Showing papers on "Ionization published in 1999"
TL;DR: In this article, a single-shot damage threshold measurement and modeling for fused silica at 800 nm as a function of pulse duration down to 20 fs was presented, and the respective roles of multiphoton ionization, tunnel ionization and impact ionization in laser damage were examined.
Abstract: We present a single-shot damage threshold measurement and modeling for fused silica at 800 nm as a function of pulse duration down to 20 fs. We examine the respective roles of multiphoton ionization, tunnel ionization, and impact ionization in laser damage. We find that avalanche predominates even in the case of sub-100-fs pulses.
525 citations
TL;DR: In this article, a rate equation for the free electron density was numerically solved to calculate the evolution of the electron density during the laser pulse and to determine the absorption coefficient and energy density of the plasma.
Abstract: The generation of plasmas in water by high-power laser pulses was investigated for pulse durations between 100 ns and 100 fs on the basis of a rate equation for the free electron density. The rate equation was numerically solved to calculate the evolution of the electron density during the laser pulse and to determine the absorption coefficient and energy density of the plasma. For nanosecond laser pulses, the generation of free electrons in distilled water is initiated by multiphoton ionization but then dominated by cascade ionization. For shorter laser pulses, multiphoton ionization gains ever more importance, and collision and recombination losses during breakdown diminish. The corresponding changes in the evolution of the free carrier density explain the reduction of the energy threshold for breakdown and of the plasma energy density observed with decreasing laser pulse duration. By solving the rate equation, we could also explain the complex pulse duration dependence of plasma transmission found in previous experiments. Good quantitative agreement was found between calculated and measured values for the breakdown threshold, plasma absorption coefficient, and plasma energy density.
510 citations
TL;DR: In this article, the authors developed a Monte Carlo code for the transport of stellar X-rays in an axially symmetric disk and showed that hard X-ray from a low-mass young stellar object (YSO) penetrate the associated accretion disk.
Abstract: We have developed a Monte Carlo code for the transport of stellar X-rays in an axially symmetric disk. The code treats Compton scattering and photoelectric absorption and follows the X-rays until they are completely absorbed. We confirm that hard X-rays from a low-mass young stellar object (YSO) penetrate the associated accretion disk. Even without the low-energy photons that are strongly attenuated by the YSO wind, the ionization rate in the inner region of the accretion disk (<1 AU) is many orders of magnitude larger than the standard ionization rate due to Galactic cosmic rays. At a fixed radius from the source, the X-ray ionization rate is a universal function of the vertical column density, independent of the structural details of the disk. The ionization rate scales with the X-ray luminosity and depends only mildly on the X-ray temperature, at least for the temperatures relevant for low-mass YSOs. Thus X-rays from a YSO can ionize regions of an accretion disk from which low-energy cosmic rays are excluded, e.g., by the action of stellar winds. Using a simple theory for the electron fraction we estimate that, for a minimum solar nebula, X-rays ionize the disk beyond 5 AU at a level sufficient to couple magnetic fields and neutral disk material. Inside this radius, the X-rays are ineffective for vertical column densities much larger than ~1025 cm-2, and thus an interior region of the disk will be uncoupled from magnetic fields. If disk accretion is mediated by MHD turbulence, as proposed by Balbus & Hawley, then our results suggest that layered accretion occurs in the inner regions of a disk ionized by X-rays, in accord with Gammie's suggestion based on cosmic-ray ionization.
327 citations
TL;DR: In this article, the global equilibrium geometries and electronic structure of neutral, cationic, and anionic aluminum clusters containing up to 15 atoms were calculated using density-functional theory with generalized gradient approximation for the exchange correlation potential.
Abstract: Density-functional theory with generalized gradient approximation for the exchange-correlation potential has been used to calculate the global equilibrium geometries and electronic structure of neutral, cationic, and anionic aluminum clusters containing up to 15 atoms The total energies of these clusters are then used to study the evolution of their binding energy, relative stability, fragmentation channels, ionization potential, and vertical and adiabatic electron affinities as a function of size The geometries are found to undergo a structural change from two dimensional to three dimensional when the cluster contains 6 atoms An interior atom emerges only when clusters contain 11 or more atoms The geometrical changes are accompanied by corresponding changes in the coordination number and the electronic structure The latter is reflected in the relative concentration of the s and p electrons of the highest occupied molecular orbital Aluminum behaves as a monovalent atom in clusters containing less than seven atoms and as a trivalent atom in clusters containing seven or more atoms The binding energy evolves monotonically with size, but Al7, Al7+, Al7−, Al11−, and Al13− exhibit greater stability than their neighbors Although the neutral clusters do not conform to the jellium model, the enhanced stability of these charged clusters is demonstrated to be due to the electronic shell closure The fragmentation proceeds preferably by the ejection of a single atom irrespective of the charge state of the parent clusters While odd-atom clusters carry a magnetic moment of 1μB as expected, clusters containing even number of atoms carry 2μB for n⩽10 and 0 μB for n>10 The calculated results agree very well with all available experimental data on magnetic properties, ionization potentials, electron affinities, and fragmentation channels The existence of isomers of Al13 cluster provides a unique perspective on the anomaly in the intensity distribution of the mass spectra The unusual stability of Al7 in neutral, cationic, and anionic form compared to its neighboring clusters is argued to be due to its likely existence in a mixed-valence state
310 citations
TL;DR: In this paper, low-frequency modes that develop as a result of an instability in a dusty rf discharge plasma were studied experimentally, leading to an empirical explanation for the instability.
Abstract: Low-frequency modes that develop as a result of an instability in a dusty rf discharge plasma were studied experimentally, leading to an empirical explanation for the instability. In the experiment, particle diameter grew with time. Two instability modes appeared after growth to a sufficient size. A filamentary mode appeared abruptly, and later a great void mode developed as a dust-free region with an intense glow inside and a sharp boundary outside. These modes were characterized by two-dimensional laser light scattering, video imaging, optical emission spectroscopy, Langmuir probe measurements, and Fourier analysis of the fluctuation spectrum. Dust growth was measured by electron microscopy and optical extinction, yielding the dust particle size and dust number density. The electron density was found to be enhanced inside the great void, due to an absence of electron depletion on the dust grains. The great void was explained by the ion drag force, which becomes stronger than the opposing Coulomb force once the particle size reaches a critical diameter. When a dust-free region develops, its electron density is enhanced, the ionization rate increases, and the ion flow that pushes particles outward is further augmented. The plasma used in the experiment grew particles by sputtering of the electrodes, although the same instabilities are expected to occur in other types of dusty plasma discharges as well.
303 citations
TL;DR: In this article, the propagation of high-power short-pulse laser beams over considerable distances in air is studied both experimentally and via numerical simulations Filaments are formed after 5-10 m and their propagation over distances in excess of 200 m is reported for the first time.
Abstract: The propagation of high-power short-pulse laser beams over considerable distances in air is studied both experimentally and via numerical simulations Filaments are formed after 5–10 m and their propagation over distances in excess of 200 m is reported for the first time The lateral dimensions of the filaments are found to range from about 100 μm to a few millimeters in diameter The early values of plasma electron density have been inferred to be a few times 1016 cm−3 using longitudinal spectral interferometry For 500 fs pulses and a wavelength of 1053 nm, the energy in the filament can be quite high initially (∼8 mJ) and is found to stabilize at about 15–2 mJ, after about 35 m A simple model based on the nonlinear Schrodinger equation coupled to a multiphoton ionization law appears to describe several experimental results quite well
283 citations
TL;DR: In this article, the authors present a new theory that incorporates history-dependent ionization coefficients, and it is shown that this model can be utilized to calculate the low-frequency properties of avalanche photodiodes (APD's) (gain, noise, and breakdown probability in the Geiger mode) and the frequency response.
Abstract: Impact ionization in thick multiplication regions is adequately described by models in which the ionization coefficients are functions only of the local electric field. In devices with thin multiplication lengths, nonlocal effects become significant, necessitating new models that account for the path that a carrier travels before gaining sufficient energy to impact ionize. This paper presents a new theory that incorporates history-dependent ionization coefficients, and it is shown that this model can be utilized to calculate the low-frequency properties of avalanche photodiodes (APD's) (gain, noise, and breakdown probability in the Geiger mode) and the frequency response. A conclusion of this work is that an ionization coefficient is not a fundamental material characteristic at a specific electric field and that any experimental determination of ionization coefficients is valid only for the particular structure on which the measurement was performed.
283 citations
TL;DR: Dusty plasmas in a gas discharge often feature a stable void, i.e., a dust-free region inside the dust cloud, under conditions relevant to both plasma processing discharges and plasma crystal experiments.
Abstract: Dusty plasmas in a gas discharge often feature a stable void, i.e., a dust-free region inside the dust cloud. This occurs under conditions relevant to both plasma processing discharges and plasma crystal experiments. The void results from a balance of the electrostatic and ion drag forces on a dust particle. The ion drag force is driven by a flow of ions outward from an ionization source and toward the surrounding dust cloud, which has a negative space charge. In equilibrium the force balance for dust particles requires that the boundary with the dust cloud be sharp, provided that the particles are cold and monodispersive. Numerical solutions of the one-dimensional nonlinear fluid equations are carried out including dust charging and dust-neutral collisions, but not ion-neutral collisions. The regions of parameter space that allow stable void equilibria are identified. There is a minimum ionization rate that can sustain a void. Spatial profiles of plasma parameters in the void are reported. In the absence of ion-neutral collisions, the ion flow enters the dust cloud's edge at Mach number M = 1. Phase diagrams for expanding or contracting voids reveal a stationary point corresponding to a single stable equilibrium void size, provided the ionization rate is constant. Large voids contract and small voids expand until they attain this stationary void size. On the other hand, if the ionization rate is not constant, the void size can oscillate. Results are compared to recent laboratory and microgravity experiments.
258 citations
TL;DR: A detailed discussion of the slowing down of fast electrons in a partly ionized gas mixture of H, H2, and He with a varying ratio of H to H2 and a fixed fraction of He is presented in this article.
Abstract: A detailed discussion is presented of the slowing down of fast electrons in a partly ionized gas mixture of H, H2, and He with a varying ratio of H to H2 and a fixed fraction of He. The mean energies and yields for ionization, electronic excitation, dissociation, and vibrational excitation and the heating efficiencies are calculated for fractional ionizations from 0 to 0.1. Analytical formulas are obtained for many of the parameters.
242 citations
CERN1
11 Dec 1999-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: In this article, the authors investigated the dependence of gain and charge collection efficiency on the external fields in a range of values for the hole diameter and pitch, and found that the collection efficiency of ionization electrons into the multiplier reaches a plateau extending to higher values of drift field the larger the GEM voltage and its optical transparency.
Abstract: We report the results of systematic investigations on the operating properties of detectors based on the gas electron multiplier (GEM). The dependence of gain and charge collection efficiency on the external fields has been studied in a range of values for the hole diameter and pitch. The collection efficiency of ionization electrons into the multiplier, after an initial increase, reaches a plateau extending to higher values of drift field the larger the GEM voltage and its optical transparency. The effective gain, fraction of electrons collected by an electrode following the multiplier, increases almost linearly with the collection field, until entering a steeper parallel plate multiplication regime. The maximum effective gain attainable increases with the reduction in the hole diameter, stabilizing to a constant value at a diameter approximately corresponding to the foil thickness. Charge transfer properties appear to depend only on ratios of fields outside and within the channels, with no interaction between the external fields. With proper design, GEM detectors can be optimized to satisfy a wide range of experimental requirements: tracking of minimum ionizing particles, good electron collection with small distortions in high magnetic fields, improved multi-track resolution and strong ion feedback suppression in large volume and time-projection chambers.
224 citations
TL;DR: In this paper, the dominant dissociative multiphoton ionization (DMI) pathway for NO2 at 375.3 nm is identified as three-photon excitation to a repulsive potential surface correlating to NO(C 2 Π)+O(3P) followed by onephoton ionization to NO+O(X 1 Π+1Σ+).
Abstract: We present the first results using a new technique that combines femtosecond pump–probe methods with energy- and angle-resolved photoelectron–photoion coincidence imaging. The dominant dissociative multiphoton ionization (DMI) pathway for NO2 at 375.3 nm is identified as three-photon excitation to a repulsive potential surface correlating to NO(C 2Π)+O(3P) followed by one-photon ionization to NO+(X 1Σ+). Dissociation along this surface is followed on a femtosecond timescale.
TL;DR: In this paper, the photodissociation and photoionization of liquid water following two-photon absorption at 266 nm was studied in the spectral range from 213 to 1108 nm with sub-picosecond time resolution.
Abstract: The photodissociation and photoionization of liquid water following two-photon absorption at 266 nm is studied in the spectral range from 213 to 1108 nm with subpicosecond time resolution. Probing in the UV enables the first direct simultaneous observation of the photoproducts eaq−, Haq, and OHaq. This makes it possible to follow the geminate recombination kinetics between the photoproducts and to determine the relative yields of the dissociation and ionization channels. The concentration of hydrated electrons deduced from the visible and near-infrared transient absorption measurements decays by 40%±2% within the first 90 ps due to recombination with OHaq and H3O+. Analyzing our measurements of the hydrated electron concentration using the independent reaction time approximation results in the relative yields of 82%±3% and 18%±3% for recombination with OHaq and with H3O+, respectively. This is in excellent agreement with the relative yield of 82%±10% for recombination with OHaq determined directly from ou...
TL;DR: In this article, a manybody Green's function method is used to calculate vertical valence ionization potentials to high accuracy for the atoms and molecules Ne, N2, F2, CO2, P2, H2O, and H2S.
Abstract: A many‐body Green’s function method is used to calculate vertical valence ionization potentials to high accuracy for the atoms and molecules Ne, N2, F2, CO2, P2, H2O, and H2S. Large basis sets including several sets of polarization functions are used in the calculations to reach the limit of the presently achievable accuracy for molecular systems. The maximum errors in the computed ionization potentials are 0.1 to 0.25 eV depending on the molecule and the basis set. The results are extremely stable, when large basis sets are used. Comparison with other methods is made.
TL;DR: The origin of ions in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is currently a matter of active research as discussed by the authors, and a number of chemical and physical pathways have been suggested for MALDI ion formation, including gas-phase photoionization, ion-molecule reactions, disproportionation, excited-state proton transfer, energy pooling, thermal ionization, and preformed ions.
Abstract: The origin of ions in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is currently a matter of active research. A number of chemical and physical pathways have been suggested for MALDI ion formation, including gas-phase photoionization, ion–molecule reactions, disproportionation, excited-state proton transfer, energy pooling, thermal ionization, and desorption of preformed ions. These pathways and others are critically reviewed, and their varying roles in the wide variety of MALDI experiments are discussed. An understanding of ionization pathways should help to maximize ion yields, control analyte charge states and fragmentation, and gain access to new classes of analytes. © 1999 John Wiley & Sons, Inc. Mass Spec Rev 17: 337–366, 1998
Book•
30 Jul 1999
TL;DR: In this article, the authors discuss the interaction of radiation with matter: energy transfer from Fast Charged Particles, ionization and excitation Phenomena, and free-Ion yield.
Abstract: Introduction. Interaction of Radiation with Matter: Energy Transfer from Fast Charged Particles. Structure of Charged Particle Tracks in Condensed Media. Ionization and Excitation Phenomena. Radiation Chemistry of Gases. The Solvated Electron. Spur Theory of Radiation Chemical Yields: Diffusion and Stochastic Models. Electron Thermalization and Related Phenomena. Electron Escape: The Free-Ion Yield. Electron Mobility in Liquid Hydrocarbons. Radiation Chemical Applications in Science and Industry.
TL;DR: In this paper, a new theory for impact ionization that utilizes history-dependent ionization coefficients to account for the nonlocal nature of the ionization process has been described, and a systematic study of the noise characteristics of GaAs homojunction avalanche photodiodes with different multiplication layer thicknesses is also presented.
Abstract: For Part I see R.J. McIntyre, ibid., vol.46, no.8, pp.1623-31 (1999). In Part I, a new theory for impact ionization that utilizes history-dependent ionization coefficients to account for the nonlocal nature of the ionization process has been described. In this paper, we will review this theory and extend it with the assumptions that are implicitly used in both the local-field theory in which the ionization coefficients are functions only of the local electric field and the new one. A systematic study of the noise characteristics of GaAs homojunction avalanche photodiodes with different multiplication layer thicknesses is also presented. It is demonstrated that there is a definite "size effect" for thin multiplication regions that is not well characterized by the local-field model. The new theory, on the other hand, provides very good fits to the measured gain and noise. The new ionization coefficient model has also been validated by Monte Carlo simulations.
TL;DR: In this article, a semi-empirical model capable of predicting the rate of tunnel ionization of N 2 and O 2 molecules interacting with strong Ti:sapphire laser pulses is derived.
TL;DR: In this article, the CCSDT-3 and CC3 parametrizations of the neutral ground state were used to obtain the vertical ionization potentials for N2, CO, and F2 with the most theoretically complete approximations.
Abstract: Vertical ionization potentials can be obtained from existing computer programs for the high-level treatment of excited states by simply including a continuum orbital in the basis set. Exploiting this feature easily allows final state energies for ionized states to be calculated at several previously untested levels of theory that go beyond the equation-of-motion coupled-cluster singles and doubles model. Values obtained for N2, CO, and F2 with the most theoretically complete approximations studied here (those based on the CCSDT-3 and CC3 parametrizations of the neutral ground state) are in excellent agreement with experiment when a large basis set is used.
TL;DR: It was demonstrated that laser mass spectrometry (REMPI-TOFMS) enables a real-time on-line trace analysis of combustion flue gases or industrial process gases.
Abstract: A newly developed, mobile laser mass spectrometer (resonance-enhanced multiphoton ionization − time-of-flight mass spectrometer, REMPI-TOFMS) was applied to on-line measurements at a waste incineration pilot plant. REMPI-TOFMS combines the optical selectivity of resonance-enhanced multiphoton ionization with a time-of-flight mass analysis to give a two-dimensional analytical method. Special care was taken to build up a sampling and inlet system suitable for on-line measurements of large, semivolatile polycyclic aromatic hydrocarbons (PAHs). An effusive molecular beam inlet in combination with a fixed frequency UV laser (Nd:YAG at 266 nm or KrF excimer at 248 nm) was used. Under these conditions, many different PAHs can be ionized selectively from the complex flue gas matrix. For example, the achieved detection limit for naphthalene is in the 10 parts-per-trillion by volume (pptv) concentration range. Calibration was performed by using external concentration standards supplied in low ppbv concentrations. T...
TL;DR: In this paper, it was shown that a high initial velocity results from incorporation of the analyte into the matrix crystals and that cooling upon expansion is effective at high initial velocities and responsible for reduced fragmentation observed in such cases compared with slow matrices.
Abstract: Since the early days of matrix-assisted laser desorption/ionization (MALDI), measurements showing that MALDI ions and neutrals have high initial velocities have led to wide acceptance of the idea that a jet of released material entrains analyte ions. The initial velocity, which could previously be determined only with large uncertainty, can be measured today with high reliability in a delayed-extraction MALDI/time-of-flight system by following the linear dependence of ion flight time vs the applied extraction delay. The detection of different initial velocities for different matrices, with and without additives, for various preparation protocols and for different classes of analytes proves that the magnitude of the initial velocity can indeed be regarded as a valuable and meaningful characteristic of the MALDI process. Based on the results reported here, it is postulated that a high initial velocity results from incorporation of the analyte into the matrix crystals and that cooling upon expansion is effective at high initial velocities and responsible for reduced fragmentation observed in such cases compared with ‘slow’ matrices. Copyright © 1999 John Wiley & Sons, Ltd.
TL;DR: An update of electron collision cross sections and electron transport parameters for CF4, CHF3, C2F6, and C3F8 was presented in this article, along with electron transport cross sections.
Abstract: An update of electron–collision cross sections and electron transport parameters is presented for CF4, CHF3, C2F6, and C3F8.
TL;DR: In this article, the chemistry of deuterium-bearing molecules in outer regions of protoplanetary disks, where comets may form, was investigated, and it was shown that molecules formed in the disk have higher D/H ratios (by which we mean abundance ratios between the singly deuterated species and the normal species) than the elemental D/C ratio in protosolar abundance, which is consistent with observations of recent comets.
Abstract: We investigate the chemistry of deuterium-bearing molecules in outer regions of protoplanetary disks, where comets may form. We find that molecules formed in the disk have higher D/H ratios (by which we mean abundance ratios between the singly deuterated species and the normal species) than the elemental D/H ratio in protosolar abundance, which is consistent with observations of recent comets. Despite the higher densities in protoplanetary disks, deuterium fractionation occurs in a similar way as in molecular clouds; because of the differences in zero-point energies and the existence of rapid ion-molecule isotopic exchange reactions, species such as H have high D/H ratios, which propagate to other molecules via gas-phase chemical reactions. Our results depend on a variety of factors such as the ionization rate and the temperature of the disk, which is somewhat uncertain because of possible variations in stellar luminosity and the existence of accretion shocks. In order to reproduce the observed cometary values of DCN/HCN and HDO/H2O, a lower ionization rate (~10-18 s-1) than is "standard" in molecular clouds is more favorable. The calculated DCN/HCN ratio also depends on whether the products assumed for the grain surface recombination reaction between HCO+ and electrons are dissociative or not.
TL;DR: In this article, a lower bound for the dissociation energy of (H2O)2:1.001 eV was derived, along with the known ionization potential of H2O, 12.615±0.13 eV (36±3 kcal/mole).
Abstract: Photoionization studies of (H2O)2 and (HF)2 producing H3O+ and H2F+ yield 7.18±0.08 eV (165.8±1.8 kcal/mole) and 4.09±0.06 eV (94.3±1.4 kcal/mole) as the proton affinities of H2O and HF, respectively. The measured ionization potential of (H2O)2, 11.21±0.09 eV, along with the known ionization potential of H2O, 12.615±0.001 eV, allow the deduction of a lower bound for the dissociation energy of (H2O)2:1.58±0.13 eV (36±3 kcal/mole). The experiments have demonstrated that photoionization of dimers is one of the most useful general methods for the determination of proton affinities.
TL;DR: In this paper, variable-temperature Hall-effect measurements were employed to optimize doping for GaN layers utilized in blue, blue-green and green light emitting diodes (LEDs).
Abstract: Variable-temperature Hall-effect measurements were employed to optimize doping for GaN layers utilized in blue, blue-green and green light emitting diodes (LEDs). N-type doping was accomplished by doping with Si, Ge, and O, and the electronic properties of these donors were studied. Si and Ge, which substitute for Ga, are shallow donors with almost identical activation energies for ionization (ca. 17 and ca. 19 meV, respectively, for a donor concentration of ca. 3×10 17 cm −3 ). O substitutes for N and introduces a slightly deeper donor level into the bandgap of GaN having an activation energy of ca. 29 meV (for a donor concentration of ca. 1×10 18 cm −3 ). Mg doping was employed to achieve p-type conductivity for GaN device layers. Mg substitutes for Ga introducing a relatively deep acceptor level. For the analysis of the variable-temperature Hall-effect data, it was found important to take the coulomb interaction between ionized acceptors into account, leading to lower activation energy with increasing degree of ionization (increasing temperature). The activation energy for ionization of Mg acceptors in GaN was thus estimated to be (208±6) meV for very low acceptor concentrations. Using optimized nitride layers, LEDs with typical external quantum efficiencies of ca. 10% in the blue and blue-green, and ca. 8% in the green wavelength range were achieved. Due to optimized doping, the forward voltages for these diodes were as low as 3.2 V at 20 mA drive current.
TL;DR: In this paper, a three-dimensional, stationary, two-fluid plasma model for electrons and one ion species was developed to understand the local interaction of Io's atmosphere with the Io plasma torus and the formation of Io ionosphere.
Abstract: A three-dimensional, stationary, two-fluid plasma model for electrons and one ion species was developed to understand the local interaction of Io's atmosphere with the Io plasma torus and the formation of Io's ionosphere. Our model calculates, self-consistently, the plasma density, the velocity and the temperatures of the ions and electrons, and the electric field for a given neutral atmosphere and imposed Io plasma torus conditions but assumes for the magnetic field the constant homogeneous Jovian field. With only photoionization in a pure SO2 atmosphere it is impossible to correctly model the plasma measurements by the Galileo spacecraft. With collisional ionization and photoionization the observations can be successfully modeled when the neutral atmospheric column density is Ncol = 6 × 1020 m−2 and the atmospheric scale height is H = 100 km. The energy reservoir of the Io plasma torus provides via electron heat conduction the necessary thermal energy for the maintenance of the collisional ionization process and thus the formation of Io's ionosphere. Anisotropic conductivity is shown numerically as well as analytically to be essential to understand the convection patterns and current systems across Io. The electric field is very greatly reduced, because the ionospheric conductances far exceed the Alfven conductance ΣA, and also strongly twisted owing to the Hall effect. We find that the electric field is twisted by an analytic angle tan Θtwist = Σ2/(Σ1 + 2ΣA) from the anti-Jupiter direction toward the direction of corotation for constant values of the Pedersen and Hall conductances Σ1 and Σ2 within a circle encompassing Io's ionosphere. Because the electron velocity is approximately equal to the E × B drift velocity, the electron flow trajectories are twisted by the same angle toward Jupiter, with E and B the electric and magnetic fields, respectively. Since Σ1 ∼ Σ2, the electron flow is strongly asymmetric during convection across Io, and the magnitude of this effect is directly due to the Hall conductivity. In contrast, the ions are diverted slightly away from Jupiter when passing Io. Large electric currents flow in Io's ionosphere owing to these substantially different flow patterns for electrons and ions, and our calculations predict that a total electric current of 5 million A was carried in each Alfven wing during the Galileo flyby. We also find a total Joule heating rate dissipated in Io's ionosphere of P = 4.2 × 1011 W.
TL;DR: In this article, the tracold potassium molecules were detected sensitively and selectively by use of two-color resonant ionization, followed by time-of-flight mass spectroscopy.
Abstract: Ultracold potassium molecules are produced in the $X^{1}\ensuremath{\Sigma}_{g}^{+}$ electronic ground state by photoassociation in a magneto-optical trap. They are detected sensitively and selectively by use of two-color resonant ionization, followed by time-of-flight mass spectroscopy. We observe deeply bound molecules with $\ensuremath{
u}\ensuremath{\cong}36$, at a temperature of about $300\ensuremath{\mu}\mathrm{K}$. Presently about 1000 molecules/sec are produced, and much higher production rates should be attainable in the future.
TL;DR: The limitation of Born's approach is demonstrated and the importance of solute-solvent interactions on the change of the pKa values emphasized.
TL;DR: In this paper, the authors present a quantitative analysis of the X-ray spectrum of the eclipsing high mass Xray binary Vela X-1 (4U 0900-40) using archival data from the ASCA Solid State Imaging Spectrometer.
Abstract: We present a quantitative analysis of the X-ray spectrum of the eclipsing high mass X-ray binary Vela X-1 (4U 0900-40) using archival data from the ASCA Solid-State Imaging Spectrometer. The observation covers a time interval centered on the eclipse of the X-ray pulsar by the companion. The spectrum exhibits two distinct sets of discrete features: (1) recombination lines and radiative recombination continua from mostly hydrogenic and helium-like species produced by photoionization in an extended stellar wind; and (2) fluorescent K-shell lines associated with near-neutral species also present in the circumsource medium. These features are superposed on a faint continuum, which is most likely nonthermal emission from the accreting neutron star that is scattered into our line of sight by free electrons in the wind. Using a detailed spectral model that explicitly accounts for the recombination cascade kinetics for each of the constituent charge states, we are able to obtain a statistically acceptable (χ = 0.88) fit to the observed spectrum and to derive emission measures associated with the individual K-shell ions of several elements. From calculations of the ionization balance using the photoionization code, XSTAR, we assign ionization parameters, ξ, to several ions, and construct a differential emission measure (DEM) distribution. The DEM distribution spans a broad range in ξ (Δ log ξ 2) and is centered around log ξ = 2.5. We find that the total emission measure of the visible portion of the highly ionized wind is ~3 × 1056 cm-3. The qualitative aspects of the inferred DEM distribution are consistent with a wind model derived from the Hatchett & McCray picture of an X-ray source immersed in a stellar wind with a generalized Castor, Abbott, & Klein velocity profile. Using this formalism, theoretical DEM distributions, parameterized only by a mass-loss rate and a wind velocity profile, are calculated and used to predict the detailed X-ray spectrum, which is then compared to the ASCA data. Again, we find a statistically acceptable fit (χ = 1.01), with a best-fit mass-loss rate of ~2.7 × 10-7 M☉ yr-1. This is approximately a factor of 10 lower than previous estimates of the mass-loss rate for the Vela X-1 companion star, which have primarily been determined from C IV and Si IV P Cygni profiles, and X-ray absorption measurements. We argue that this discrepancy can be reconciled if the X-ray-irradiated portion of the wind in Vela X-1 is structurally inhomogeneous, consisting of hundreds of cool, dense clumps embedded in a hotter, more ionized gas. Most of the mass is contained in the clumps, while most of the wind volume (>95%) is occupied by the highly ionized component. We show quantitatively that this interpretation is also consistent with the presence of the X-ray fluorescent lines in the ASCA spectrum, which are produced in the cooler, clumped component.
TL;DR: In this article, the coupling mechanism between an intense (∼1013 W cm-2, 780 nm) near-infrared radiation field of duration 50−200 fs with molecules having 5−50 atoms is considered.
Abstract: The coupling mechanism between an intense (∼1013 W cm-2, 780 nm) near-infrared radiation field of duration 50−200 fs with molecules having 5−50 atoms is considered in this article. In general, the interaction of intense radiation fields with molecules can result in both electron emission and subsequent dissociation. For the laser excitation scheme employed here, intact ions are observed in addition to dissociative ionization channels for all classes of molecules investigated to date. An excitation mechanism is considered where the electric field of the laser mediates the coupling between the radiation and the molecule. This field-induced ionization is compared with the more common frequency-mediated coupling mechanism found in multiphoton processes. Measurements of intense-laser photoionization probability are presented for several series of molecules. An outline of our structure-based model is presented to enable calculation of relative tunneling rates and prediction of the laser−molecule coupling mechan...
TL;DR: In this article, the authors review the theoretical tools needed for the description of the molecular continuum with B-splines and discuss what is probably the most successful application up to date: the photoionization of H2, including resonant effects and simultaneous dissociation.
Abstract: One of the most significant developments in modern computational physics has been the introduction of B-spline basis sets to evaluate continuum states of atoms and molecules. B-splines were first used in the early 1970s to describe the hydrogen atom and, since then, most applications have focused on atomic systems. Very recently, B-splines have emerged as powerful tools to describe the molecular continuum as well. Ionization and dissociation can now be studied with an accuracy comparable to that achieved in recent experiments. In this paper, we review the theoretical tools needed for the description of the molecular continuum with B-splines and discuss what is probably the most successful application up to date: the photoionization of H2, including resonant effects and simultaneous dissociation. Recent progress for more complicated molecules is also reviewed.