Showing papers on "Ionization published in 1992"

Kinetic scheme of the non-equilibrium discharge in nitrogen-oxygen mixtures

Abstract: A kinetic scheme for non-equilibrium discharge in nitrogen-oxygen mixtures is developed, which almost wholly describes chemical transformations of particles in the cold (200 K

Chemical Ionization Mass Spectrometry

Abstract: INTRODUCTION. PREFACE. MOLECULAR MASS DETERMINATION. STRUCTURE ELUCIDATION. IDENTIFICATION AND QUANTITATION. SCOPE OF THE PRESENT WORK. FUNDAMENTALS OF GAS PHASE ION CHEMISTRY. ION/MOLECULE COLLISION RATES. Langevin Ion-Induced Dipole Theory. Average Dipole Orientation (ADO) Theory. POSITIVE ION/MOLECULE REACTIONS. Charge Exchange. Proton and Hydrogen Atom Transfer. Negative Ion Transfer Reactions. Condensation Reactions. Clustering or Association Reactions. NEGATIVE ION/MOLECULE REACTIONS. Electron/Molecule Interactions. Associative Detachment Reactions. Displacement and Elimination Reactions. Proton Transfer Reactions. Charge Exchange Reactions. Association Reactions. THERMOCHEMICAL PROPERTIES OF GAS-PHASE IONS. Gas Phase Basicities: Proton Affinities. Hydride Ion Affinities. Electron Affinities. Gas Phase Acidities: Proton Affinities of Anions. INSTRUMENTATION FOR CHEMICAL IONIZATION. MEDIUM PRESSURE MASS SPECTROMETRY. ATMOSPHERIC PRESSURE IONIZATION (API) MASS SPECTROMETRY. FOURIER TRANSFORM MASS SPECTROMETRY (FTMS). QUADRUPOLE ION TRAP MASS SPECTROMETRY. PULSED POSITIVE ION-NEGATIVE ION CHEMICAL IONIZATION. SAMPLE INTRODUCTION IN CHEMICAL IONIZATION MASS SPECTROMETRY. INSTRUMENTATION FOR COLLISION-INDUCED DISSOCIATION (CID) STUDIES. CHEMICAL IONIZATION REAGENT SYSTEMS. POSITIVE ION REAGENT SYSTEMS. Bronsted Acid Reagent Systems. Hydride Ion Abstraction Reagent Systems. Charge Exchange Reagent Systems. Condensation Reaction Reagents. NO as a Positive Ion Reagent System. Tetramethylsilane Reagent System. Metal Ion Reagent Systems. Miscellaneous Positive Ion Reagent Systems. NEGATIVE ION REAGENT GAS SYSTEMS. Electron Capture Reagent Systems. Bronsted Base Reagent Systems. 0- as a Reagent Ion. 02- as a Reagent Ion. SENSITIVITY OF CHEMICAL IONIZATION. CHEMICAL IONIZATION MASS SPECTRA. ALKANES (CnH2n+2). ALKENES AND CYCLOALKANES (CnH2n). ALKYNES, ALKADIENES, AND CYCLOALKENES (CnH2n-2). AROMATIC HYDROCARBONS. ALCOHOLS. ETHERS. ALDEHYDES AND KETONES. CARBOXYLIC ACIDS AND ESTERS. AMINES. NITRO COMPOUNDS. HALOGENATED COMPOUNDS. AMINO ACIDS AND DERIVATIVES. CARBOHYDRATES AND DERIVATIVES. SELECTED TOPICS IN CHEMICAL IONIZATION MASS SPECTROMETRY. ISOTOPE EXCHANGE REACTIONS IN CHEMICAL IONIZATION STUDIES. STEREOCHEMICAL EFFECTS IN CHEMICAL IONIZATION MASS SPECTRA. TANDEM MASS SPECTROMETRY AND CHEMICAL IONIZATION. REACTIVE COLLISIONS IN QUADRUPOLE COLLISION CELLS.

Iron ionization and recombination rates and ionization equilibrium

Abstract: In the past few years important progress has been made on the knowledge of ionization and recombination rates of iron, an astrophysically abundant heavy element and a major impurity in laboratory fusion devices. We make a critical review of the existing data on ionization and dielectronic recombination and present new computations of radiative recombination rate coefficients of Fe(+14) through Fe(+25) using the photoionization cross sections of Clark et al. (1986). We provide analytical fits to the recommended data (direct ionization and excitation-autoionization cross sections; radiative and dielectronic recombination rate coefficients). Finally we determine the iron ionic fractions at ionization equilibrium and compare them with previous computations as well as with observational data.

Solar photo rates for planetary atmospheres and atmospheric pollutants

Abstract: Unattenuated solar photo rate coefficients and excess energies for dissociation, ionization, and dissociative ionization are presented for atomic and molecular species that have been identified or are suspected to exist in the atmospheres of planets, satellites (moons), comets, or as pollutants in the Earth atmosphere. The branching ratios and cross sections with resonances have been tabulated to the greatest detail possible and the rate coefficients and excess energies have been calculated from them on a grid of small wavelength bins for the quiet and the active Sun at 1 AU heliocentric distance.

Thin films from energetic cluster impact: A feasibility study

Abstract: An intense, continuous beam of metal clusters and cluster ions is produced by combining a magnetron sputter discharge with a gas aggregation source. The average cluster size can be varied between 50 and more than 106 atoms per cluster. The sputter discharge is also used to ionize the clusters; between 30% and 80% of them carry a charge without further electron‐impact ionization. Mon− clusters with n≊1200 were separated from the neutral clusters, accelerated, and deposited on a polished Cu substrate. Above a kinetic energy of 6 keV, highly reflecting, strongly adhering thin films are formed on room‐temperature substrates. The films can be mechanically polished, which increases the reflectivity from 95% to 97% at 10.6 μm. Rutherford backscattering spectroscopy data reveal that less than 0.5% argon is incorporated into the films. The standard structure zone model of Movchan, Demchishin, and Thornton [in B. Chapman, Glow Discharge Processes (Wiley, New York, 1982)] is not applicable. The impact of an energetic cluster leads locally to a sudden increase of pressure and temperature. A tiny, high‐temperature spot is formed at each impact of an energetic cluster. The high local temperature present for several picoseconds leads to the observed film properties. The main advantage of the method seems to be that excellent thin films can be produced on room‐temperature substrates. The name ‘‘energetic cluster impact’’ is proposed for this new deposition method.

Observation of nonsequential double ionization of helium with optical tunneling.

Abstract: We have measured the ion yields for helium ionized by 120 fs, 614 nm laser pulses for intensities near ${10}^{16}$ W/${\mathrm{cm}}^{2}$. We have found that for these ultrashort pulses the ${\mathrm{He}}^{+2}$ data exhibit a feature which saturates in parallel with the ${\mathrm{He}}^{+}$ signal indicating that the ionization may proceed nonsequentially. We propose a new mechanism, which can exist only in the tunneling regime, for such nonsequential ionization.

Non-LTE Radiating Acoustic Shocks and CA II K2V Bright Points

Abstract: We present, for the first time, a self-consistent solution of the time-dependent 1D equations of non-LTE radiation hydrodynamics in solar chromospheric conditions. The vertical propagation of sinusoidal acoustic waves with periods of 30, 180, and 300 s is calculated. We find that departures from LTE and ionization recombination determine the temperature profiles of the shocks that develop. In LTE almost all the thermal energy goes into ionization, so the temperature rise is very small. In non-LTE, the finite transition rates delay the ionization to behind the shock front. The compression thus goes into thermal energy at the shock front leading to a high temperature amplitude. Further behind the shock front, the delayed ionization removes energy from the thermal pool, which reduces the temperature, producing a temperature spike. The 180 s waves reproduce the observed temporal changes in the calcium K line profiles quite well. The observed wing brightening pattern, the violet/red peak asymmetry and the observed line center behavior are all well reproduced. The short-period waves and the 5 minute period waves fail especially in reproducing the observed behavior of the wings.

Electrochemical Origin of Radical Cations Observed in Electrospray Ionization Mass Spectra

Abstract: A variety of experimental data is presented that implicates electrochemical oxidation of analytes in the electrospray (ES) needle as the mechanism for formation of molecular radical cations observed in the ES ionization mass spectra of alkylsubstituted metalloporphyrins, polycyclic aromatic hydrocarbons (PAH`s), and other compound types. Analyte structural characteristics and solution-phase half-wave oxidation potentials (which correlate with gas-phase ionization energies) can be used to evaluate the likelihood of forming and observing a particular compound as a radical cation. Use of an appropriate solvent is critical in the observation of radical cations generated by the ES process. In addition to dissolving the analyte and providing a stable electrospray, the solvent-(s) must {open_quotes}stabilize{close_quotes} or otherwise protect the radical cation from reactions in solution. Appropriate solvent systems (e.g., methylene chloride/0.1% trifluoroacetic acid) are much the same as used in traditional studies of electrochemically generated radical cations. The ability to produce radical cations in the ES process expands the utility of ES ionization mass spectrometry to include compound classes not normally amenable to the technique (e.g., neutral, nonpolar compounds such as PAH`s) and provides for generation of a different type of molecular species than normally produced in positive-ion ES ionization (i.e., M{sup {sm_bullet}+} versus (Mmore » + H){sup +}, (M + Na){sup +}, etc.).« less

Multiphoton ionization studies of clusters of immiscible liquids. I. C6H6–(H2O)n, n=1,2

Abstract: Resonant two‐photon ionization (R2PI) time‐of‐flight mass spectroscopy is used to record S0–S1 spectra of the neutral complexes C6H6–H2O, C6H6 –HDO, C6H6–D2O, C6H6–(H2O)2, and C6H6–(D2O)2. In C6H6–H2O, the lack of an S0–S1 origin transition and the presence of a splitting at 610 (which is absent in C6H6 –HDO) provide vibronic level evidence that the water molecule is on the sixfold axis undergoing internal rotation about that axis. Rotational band contour analysis of the 610 transitions of the isotopomers confirms this picture and also determines a ground state center‐of‐mass separation between C6H6 and D2O of 3.32±0.07 A, very close to that predicted by ab initio calculations. R2PI scans of the van der Waals structure in the isotopic series C6H6–H2O, C6H6 –HDO, and C6H6–D2O provide tentative assignments for three of the six van der Waals modes in the complex. In C6H6–(H2O)2, rotational band contour analysis of the origin transition provides a best‐fit structure in which the two water molecules reside on ...

Asymmetric photoelectron angular distributions from interfering photoionization processes

Abstract: We have measured asymmetric photoelectron angular distributions for atomic rubidium. Ionization is induced by a one-photon interaction with 280 nm light and by a two-photon interaction with 560 nm light. Interference between the even- and odd-parity free-electron wave functions allows us to control the direction of maximum electron flux by varying the relative phase of the two laser fields.

Thresholds of impact ionization in semiconductors

Abstract: Using a first‐principles approach to the ionization rate, we re‐examine some of the prejudices concerning impact ionization and offer a new view of the role of thresholds We also discuss trends of ionization coefficients related to the energy band structure for silicon and a number of III‐V compounds

Vacuum ultraviolet photoionization mass spectrometric study of C60

Abstract: Gaseous C60 has been studied by photoionization mass spectrometry between the ionization threshold and 40.8 eV. An adiabatic threshold of 7.57±0.01 eV is observed, which may be slightly low due to hot bands. The energy derivative of the photoion yield curve is in rough agreement with the He i photoelectron spectrum on the positions of some peaks, but others are weak or absent. The discrepancy is not attributed to autoionization, but rather to selection rules governing the ejection of low energy electrons into high angular momentum waves. C++60 is observed at higher energies, and becomes ∼0.6 as intense as C+60 at 40.8 eV. The photoion yield curve of C++60, approximately linear well above threshold, appears to exhibit curvature near threshold, thwarting an attempt to make a distinction between two alternative values of the second ionization potential. Fragmentation to form C+58 is only observed at the highest energy, 40.8 eV. The unimolecular decay is modelled by quasiequilibrium theory. In this model, the...

Ionization and dissociation of diatomic molecules in intense infrared laser fields

Abstract: We report the first observation of infrared multiphoton dissociation of a strongly bound diatomic molecule, HCl+. The dissociation is explained by a charge–resonance coupling of electronic states of the molecular ion HCl+. This coupling results in Stark shifts which depend on the internuclear separation thereby changing the molecular bonding. Using a barrier suppression model, we obtain good agreement with the observed dissociation threshold. We show the close relationship between barrier suppression and chaotic dissociation. We also report the first quantitative theoretical and experimental study of infrared multiphoton ionization of a small molecule, HCl. Based on tunnel ionization, we develop a molecular ionization model that incorporates both the large Stark shifts of the molecular ion and the associated large induced dipole moments. The model agrees with the experiment for the multiphoton ionization of HCl. It allows us to derive a general expression for the maximum intensity which can be applied to ...

Ionization of atoms in the tunnelling regime with experimental evidence using Hg atoms

Abstract: Some 'Keldysh-like' theories are analysed leading to a more pragmatic definition of the tunnelling regime of ionization of atoms. Rather than using the more extreme definition of tunnelling (i.e. gamma <<1, F<

Laser-induced molecular alignment probed by a double-pulse experiment

Abstract: The authors have studied the multielectron dissociative ionization of CO using a linearly polarized YAG laser delivering 1015 W cm-2 at 1064 nm, with a pulse duration of 30 ps. By firing two identical laser pulses, with crossed polarizations and a time delay of 800 ps, they show that an intense laser field forces all the molecules to align along its polarization vector. The molecular confinement increases with the dissociation threshold energies of the decay paths involved. Surprisingly enough, the different molecular decay paths observed with a 30 ps pulse release the same 'magic' kinetic energies as already observed with 100 fs, 600 fs and 2 ps pulse durations.

A model of the ionosphere of Titan

Abstract: A 1D model is developed to study both the composition and density of Titan's structure. Ionization rates due both to photoionization by solar EUV flux and to electron impact ionization by photoelectrons and Saturnian magnetospheric electrons are included. The major neutral species (nitrogen and methane) are ionized to produce N2(+), N(+), CH4(+), CH3(+), CH2(+), and CH(+) ions. The total external pressure upstream of Titan at the time of the Voyager encounter is of the order of the maximum ionospheric thermal pressure. It is argued that the solar wind interaction with Venus during periods of high solar wind dynamic pressure might provide a good analogy for the interaction of Titan with the Saturnian magnetospheric plasma.

Electron impact ionization of silver clusters AG n , n≦36

Abstract: Electron impact ionization of gas phase silver clusters Ag n ,n≦36 has been achieved in the threshold region. The vertical ionization potentials in this region clearly demonstrate the evidence of shell effects as well as a distinct even-odd oscillation up ton≃20. Their general size dependence is somewhat different from that of the alkali metal clusters due to the presence of thed-electrons.

Detailed simulations of plasma-induced spectral blueshifting

Abstract: A one-dimensional model, including pulse propagation and detailed ionization dynamics, is developed in order to calculate the spectrum of an intense femtosecond laser pulse transmitted through an atmospheric-density noble gas. The spectrum is broadened and blueshifted due to self-phase-modulation resulting from a combination of optical-field-induced ionization and collisional ionization. Different features in the spectrum can be used to indicate the relative importance of the two ionization mechanisms. The calculations are in good agreement with the recent experimental results of Wood, Siders, and Downer [Phys. Rev. Lett. 67, 3523 (1991)], but indicate that a significant degree of defocusing occurs as a result of plasma generation. The simulations could possibly be used as a test of various theoretical models for optical-field-induced ionization.

Experiments and simulations of tunnel-ionized plasmas.

Abstract: The tunneling-ionization model predicts that fully ionized plasmas with controllable perpendicular ({ital T}{sub {perpendicular}}) and negligible longitudinal temperature ({ital T}{sub {parallel}}) can be produced. The validity of these predictions has been studied through experiments and supporting theory and simulations. Emission of odd harmonics of the laser frequency, indicative of a stepwise ionization process, has been observed. X-ray measurements show that the plasma temperature is higher for a circularly polarized laser-produced plasma compared to when linear polarization is used. Analytically we find that the growth of the stimulated Raman (SRS) and Compton scattering (SCS) instabilities are suppressed during the ionization phase. A higher {ital T}{sub {parallel}} than expected from the single-particle-tunneling model was observed after the ionization phase through SCS fluctuation spectra. The maximum achievable plasma density is found to be limited by ionization induced refraction. One-dimensional (1D) simulations show that, after the ionization phase, the initial {ital T}{sub {parallel}} is low as expected from the single particle model and SRS density fluctuations grow to large values. In 2D simulations, however, {ital T}{sub {parallel}} at the end of the ionization phase is already much higher and only SCS is seen to grow. The simulations indicate that stochastic heating and the Weibelmore » instability play an important role in plasma heating in all directions and in making the plasma isotropic. Two-dimensional simulations also confirm that refraction plays a crucial role in determining the maximum electron density that can be obtained in such plasmas.« less

Dissociation of state-selected complex ions studied by mass-selective pulsed field threshold ionization spectroscopy

Abstract: Pulsed field threshold ionization leads to a vibrationally resolved spectrum of the X 2E1g state of the benzene cation. It is further used to investigate the state‐selective dissociation of the benzene–Ar+ complex. Exclusive detection of threshold ions with a resolution of <9 cm−1 is achieved by kinetic energy analysis in a reflectron mass spectrometer.

Ab initio molecular orbital calculations on DNA base pair radical ions: effect of base pairing on proton-transfer energies, electron affinities, and ionization potentials

Abstract: Ab initio molecular orbital calculations have been performed in this study to estimate proton-transfer energies in DNA complementary base pair radical ions and the effect of base pairing on ionization potentials and electron affinities. The calculated (3-21G) adiabatic proton-transfer energy profile for the neutral GC is found to have a single potential minimum, i.e., no stable proton-transfer state, whereas the GC{sup {sm_bullet}{minus}} shows a second potential minimum which favors proton transfer ({Delta}E = {minus}5 kcal). High level calculations (6-31 + G(d)) of various uncommon protonation states of the DNA bases and DNA base radical ions were performed to estimate the energy for proton transfer in {sup {sm_bullet}}T{sup +}A, {sup {sm_bullet}}C{sup +}G, TA{sup {sm_bullet}{minus}}, and CG{sup {sm_bullet}{minus}}. All transfers are energetically unfavorable, but proton transfer in the AT cation radical and anion radical is only slightly endothermic. Base pairing is not found to significantly affect the ionization potential of A or T in the AT base pair. However, base pairing lowers guanine`s ionization potential by 0.54 eV while raising cytosine`s ionization potential by 0.58 eV. Base pairing reverses the order of the ionization potentials and electron affinities of thymine and cytosine which makes cytosine the most electron affinic DNA base andmore » the least likely to be ionized. The order of ionization potentials in base pairs calculated at the 3-21G level is C> T >> A > G. Further investigation was performed on stacked four base (AT/GC) configurations. A 3-21G calculation of the anion radical of the stacked system with the neutral base pair geometries shows the electron localizes on thymine. However, on relaxation of the nuclear framework of the AT/GC system, the electron is found to preferentially localize on cytosine. 62 refs., 6 figs., 7 tabs.« less

The role of the equatorial electrojet in the evening ionosphere

Abstract: This paper focuses on the role of the equatorial E region in the electrodynamics of the evening ionosphere. The influence and reaction of the electrojet current on the equatorial ionosphere at sunset is investigated using a field line integrated, one-dimensional, electrodynamic model. The one-dimensional, time-varying model predicts the divergence of the horizontal current of the equatorial electrojet for a given time variation of the horizontal electric field. The negative divergence of the horizontal current during the evening hours provides a net upward current out of the equatorial E region into the integrated ionosphere of higher equatorial altitudes (and equivalent latitudes). This upward current affects the vertical electric field magnitudes and subsequent horizontal plasma drifts of the overlying ionosphere. The model allows for chemical recombination and dynamic redistribution of ionization within the electrojet region under the assumption that the profile of the ionization density along a field line is proportional to the chemical equilibrium profile. The eastward horizontal electric field and the net upward current during the 2 hours after sunset combine to lift the ionization out of the E region resulting in ionization densities less than the equilibrium values. As the ionization densities (conductivities) are reduced, the electrodynamics of the equatorial ionosphere is altered. This model of the equatorial electrojet current divergence can be used as a lower boundary to global, two-dimensional models of the equatorial electric fields. Finally, it is proposed that the equatorial electrojet current near sunset has a significant role in the determination of the postsunset enhancement of the horizontal electric field. The electrojet region provides the best avenue for current to be channeled from the dayside to meet the vertical current demands of the F region neutral wind dynamo after sunset. The conductivity reduction in the E region due to the recombination of ionization and the plasma uplift enhances the horizontal (eastward) electric field and thereby increases the speed of the uplift. Thus the dynamic adjustment of ionization has an unstable, feedback relationship with the electric fields which may explain the night to night variability of the horizontal electric field enhancement.

Electronic Structures and Related Properties. Electron Impact Ionization Potentials of Gold and Silver Clusters Men, n ≤ 22

Abstract: Small noble metal clusters were produced by a gas aggregation technique and were investigated mass spectrometrically using electron ionizing energies ranging from 100 eV down to the threshold region. For energies above approximately 20 eV electronic effects are displayed, that characterize a stable cationic distribution, i.e. enhanced stability is found for odd numbered clusters posessing an even number of electrons, furthermore, shell closing appears at n = 3, 9, 21… On the other hand, the electronic effects found while investigating the threshold energy region have to be attributed to neutral clusters, i.e. even numbered clusters are more stable.–The vertical ionization potentials in this region clearly demonstrate the manifestations of shell effects as well as distinct even-odd oscillations. For both Ag and Au a dramatical drop in the ionization potential is observed from dimer to trimer. The overall size-dependence of the ionization energies differs from that of alkali clusters due to the presence of d-electrons.

Two-dimensional modelling of the dielectric barrier discharge in air

Abstract: A self-consistent two-dimensional modelling of microdischarges in devices in which one of the electrodes is covered with a dielectric is presented. The discharge development can be divided into four phases, a Townsend, an ionization wave or streamer, a cathode layer formation, and a decay phase. While during the Townsend phase the initial field strength distribution is hardly distorted, an ionization wave propagates towards the cathode during the following phase. On the wave reaching the cathode, a cathode layer develops. Its radical extension is determined by the increase of the current. During the decay phase the distributions of, for example, field strength and charge carriers are nearly frozen. The discharge fades because of the slow decrease of the field strength within its column. Energy and temperature distributions of microdischarge channels in air at atmospheric pressure are given.

Appearance mass spectrometry of neutral radicals in radio frequency plasmas

Abstract: High‐sensitivity space‐resolved measurements of the number density of neutral free radicals in reactive plasmas have been accomplished by appearance mass spectrometry. This technique is based on several electron volts difference among the appearance potentials for electron‐impact ionization. First, some difficulties accompanied with the appearance mass spectrometry are considered, especially on the influence of excited parent molecules and radical sticking. Second, spatial distributions of neutral radical CH3 and CH2 in a parallel plate rf discharge of methane at a pressure of 0.01–0.3 Torr have been measured. The results are well described by a one‐dimensional numerical modeling whose input parameters are provided by comprehensive measurements of electron energy distribution, ionic composition, and radical sticking coefficients. Third, the density distributions of CF3 and CF2 radicals in a fluorocarbon etching plasma at a CF4 pressure of 0.01–0.2 Torr have been measured. Finally, several applications of ...

On the release and ionization efficiency of catcher-ion-source systems in isotope separation on-line

Abstract: It is shown that the critical steps in isotope separation on-line - diffusion, effusion, and ionization - can be studied in a very efficient way by stimulating the separation process using beam particles of the UNILAC accelerator as tracer instead of reaction products. The analysis of the measured release profiles readily yields the ionization efficiency and the half-life dependence of the separation efficiency on an absolute scale. For the cases in which surface desorption is a fast process compared to solid state diffusion, additionally the decisive delay parameters μ 0 for diffusion and ν for effusion can be extracted. These are closely related to material constants such as diffusion coefficients and enthalpies for surface adsorption and in favourable cases permit extraction of the Arrhenius coefficients. Thus the assumed release model can be tested by comparison with literature values or, in turn, enables the conversion of known material constants into estimates for the half-life dependence of the separation efficiency. Since ion sources have reached a high degree of sophistication, the ionization efficiency is hardly ever the limiting factor of on-line mass separation. This is in general also true for the diffusion step, if the distribution of the reaction products in the catcher is either homogeneous or very close to the surface. As a consequence it turns out that effusion decisively causes the limitations in mass separation on-line, i.e. surface adsorption for refractory elements and the intrinsic delay for very short half-lives.