Showing papers on "Ionization published in 2005"

A comprehensive range of X-ray ionized-reflection models

Abstract: X-ray ionized reflection occurs when a surface is irradiated with X-rays so intense that its ionization state is determined by the ionization parameter ξ ∝ F/n, where F is the incident flux and n the gas density. It occurs in accretion, on to compact objects including black holes in both active galaxies and stellar-mass binaries, and possibly in gamma-ray bursts. Computation of model reflection spectra is often time consuming. Here we present the results from a comprehensive grid of models computed with our code, which has now been extended to include what we consider to be all energetically important ionization states and transitions. This grid is being made available as an ionized-reflection model, REFLION, for XSPEC. Ke yw ords: accretion, accretion discs ‐ line: formation ‐ radiative transfer ‐ galaxies: active ‐ X-rays: general.

Cross Sections for Electron Collisions with Water Molecules

Abstract: Cross section data have been compiled from the literature (to the end of 2003) for electron collisions with water (H2O) molecules. All major collision processes are reviewed including: total scattering, elastic scattering, momentum transfer, excitation of rotational, vibrational, and electronic states, ionization, electron attachment, dissociation, and emission of radiation. In each case we assess the collected data and provide a recommendation of the values of the cross section to be used. They are presented in a tabular form. Isotope effects (H2O versus D2O) are discussed as far as information is available.

Analysis of solids, liquids, and biological tissues using solids probe introduction at atmospheric pressure on commercial LC/MS instruments.

Abstract: Direct analysis of samples using atmospheric pressure ionization (API) provides a more rapid method for analysis of volatile and semivolatile compounds than vacuum solids probe methods and can be accomplished on commercial API mass spectrometers. With only a simple modification to either an electrospray (ESI) or atmospheric pressure chemical ionization (APCI) source, solid as well as liquid samples can be analyzed in seconds. The method acts as a fast solids/liquid probe introduction as well as an alternative to the new direct analysis in real time (DART) and desorption electrospray ionization (DESI) methods for many compound types. Vaporization of materials occurs in the hot nitrogen gas stream flowing from an ESI or APCI probe. Ionization of the thermally induced vapors occurs by corona discharge under standard APCI conditions. Accurate mass and mass-selected fragmentation are demonstrated as is the ability to obtain ions from biological tissue, currency, and other objects placed in the path of the hot ...

Quantum interference during high-order harmonic generation from aligned molecules

Abstract: High-order harmonic generation (HHG) from atoms and molecules offers potential application as a coherent ultrashort radiation source in the extreme ultraviolet and soft X-ray regions1,2,3. In the three-step model4,5,6 of HHG, an electron tunnels out from the atom and may recombine with the parent ion (emitting a high-energy photon) after undergoing laser-driven motion in the continuum. Aligned molecules7,8,9,10,11 can be used to study quantum phenomena in HHG associated with molecular symmetries; in particular, simultaneous observations of both ion yields and harmonic signals under the same conditions serve to disentangle the contributions from the ionization and recombination processes. Here we report evidence for quantum interference of electron de Broglie waves12,13,14 in the recombination process of HHG from aligned CO2 molecules. The interference takes place within a single molecule and within one optical cycle. Characteristic modulation patterns of the harmonic signals measured as a function of the pump–probe delay are explained with simple formulae determined by the valence orbital of the molecules. We propose that simultaneous observations of both ion yields and harmonic signals can serve as a new route to probe the instantaneous structure of molecular systems.

Empirical formula for static field ionization rates of atoms and molecules by lasers in the barrier-suppression regime

Abstract: We propose an empirical formula for the static field ionization rates of atoms and molecules by extending the well-known analytical tunnelling ionization rates to the barrier-suppression regime. The validity of this formula is checked against ionization rates calculated from solving the Schrodinger equation for a number of atoms and ions. The empirical formula retains the simplicity of the original tunnelling ionization rate expression but can be used to calculate the ionization rates of atoms and molecules by lasers at high intensities.

Electrohydrodynamic force and aerodynamic flow acceleration in surface dielectric barrier discharge

Abstract: Surface discharges created in dielectric barrier discharge (DBD) configurations have been proposed as actuators for flow control in aerodynamic applications. We focus on DBDs operating in a glow regime, i.e., where the discharge is sustained by ion-induced secondary electron emission from the surface and volume ionization. After a brief discussion of the force per unit volume acting on the flow and due to the momentum transfer from charged particles to neutral molecules, we present calculations of this force based on a two-dimensional fluid model of the surface discharge. We show that this force is of the same nature as the electric wind in a corona discharge. However, the force in a DBD is localized in the cathode sheath region of the discharge expanding along the dielectric surface. While its intensity is much larger than the analogous force in a direct-current corona discharge, it is active during less than one hundred nanoseconds for each discharge pulse and the time-averaged forces in the two cases a...

Correlated multielectron systems in strong laser fields: A multiconfiguration time-dependent Hartree-Fock approach

Abstract: The multiconfiguration time-dependent Hartree-Fock approach for the description of correlated few-electron dynamics in the presence of strong laser fields is introduced and a comprehensive description of the method is given. Total ionization and electron spectra for the ground and first excited ionic channels are calculated for one-dimensional model systems with up to six active electrons. Strong correlation effects are found in the shape of photoelectron peaks and the dependence of ionization on molecule size.

Solar extreme‐ultraviolet irradiance for general circulation models

Abstract: [1] Recent measurements of the solar extreme-ultraviolet spectrum provide high-resolution spectral irradiance that can be used for calculating ionization and dissociation rates in the upper atmosphere and for providing improved proxy-based models of the solar spectrum. These are crucial inputs for global time-dependent general circulation models of the thermosphere and ionosphere, but computational economies require that a lower-resolution spectrum be used in the calculations without excessive loss of accuracy. The problem is compounded by the photoelectrons generated by ionization, which cause further ionization and dissociation of atmospheric gases. We describe a method for using solar spectral measurements or models to calculate ionization and dissociation rates throughout the upper atmosphere, including photoelectron effects, that is more accurate and more efficient than its predecessors. Examples of use with measurements from the Solar EUV Experiment on the TIMED satellite and with the EUVAC model are given, and an example calculation using the National Center for Atmospheric Research thermosphere-ionosphere-electrodynamics general circulation model is shown.

Intense-field many-body S-matrix theory

Abstract: Intense-field many-body S-matrix theory (IMST) provides a systematic ab initio approach to investigate the dynamics of atoms and molecules interacting with intense laser radiation. We review the derivation of IMST as well as its diagrammatic representation and point out its advantage over the conventional 'prior' and 'post' expansions which are shown to be special cases of IMST. The practicality and usefulness of the theory is illustrated by its application to a number of current problems of atomic and molecular ionization in intense fields. We also present a consistent S-matrix formulation of the quantum amplitude for high harmonic generation (HHG) and point out some of the most general properties of HHG radiation emitted by a single atom as well as its relation to coherent emission from many atoms. Experimental results for single and double (multiple) ionization of atoms and the observed distributions of coincidence measurements are analysed and the dominant mechanisms behind them are discussed. Ionization of more complex systems such as diatomic and polyatomic molecules in intense laser fields is analysed as well using IMST and the results are discussed with special attention to the role of molecular orbital symmetry and molecular orientation in space. The review ends with a summary and a brief outlook.

Glow and Townsend dielectric barrier discharge in various atmosphere

Abstract: The electrical characteristics of homogeneous dielectric barrier discharges in helium, argon and nitrogen are presented and discussed. From the evolution of the discharge current as a function of the voltage applied to the gas it is shown that (i) in helium and argon, during the current increase, the discharge transits from a non-self-sustained discharge to a Townsend discharge and then a subnormal glow discharge (atmospheric pressure glow discharge) (ii) in nitrogen the ionization level is too low to induce a localization of the electrical field and the glow regime cannot be achieved. The discharge is a Townsend discharge (atmospheric pressure Townsend discharge). The characteristics of this specific discharge are described including the time variation of the density of electron, ion, metastable state and electrical field.

Anatomy of strong field ionization

Abstract: Strong field ionization is a starting point for a rich set of physical phenomena associated with attosecond science. This paper provides an introductory overview of the basic theory of strong field ionization and focuses on (i) the physics and the dynamics of the electron transition to the continuum, and (ii) the shape of the electron wavepacket as it appears in the continuum.

Ionization of sputtered metals in high power pulsed magnetron sputtering

Abstract: The ion to neutral ratio of the sputtered material have been studied for high power pulsed magnetron sputtering and compared with a continuous direct current (dc) discharge using the same experimental setup except for the power source. Optical emission spectroscopy (OES) was used to study the optical emission from the plasma through a side window. The emission was shown to be dominated by emission from metal ions. The distribution of metal ionized states clearly differed from the distribution of excited states, and we suggest the presence of a hot dense plasma surrounded by a cooler plasma. Sputtered material was ionized close to the target and transported into a cooler plasma region where the emission was also recorded. Assuming a Maxwell–Boltzmann distribution of excited states the emission from the plasma was quantified. This showed that the ionic contribution to the recorded spectrum was over 90% for high pulse powers. Even at relatively low applied pulse powers, the recorded spectra were dominated by emission from ions. OES analysis of the discharge in a continuous dc magnetron discharge was also made, which demonstrated much lower ionization.

Dipole-dipole excitation and ionization in an ultracold gas of Rydberg atoms.

Abstract: In cold dense Rydberg atom samples, the dipole-dipole interaction strength is effectively resonant at the typical interatomic spacing in the sample, and the interaction has a $1/{R}^{3}$ dependence on interatomic spacing $R$. The dipole-dipole attraction leads to ionizing collisions of initially stationary atoms, which produces hot atoms and ions and initiates the evolution of initially cold samples of neutral Rydberg atoms into plasmas. More generally, the strong dipole-dipole forces lead to motion, which must be considered in proposed applications.

Photoionization mass spectrometer for studies of flame chemistry with a synchrotron light source

Abstract: A flame-sampling molecular-beam photoionization mass spectrometer, recently designed and constructed for use with a synchrotron-radiation light source, provides significant improvements over previous molecular-beam mass spectrometers that have employed either electron-impact ionization or vacuum ultraviolet laser photoionization. These include superior signal-to-noise ratio, soft ionization, and photon energies easily and precisely tunable [E∕ΔE(FWHM)≈250–400] over the 7.8–17-eV range required for quantitative measurements of the concentrations and isomeric compositions of flame species. Mass resolution of the time-of-flight mass spectrometer is m∕Δm=400 and sensitivity reaches ppm levels. The design of the instrument and its advantages for studies of flame chemistry are discussed.

The Lyman alpha forest opacity and the metagalactic hydrogen ionization rate at z~ 2-4

Abstract: Estimates of the metagalactic hydrogen ionization rate from the Lya forest opacity in QSO absorption spectra depend on the complex density distribution of neutral hydrogen along the line of sight. We use a large suite of high-resolution hydrodynamical simulations to investigate in detail the dependence of such estimates on physical and numerical parameters in the context of ACDM models. Adopting fiducial values for cosmological parameters together with published values of the temperature of the IGM and the effective optical depth, the metagalactic ionization rates that reproduce the Lya effective optical depth at z = [2, 3, 4] are Γ HI = [1.3± 0.8 0.5 , 0.9 ± 0.3, 1.0± 0.5 0.3 ] × 10 -12 s -1 , respectively. The errors include estimates of uncertainties in the relevant physical parameters and the numerical accuracy of the simulations. We find the errors are dominated by the uncertainty in the temperature of the low-density IGM. The estimated metagalactic hydrogen ionization rate for the neutral hydrogen distribution in the current concordance ACDM model is more than four times the value inferred for that in an Einstein-de Sitter model of the same rms-density-fluctuation amplitude σ 8 . The estimated ionization rate is also more than double that expected from updated estimates of the emissivity of observed QSOs alone. A substantial contribution from galaxies appears to be required at all redshifts.

High-temperature mass spectrometry: Instrumental techniques, ionization cross-sections, pressure measurements, and thermodynamic data (IUPAC Technical Report)

Abstract: An assessment of high-temperature mass spectrometry and of sources of inaccuracy is made. Experimental, calculated, and estimated cross-sections for ionizalion of atoms and inorganic molecules typically present in high-temperature vapors are summarized. Experimental cross-sections determined for some 56 atoms are generally close to theoretically calculated values, especially when excitation-autoionization is taken into account. Absolute or relative cross-sections for formation of parent ions were measured for ca. 100 molecules. These include homonuclear diatomic and polyatomic molecules, oxides, chalcogenides, halides, and hydroxides. Additivity of atomic cross-sections supplemented by empirical corrections provides fair estimates of molecular cross-sections. Causes of uncertainty are differences in interatomic distances and in shapes of potential energy curves (surfaces) of neutral molecules and of molecular ions and tendency toward dissociative ionization in certain types of molecules. Various mass spectrometric procedures are described that render the accuracy of measured thermodynamic properties of materials largely independent of ionization cross-sections. This accuracy is comparable with that of other techniques applicable under the conditions of interest, but often only the mass spectrometric procedure is appropriate at high temperatures.

Production of Doubly Charged Helium Ions by Two-Photon Absorption of an Intense Sub-10-fs Soft X-Ray Pulse at 42 eV Photon Energy

Abstract: We report on the observation of doubly charged helium ions produced by a nonlinear interaction between a helium atom and photons with a photon energy of 42 eV which are generated with the 27th harmonic of a femtosecond pulse from a Ti:sapphire laser. The number of ions is proportional to the square of the intensity of the 27th harmonic pulse, and thus two-photon double ionization should be dominantly induced as compared with other nonlinear processes accompanying sequential ionization via a singly charged ion. This phenomenon is utilized to measure the pulse duration of the 27th harmonic pulse by using an autocorrelation technique, for the first time to our knowledge, and as a result a duration of 8 fs is found.

Spectral discrimination of Čerenkov radiation in scintillating dosimeters

Abstract: Radiation therapy accelerators require highly accurate dose deposition and the output must be monitored frequently and regularly. Ionization chambers are the primary tool for this control, but their size, their high voltage needed, and the correction needed for electrons make them unsuitable for use during patient treatment. We have developed a small (1-mm-diam and 1-mm-long active part), flexible, and water-equivalent dosimeter. It is suitable for photon and electron beams without corrections, and performs on line dose measurements. This detector is based on only one scintillating fiber and a CCD camera. A new signal processing is used to remove the effect of Cerenkov radiation background, which only requires a preliminary calibration. Central-axis depth-dose distribution comparisons have been achieved with standard ionization chambers, over a range from 8 to 25 MV photons and from 6 to 21 MeV electrons in order to validate this calibration. Results show a very good agreement, with less than 1% difference between the two detectors.

Simultaneous X-ray and UV spectroscopy of the Seyfert galaxy NGC 5548 - II. Physical conditions in the X-ray absorber

Abstract: We present the results from a 500 ks Chandra observation of the Seyfert 1 galaxy NGC 5548. We detect broadened (full width half maximum = 8000 km s −1 ) emission lines of O  and C  in the spectra, similar to those observed in the optical and UV bands. The source was continuously variable, with a 30% increase in luminosity in the second half of the observation. The gradual increase in luminosity occurred over a timescale of ∼300 ks. No variability in the warm absorber was detected between the spectra from the first 170 ks and the second part of the observation. The longer wavelength range of the LETGS resulted in the detection of absorption lines from a broad range of ions, in particular of C, N, O, Ne, Mg, Si, S and Fe. The velocity structure of the X-ray absorber is consistent with the velocity structure measured simultaneously in the ultraviolet spectra. We find that the highest velocity outflow component, at −1040 km s −1 , becomes increasingly important for higher ionization parameters. This velocity component spans at least three orders of magnitude in ionization parameter, producing both highly ionized X-ray absorption lines (Mg  ,S i) as well as UV absorption lines. A similar conclusion is very probable for the other four velocity components. Based upon our observations, we argue that the warm absorber probably does not manifest itself in the form of photoionized clumps in pressure equilibrium with a surrounding wind. Instead, a model with a continuous distribution of column density versus ionization parameter gives an excellent fit to our data. From the shape of this distribution and the assumption that the mass loss through the wind should be smaller than the accretion rate onto the black hole, we derive upper limits to the solid angle as small as 10 −4 sr. From this we argue that the outflow occurs in density-stratified streamers. The density stratification across the stream then produces the wide range of ionization parameter observed in this source. We determine an upper limit of 0.3 Myr −1 for the mass loss from the galaxy due to the observed outflows.

Simultaneous X-ray and UV spectroscopy of the Seyfert 1 galaxy NGC 5548.II. Physical conditions in the X-ray absorber

Abstract: We present the results from a 500 ks Chandra observation of the Seyfert 1 galaxy NGC 5548. We detect broadened emission lines of O VII and C VI in the spectra, similar to those observed in the optical and UV bands. The source was continuously variable, with a 30 % increase in luminosity in the second half of the observation. No variability in the warm absorber was detected between the spectra from the first 170 ks and the second part of the observation. The velocity structure of the X-ray absorber is consistent with the velocity structure measured simultaneously in the ultraviolet spectra. We find that the highest velocity outflow component, at -1040 km/s, becomes increasingly important for higher ionization parameters. This velocity component spans at least three orders of magnitude in ionization parameter, producing both highly ionized X-ray absorption lines (Mg XII, Si XIV) as well as UV absorption lines. A similar conclusion is very probable for the other four velocity components. Based upon our observations, we argue that the warm absorber probably does not manifest itself in the form of photoionized clumps in pressure equilibrium with a surrounding wind. Instead, a model with a continuous distribution of column density versus ionization parameter gives an excellent fit to our data. From the shape of this distribution and the assumption that the mass loss through the wind should be smaller than the accretion rate onto the black hole, we derive upper limits to the solid angle as small as 10^{-4} sr. From this we argue that the outflow occurs in density-stratified streamers. The density stratification across the stream then produces the wide range of ionization parameter observed in this source. Abridged.

Modeling of ionization produced by fast charged particles in gases

Abstract: A computer modeling of ionization is necessary for the simulation of gaseous detectors of fast charged particles. The interactions of the incident particle with matter are well described by the photoabsorption ionization (PAI) model, which is based on the relation between the energy deposited by the fast charged particle in a medium and the photoabsorption cross-section of this medium. Some modification of the PAI model energy-transfer cross-section allows to distinguish the interactions with different atomic shells and to determine the energy of the primary photoelectrons and possible atomic relaxation cascades. Further simulation of paths and absorption of secondary particles results in a realistic reproduction of the space distributions and amount of initial ionization.

Laser ablation of Cu and plume expansion into 1 atm ambient gas

Abstract: A one-dimensional gas-dynamic model is presented for the laser ablation of Cu and the expansion of the Cu vapor in a background gas (He) at 1atm. The ionization of Cu and He, the inverse bremsstrahlung absorption processes and photoionization process, and the back flux onto the target are considered simultaneously. The binary diffusion, the viscosity, and the thermal conduction including the electron thermal conduction are considered as well. Numerical results show that the consideration of ionization and laser absorption in the plume greatly influences the gas dynamics. The ionization of Cu enables the recondensation at the target surface to happen even during the laser pulse. The ionization degree of Cu and He may change greatly with the location in the plume. For laser irradiances ranging from 2to9×1012W∕m2, the simulations show that the second-order ionization of Cu competes with the first-order ionization. In the region close to the target surface, the first-order ionization of Cu dominates. In the c...

Ordered silicon nanocavity arrays in surface-assisted desorption/ionization mass spectrometry.

Abstract: We report here a simple method to generate ordered nanocavity arrays on a Si wafer and use it in surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). A close-packed SiO2 nanosphere array was first deposited on a low-resistivity Si wafer using a convective self-assembly method. The nanoparticle array was then used as a mask in a reactive ion etching (RIE) process to selectively remove portions of the Si surface. Subsequent sonication removed those physically adsorbed SiO2 nanoparticles and exposed an ordered nanocavity array underneath. The importance of this approach is its capability of systematically varying surface geometries to achieve desired features, which makes detailed studies of the impacts of surface features on the desorption/ionization mechanism feasible. We demonstrated that the in-plane width and out-of-plane depth of the cavities were adjustable by varying etching times, and the intercavity spacing was controllable by varying the number of particle layers deposited. M...

Ultrashort pulse non-linear optical absorption in transparent media.

Abstract: A focused ultrashort pulse can reach high enough intensity that non-linear ionization dominates its interaction with transparent media while still having relatively low fluence. In this case, the energy extracted from the beam can counter self-focusing by energy depletion and plasma formation, providing controlled energy deposition that can modify the material in a highly local manner. We demonstrate that non-linear absorption limits the intensity that can be reached and that the energy is deposited prior to the focus. We model the energy distribution, and predict and measure the energy transmitted through the focus. We establish the threshold intensity for non-linear ionization in dielectrics at ~1013 W cm-2. We use the refractive index modification that the non-linear ionization causes in glass to image the spatial distribution of the energy deposition.