Showing papers on "Ionization published in 1988"
500 citations
TL;DR: In this article, a free energy minimization method for computing the dissociation and ionization equilibrium of a multicomponent gas is discussed, which includes terms representing the translational free energy of atoms, ions, and molecules.
Abstract: A free-energy-minimization method for computing the dissociation and ionization equilibrium of a multicomponent gas is discussed. The adopted free energy includes terms representing the translational free energy of atoms, ions, and molecules; the internal free energy of particles with excited states; the free energy of a partially degenerate electron gas; and the configurational free energy from shielded Coulomb interactions among charged particles. Internal partition functions are truncated using an occupation probability formalism that accounts for perturbations of bound states by both neutral and charged perturbers. The entire theory is analytical and differentiable to all orders, so it is possible to write explicit analytical formulas for all derivatives required in a Newton-Raphson iteration; these are presented to facilitate future work. Some representative results for both Saha and free-energy-minimization equilibria are presented for a hydrogen-helium plasma with N(He)/N(H) = 0.10. These illustrate nicely the phenomena of pressure dissociation and ionization, and also demonstrate vividly the importance of choosing a reliable cutoff procedure for internal partition functions.
385 citations
377 citations
TL;DR: An intensity-dependent ponderomotive shift of the ionization threshold is demonstrated, a free-electron scaling of the number of ATI peaks with intensity and frequency of the field is found, and the numerical simulations with two simple Keldysh-type models are compared.
Abstract: We study above-threshold ionization (ATI) of a one-dimensional model atom in a time-varying external laser field. The time-dependent Schr\"odinger equation is integrated in space and time using the Crank-Nicholson method, and the photoelectron energy spectrum is then computed by projecting the wave function onto the energy eigenstates of the time-independent zero-field Hamiltonian. We demonstrate an intensity-dependent ponderomotive shift of the ionization threshold, find a free-electron scaling of the number of ATI peaks with intensity and frequency of the field, and contrast the numerical simulations with two simple Keldysh-type models. For certain field parameters we encounter turn-on transients in the form of ``energy-nonconserving'' substructure within each ATI peak. Effects on the results of the physical parameters such as length and shape of the laser pulse on one hand, and of the iteration parameters on the other, are discussed in detail.
347 citations
TL;DR: In this paper, the authors reported the discovery of high-lying, very-long-lived Rydberg states in NO and determined the ionization potential of NO to be 74721.7 ± 0.4 cm−1.
310 citations
TL;DR: In this article, a compilation of absolute total photoabsorption and partial-channel photoionization cross sections for the valence shells of selected molecules is provided, including simple hydrides (H2O, NH3, CH4), hydrogen halides (HF, HCl, HBr, HI), sulfur compounds (H 2S, CS2, OCS, SO2, SF6), and chlorine compounds (Cl2, CCl4).
Abstract: A compilation is provided of absolute total photoabsorption and partial‐channel photoionization cross sections for the valence shells of selected molecules, including diatomics (H2, N2, O2, CO, NO) and triatomics (CO2, N2O), simple hydrides (H2O, NH3, CH4), hydrogen halides (HF, HCl, HBr, HI), sulfur compounds (H2S, CS2, OCS, SO2, SF6), and chlorine compounds (Cl2, CCl4). The partial‐channel cross sections presented refer to production of the individual electronic states of molecular ions and also to production of parent and specific fragment ions, as functions of incident photon energy, typically from ∼20 to 100 eV. Total photoabsorption cross sections above the first ionization threshold are reported from conventional optical measurements obtained using line and continuum sources and from ‘‘equivalent‐photon’’ dipole (e,e) electron scattering experiments. Partial photoionization cross sections for production of electronic states of molecular ions are obtained from photoelectron spectroscopy and from dip...
288 citations
TL;DR: This is the first of two papers that develop the theory of oscillatory spectra, which is found that the absorption as a function of energy is a superposition of sinusoidal oscillations.
Abstract: This is the first of two papers that develop the theory of oscillatory spectra. When an atom is placed in a magnetic field, and the absorption spectrum into states close to the ionization threshold is measured at finite resolution, so that individual energy levels are not resolved, it is found that the absorption as a function of energy is a superposition of sinusoidal oscillations. These papers present a quantitative theory of this phenomenon. In this first paper, we describe the physical ideas underlying the theory in the simplest possible way, and we present our first calculations based upon the theory. In the second paper, the theory is developed in full detail, proofs of all of the assertions are given, and we describe the algorithm that was used to make the calculations.
279 citations
Abstract: A pulsed electron beam and ion extraction method is used to measure normalized values of partial ionization cross sections for rare gases from threshold to 1000 eV. Cross sections obtained for singly ionized species are used to calibrate the mass transmission efficiency of the ion extraction/analyzer/detection system by the relative flow technique, and this mass transmission curve is then used to determine the absolute cross sections of the multiply ionized species. Total ion cross sections are found by summation of the individual partial cross sections with proper weighting for charge.
255 citations
TL;DR: In this paper, experimental and theoretical cross-section data for electron impact ionization of atoms and ions from fluorine to nickel has been assessed and earlier recommendations for light atoms and ion have been revised.
Abstract: Experimental and theoretical cross‐section data for electron impact ionization of atoms and ions from fluorine to nickel has been assessed and earlier recommendations for light atoms and ions have been revised. Based on this assessment and, in the absence of any data, on the classical scaling laws a recommended cross section has been produced for each species. This has been used to evaluate recommended Maxwellian rate coefficients over a wide range of temperatures. Convenient analytic expressions have been obtained for the recommended cross sections and rate coefficients. The data are presented in both graphical and tabular form and estimates of the reliability of the recommended data are given.
249 citations
TL;DR: In this article, the authors examined the bonding properties of small boron cluster cations (BZTl3+) by measurement of appearance potentials and fragmentation patterns for collision-induced dissociation (CID) with Xe.
Abstract: Bonding in small boron cluster cations (BZTl3+) is examined by measurement of appearance potentials and fragmentation patterns for collision-induced dissociation (CID) with Xe. Cluster stabilities are generally found to increase with increasing cluster size; however, there are large fluctuations from the overall trend. The lowest energy fragmentation channel for all size cluster ions is loss of a single atom. Clusters smaller than six atoms preferentially lose B', while for the larger clusters the charge remains on the BW1+ fragment. The results are used to estimate cluster ionization potentials and geometries. Comparison of measured stabilities with "magic" numbers in the cluster ion size distribution and with total CID cross sections shows that neither is a reliable indicator of stability. We also report on ab initio calculations for both neutral and ionic BI4. The results include cluster geometries, ionization potentials, charge distributions, dissociation energies, and bonding character. The results for IPS, geometries, and De)s are compared with experiment.
235 citations
TL;DR: The two density-functional methods of calculating excitation energies proposed in the preceding paper, combined with the recently formulated quasi-local-density approximation for the equiensemble exchange-correlation energy functional are applied to the He atom.
Abstract: The two density-functional methods of calculating excitation energies proposed in the preceding paper, combined with the recently formulated quasi-local-density approximation for the equiensemble exchange-correlation energy functional [W. Kohn, Phys. Rev. A 34, 737 (1986)], are applied to the He atom. Although the splittings between nearly degenerate levels with different angular momenta are badly overestimated, in both approaches the averages over angular momentum and spin of the experimental excitation energies measured from the ionization threshold are reproduced within a few percent. The computed self-consistent ensemble-averaged densities and the Kohn-Sham potentials associated with them are discussed.
TL;DR: In this article, a new approach to ionizing radiation detection is proposed, where the amount of ionization produced in a detector medium is measured by the heat generated as the charged carriers are drifted across the device under an applied voltage.
Abstract: A new approach to ionizing radiation detection is proposed. The amount of ionization produced in a detector medium is measured by the heat generated as the charged carriers are drifted across the device under an applied voltage. The amount of energy generated can be orders of magnitude larger than that deposited by the radiation itself. A dramatic increase in detector mass can be achieved compared to simple calorimetric particle detectors for equivalent energy thresholds. It is possible to obtain a sensitivity level sufficient for single‐carrier detection. The principle of operation has been demonstrated with an experimental device operated at a temperature of 1.8 K, and improved performance is expected at lower temperatures.
TL;DR: In this article, the authors used tips as sources for free electrons in an STM-like setup to obtain high-resolution images of surfaces with an electron beam of only 15 eV primary energy.
Abstract: Field-ion techniques have been used to create physical point sources for ions and electrons with emission areas and angles orders of magnitude smaller than in any other available source. The monatomic pyramidal tips emit electrons or ionize noble-gas atoms originating from the single front atom. The angular divergence from the normal direction above the single W atom is less than 0.5° for ion beams produced by field ionization. The angular spread for emission from small clusters is somewhat larger for field ionization and electron emission. By employing tips as sources for free electrons in an STM-like setup, we were able to obtain high-resolution images of surfaces with an electron beam of only 15 eV primary energy. The image information is contained in the yield of the secondary electrons created at the sample surface.
TL;DR: In this article, a new atmospheric sampling ion source, based on the establishment of a glow discharge in ambient air drawn into a region of reduced pressure, is described, which exhibits a very short memory and is extremely sensitive for compounds with high proton affinities.
Abstract: A new atmospheric sampling ion source, based on the establishment of a glow discharge in ambient air drawn into a region of reduced pressure, is described. The source is simple, rugged, and relatively maintenance-free, exhibits a very short memory, and is extremely sensitive for compounds with high proton affinities, high electron affinities, high gas-phase acidities, and/or low ionization potentials. The effects of discharge voltage and ion source pressure on the nature of the mass spectra observed are described. These operating parameters affect the absolute number of ions observed and, particularly for positive ions, affect the distribution of the reagent ions and the degree of fragmentation. For illustrative purposes, the limit of detection of dynamic range of the ion source coupled with a mass spectrometer are discussed for 2,4,6-trinitrotoluene. For the present system, a detection limit of 1-2 parts per trillion and a linear dynamic range of at least 6 orders of magnitude are observed.
TL;DR: It is found that a substantial fraction of the B-field associated with the initial wave can be sustained in the plasma as a time-independent B field, which results in an upshifting of the wave frequency, which can be varied by changing of the plasma density.
Abstract: We investigate the effects of quickly creating a plasma around a monochromatic electromagnetic source wave, on time scales on the order of a cycle of the wave. It is found that this results in an upward shifting of the wave frequency, which can be varied by changing of the plasma density. It is also found that a substantial fraction of the magnetic field associated with the initial wave can be sustained in the plasma as a time-independent magnetic field. Computer simulations have been used to study this process in detail, including the effects of finite ionization time. For long ionization times, strong plasma heating results.
TL;DR: This reprint explores the excitement and detection of an atomic-electron wave packet that is localized in the polar and azimuthal angles that is formed through the coherent superposition of Rydberg states of atomic sodium.
Abstract: We have observed a radially localized electron wave packet in a direct pump-probe experiment with 6-ps laser pulses. Use is made of the fact that both excitation and ionization take place predominantly close to the atomic core. A wave packet is created by coherently exciting Rydberg states of rubidium atoms around $n=41$ with a pump pulse, and detected through ionization with a delayed probe pulse. The wave packet is observed to return to the core twice before the spreading becomes too large.
TL;DR: A formula is derived for oscillations in the near-threshold absorption spectrum of an atom in a magnetic field that is similar to (cylindrically modified) Coulomb-scattering waves.
Abstract: A formula is derived for oscillations in the near-threshold absorption spectrum of an atom in a magnetic field. Three approximations are used. (1) Near the atomic nucleus, the diamagnetic field is negligible. (2) Far from the nucleus, the waves propagate semiclassically. (3) Returning waves are similar to (cylindrically modified) Coulomb-scattering waves. With use of these approximations, together with the physical picture described in the accompanying paper, an algorithm is specified for calculation of the spectrum.
TL;DR: In this paper, the authors measured the reactivity of aluminum clusters toward several molecules under thermal conditions, and observed that the reactionivity exhibits a unique dependence on the number of aluminum atoms in the cluster.
Abstract: Using the pulsed cluster beam flow reactor technique, the authors have measured the reactivity of aluminum clusters toward several molecules under thermal conditions. For each different molecule they observe that the reactivity exhibits a unique dependence on the number of aluminum atoms in the cluster. The overall reactivity of aluminum clusters toward different molecules is ordered roughly as O/sub 2/ > CH/sub 3/OH > CO > D/sub 2/O > D/sub 2/ > CH/sub 4/ with CH/sub 4/ showing no reaction under these experimental conditions. In addition, they are able to place upper and lower bounds on the ionization thresholds for the smaller aluminum clusters.
TL;DR: In this paper, the continuum-distorted-wave-eikonal-initial-state model is extended to describe single-electron ionisation by impact of bare projectiles on multielectronic targets.
Abstract: The continuum-distorted-wave-eikonal-initial-state model is extended to describe single-electron ionisation by impact of bare projectiles on multielectronic targets. Applications are given for collisions between multicharged ions and helium. Double differential, single differential and total cross sections are calculated. Experimental data and present theoretical results show deviations from the square of the projectile charge dependence predicted by the first Born approximation.
TL;DR: In this paper, an improved set of electron collision cross sections is derived for SF6 and used to calculate transport, ionization, attachment, and dissociation coefficients for pure SF 6 and mixtures of SF6 with N2, O2, and Ne.
Abstract: An improved set of electron‐collision cross sections is derived for SF6 and used to calculate transport, ionization, attachment, and dissociation coefficients for pure SF6 and mixtures of SF6 with N2, O2, and Ne. The SF6 cross sections differ from previously published sets primarily at very low and high electron energies. At energies below 0.03 eV the attachment cross section is adjusted to fit recent electron swarm experiments, while the elastic momentum transfer cross section is increased to the theoretical limit. At high energies an allowance is made for the excitation of highly excited levels as observed in electron beam experiments. The cross‐section sets used for the admixed gases have previously been published. Electron kinetic energy distributions computed from numerical solutions of the electron‐transport (Boltzmann) equation using the two‐term, spherical harmonic expansion approximation were used to obtain electron‐transport and reaction coefficients as functions of E/N and the fractional concen...
TL;DR: The density and temperature of the low-energy electron gas in the negative glow is determined by combining information from the experiments and Monte Carlo simulations.
Abstract: Various laser diagnostics are used to study the cathode-fall and negative-glow regions of a He glow discharge with a cold Al cathode. The electric field and absolute metastable densities are mapped and the gas temperature is measured over a range of current densities from a near-normal (173 V) to a highly abnormal (600 V) cathode fall. These measurements are analyzed to yield the current balance at the cathode surface, the ionization rate in the cathode-fall region, and the metastable production rate in the cathode-fall and negative-glow regions. The experimental results compare favorably with the results of Monte Carlo simulations. The density and temperature of the low-energy electron gas in the negative glow is determined by combining information from the experiments and Monte Carlo simulations.
TL;DR: In this paper, it is shown that if the field strength is less than a so-called delocalization border, the distribution over unperturbed levels is exponential in the number of absorbed photons and the corresponding localization length is determined.
Abstract: The quantum localization phenomenon that strongly limits any quantum process of diffusive ionization that may be started in systems subjected to a periodic perturbation is discussed. In the case of a highly excited hydrogen atom in a monochromatic field, this phenomenon is theoretically analyzed by reducing the dynamics to appropriate mappings. It is shown that if the field strength is less than a so-called delocalization border, the distribution over unperturbed levels is exponential in the number of absorbed photons and the corresponding localization length is determined. Using the mapping description, it is shown that the excitation process occurring in a two-dimensional atom proceeds essentially along the same lines as in the one-dimensional model. These predictions are supported by results of numerical simulation, and the possibility of their experimental verification is discussed. >
TL;DR: In this article, the gas phase optical spectrum of jet-cooled Pt2 has been investigated over the range of 11,300 to 26,300 cm−1 using resonant two-photon ionization spectroscopy in combination with time-of-flight mass spectrometry.
Abstract: The gas phase optical spectrum of jet‐cooled Pt2 has been investigated over the range of 11 300 to 26 300 cm−1 using resonant two‐photon ionization spectroscopy in combination with time‐of‐flight mass spectrometry. Numerous vibronic bands are observed. Analysis of the data gives the location of some 26 excited electronic states, which are characterized by the frequencies of their origin bands, vibrational frequencies, and anharmonicities. Variation of the second color in a two‐color resonant two‐photon ionization scheme has determined the ionization threshold of Pt2 to be 8.68±0.02 eV. The observation of the onset of predissociation, characterized by a sharp drop in excited state lifetime, places the dissociation energy of Pt2 at 3.14±0.02 eV. In combination with the Pt atomic ionization potential of 8.8±0.2 eV, these results give the bond strength of Pt+2 as D0(Pt−Pt+)=3.26±0.24 eV. The strength of the chemical bond in Pt2, as compared to Au2, demonstrates that there are significant 5d contributions to the chemical bonding in Pt2.
TL;DR: In this paper, the photoelectron impact to EUV photoionization rate ratios for atmospheric O, O2, and N2 was analyzed, and it was found that the ratios vary greatly with altitude.
Abstract: This paper presents calculations performed on the photoelectron impact to EUV photoionization rate ratios for atmospheric O, O2, and N2, and also for the production of N(+) by photodissociative ionization of N2. It was found that the ratios vary greatly with altitude. At high altitudes in the absence of photoelectron transport, the O(+) and N2(+) ionization rate ratios are about 0.35, but they increase with increasing optical depth; in the vicinity of the ionization peak, the photoelectron rate can exceed the EUV ionization rate for O(+) and N2(+). At high altitudes, the O2(+) ionization rate ratio is about half that of O(+); this ratio increases with increasing optical depth to reach a peak of about 0.4. There are also seasonal variations in the ratios at high altitudes depending on the magnitude of the conjugate photoelectron flux.
TL;DR: Calculations of the single and multiphoton ionization of xenon valence-shell electrons are reported for several wavelengths and laser intensities, and departures from perturbation theory at high intensities are demonstrated and discussed.
Abstract: Calculations of the single and multiphoton ionization of xenon valence-shell electrons are reported for several wavelengths and laser intensities. The model follows a single valence electron in the field of an effective core potential, the remaining valence electrons, frozen in their ground-state orbitals, and a linearly polarized laser field. Ionization rates and cross sections are determined using a direct numerical solution of the time-dependent Schr\"odinger equation using a finite-difference technique. Excellent agreement with experimental rates and previous theoretical cross sections has been obtained. Bound states, shifted into resonance by the ac Stark effect, are found to affect the ionization dynamics. Departures from perturbation theory at high intensities are demonstrated and discussed.
TL;DR: In this article, an atomic electron wave packet was localized in polar and azimuthal angles by coherent superposition of Rydberg states of atomic sodium atoms in the presence of a strong dc field.
Abstract: We have excited and detected an atomic electron wave packet that is localized in the polar and azimuthal angles. The wave packet is formed through the coherent superposition of Rydberg states of atomic sodium. The superposition is achieved by short-pulse optical excitation of the atom in the presence of a strong rf field. The wave packet is detected by dc field ionization. The behavior of this wave packet in a strong dc field is much different from that of an eigenstate. This behavior agrees well with a simple classical model.
TL;DR: In this article, x-ray absorption measurements on films deposited onto substrates at room temperature were performed in order to identify the presence of short-range crystalline order in the films and to rule out the ejection of stoichiometric clusters of material from the pellet during the laser ablation/deposition process instead, binary and ternary suboxides are emitted from the targe.
Abstract: The pulsed laser thin‐film deposition process can enable preparation of thin films of complex composition with good control over the film stoichiometry The film compositions are similar to that of the target pellet and as a consequence this technique appears to be an ideal method for preparing high Tc thin films on a variety of substratesThe factors which contribute to this beneficial phenomenon have been explored by a laser ionization mass spectrometry (LIMS) and a post ablation ionization (PAI) neutral velocity analysis technique in order to determine the mass and velocities of the laser ejected material In addition, x‐ray absorption measurements on films deposited onto substrates at room temperature were performed in order to identify the presence of short‐range crystalline order in the films Both of these studies rule out the ejection of stoichiometric clusters of material from the pellet during the laser ablation/deposition process Instead, binary and ternary suboxides are emitted from the targe
TL;DR: Polycyclic aromatic hydrocarbons in C1, C2, and C3 carbonaceous chondrite and in some ordinary chondrites have been directly analyzed by two-step laser desorption/ laser multiphoton ionization mass spectrometry, a selective and sensitive method requiring only milligram samples.
Abstract: Polycyclic aromatic hydrocarbons in C1, C2, and C3 carbonaceous chondrites and in some ordinary chondrites have been directly analyzed by two-step laser desorption/ laser multiphoton ionization mass spectrometry, a selective and sensitive method requiring only milligram samples. At the ionization wavelength of 266 nanometers, parent ion peaks of polycyclic aromatic hydrocarbons dominate the mass spectra. Quantitative analysis is possible; as an example, the concentration of phenanthrene in the Murchison meteorite was determined to be 5.0 parts per million.
TL;DR: In this paper, the authors present a theoretical analysis of high-frequency, diffusion-controlled discharges in which the ionization is provided by direct electron impact with gas atoms within the discharge volume.
Abstract: This paper presents a theoretical analysis of high-frequency, diffusion-controlled discharges in which the ionization is provided by direct electron impact with gas atoms within the discharge volume. The theoretical formulation uses the electron Boltzmann equation along with two-moment, hydrodynamic-type equations for the electrons and the ions. The treatment followed covers a wide range of applied field frequencies, ω/2π, from ω νc to ω νc, where νc is the electron-neutral collision frequency for momentum transfer; it is, however, limited to situations for which ω τ−1, where τ is the characteristic electron energy relaxation time by collisions. It is shown that, under steady-state conditions, the maintenance field and the average absorbed power per electron are practically independent of the field configuration and essentially determined by the gas pressure, the configuration and the dimensions of the discharge vessel, and ω, and not by the applied HF power level. Both the maintenance field and the average absorbed power per electron are shown to obey similarity laws as a function of the discharge operating parameters.
TL;DR: In this article, gas phase spectroscopic investigations of the jet-cooled aluminum trimer are reported using the technique of resonant two-photon ionization with mass spectrometric detection.
Abstract: Gas phase spectroscopic investigations of the jet‐cooled aluminum trimer are reported using the technique of resonant two‐photon ionization with mass spectrometric detection. A discrete band system in the 5200–6100 A region is observed, consisting of an extended vibrational progression in a single vibrational mode. In addition, an apparent continuum absorption is observed which gradually grows in toward shorter wavelengths. The apparent continuum exhibits a long lifetime, 24–35 μs, which is most unusual and indicates that the continuum arises from spectral congestion and not lifetime broadening. At 19 378 cm−1 both the discrete and the continuum absorptions terminate abruptly, indicating the onset of dissociation above this energy. Although it is not certain that dissociation above this energy leads to ground electronic state Al2, this measurement nevertheless places an upper limit on D0(Al2–Al) of 2.40 eV.