydrogen to the Balmer α photon emission rate and the effective rate coefficients for radiation and energy losses are also presented.