Showing papers on "Ionization published in 2001"

Photoelectric Emission from Interstellar Dust: Grain Charging and Gas Heating

Abstract: We model the photoelectric emission from and charging of interstellar dust and obtain photoelectric gas heating efficiencies as a function of grain size and the relevant ambient conditions. We employ improved estimates for photoelectric thresholds, yields, and electron capture rates. Using realistic grain size distributions, we evaluate the net gas heating rate for various interstellar environments and find less heating for dense regions characterized by RV = 5.5 than for diffuse regions with RV = 3.1. We provide fitting functions that reproduce our numerical results for photoelectric heating and recombination cooling for a wide range of interstellar conditions. Finally, we investigate the potential importance of photoelectric heating in H II regions, including the warm ionized medium. We find that photoelectric heating could be comparable to or exceed heating due to photoionization of H for high ratios of the radiation intensity to the gas density. We also find that photoelectric heating by dust can account for the observed variation of temperature with distance from the Galactic midplane in the warm ionized medium.

Supernova Remnants in the Sedov Expansion Phase: Thermal X-Ray Emission

Abstract: Improved calculations of X-ray spectra for supernova remnants (SNRs) in the Sedov-Taylor phase are reported, which for the first time include reliable atomic data for Fe L-shell lines. This new set of Sedov models also allows for a partial collisionless heating of electrons at the blast wave and for energy transfer from ions to electrons through Coulomb collisions. X-ray emission calculations are based on the updated Hamilton-Sarazin spectral model. The calculated X-ray spectra are successfully interpreted in terms of three distribution functions: the electron temperature and ionization timescale distributions, and the ionization timescale-averaged electron temperature distribution. The comparison of Sedov models with a frequently used single nonequilibrium ionization (NEI) timescale model reveals that this simple model is generally not an appropriate approximation to X-ray spectra of SNRs. We find instead that plane-parallel shocks provide a useful approximation to X-ray spectra of SNRs, particularly for young SNRs. Sedov X-ray models described here, together with simpler plane shock and single-ionization timescale models, have been implemented as standard models in the widely used XSPEC v11 spectral software package.

Far-Infrared Spectroscopy of Normal Galaxies: Physical Conditions in the Interstellar Medium

Abstract: The most important cooling lines of the neutral interstellar medium (ISM) lie in the far-infrared (FIR). We present measurements by the Infrared Space Observatory Long Wavelength Spectrometer of seven lines from neutral and ionized ISM of 60 normal, star-forming galaxies. The galaxy sample spans a range in properties such as morphology, FIR colors (indicating dust temperature), and FIR/blue ratios (indicating star formation activity and optical depth). In two-thirds of the galaxies in this sample, the [C II] line flux is proportional to FIR dust continuum. The other one-third show a smooth decline in L[C II]/LFIR with increasing Fν(60 μm)/Fν(100 μm) and LFIR/LB, spanning a range of a factor of more than 50. Two galaxies at the warm and active extreme of the range have L[C II]/LFIR < 2 × 10-4 (3 σ upper limit). This is due to increased positive grain charge in the warmer and more active galaxies, which leads to less efficient heating by photoelectrons from dust grains. The ratio of the two principal photodissociation region (PDR) cooling lines L[O I]/L[C II] shows a tight correlation with Fν(60 μm)/Fν(100 μm), indicating that both gas and dust temperatures increase together. We derive a theoretical scaling between [N II] (122 μm) and [C II] from ionized gas and use it to separate [C II] emission from neutral PDRs and ionized gas. Comparison of PDR models of Kaufman et al. with observed ratios of (1) L[O I]/L[C II] and (L[C II] + L[O I])/LFIR and (2) L[O I]/LFIR and Fν(60 μm)/Fν(100 μm) yields far-UV flux G0 and gas density n. The G0 and n values estimated from the two methods agree to better than a factor of 2 and 1.5, respectively, in more than half the sources. The derived G0 and n correlate with each other, and G0 increases with n as G0 ∝ nα, where α ≈ 1.4 . We interpret this correlation as arising from Stromgren sphere scalings if much of the line and continuum luminosity arises near star-forming regions. The high values of PDR surface temperature (270-900 K) and pressure (6 × 104-1.5 × 107 K cm-3) derived also support the view that a significant part of grain and gas heating in the galaxies occurs very close to star-forming regions. The differences in G0 and n from galaxy to galaxy may be due to differences in the physical properties of the star-forming clouds. Galaxies with higher G0 and n have larger and/or denser star-forming clouds.

Assessment of W1 and W2 theories for the computation of electron affinities, ionization potentials, heats of formation, and proton affinities

Abstract: The performance of two recent ab initio computational thermochemistry schemes, W1 and W2 theory [J. M. L. Martin and G. de Oliveira, J. Chem. Phys. 111, 1843 (1999)], is assessed for an enlarged sample of thermochemical data consisting of the ionization potentials and electron affinities in the G2-1 and G2-2 sets, as well as the heats of formation in the G2-1 and a subset of the G2-2 set. We find W1 theory to be several times more accurate for ionization potentials and electron affinities than commonly used (and less expensive) computational thermochemistry schemes such as G2, G3, and CBS-QB3: W2 theory represents a slight improvement for electron affinities but no significant one for ionization potentials. The use of a two-point A+B/L5 rather than a three-point A+B/CL extrapolation for the self-consistent field (SCF) component greatly enhances the numerical stability of the W1 method for systems with slow basis set convergence. Inclusion of first-order spin–orbit coupling is essential for accurate ioniza...

Nonadiabatic tunnel ionization: Looking inside a laser cycle

Abstract: We obtain a simple closed-form analytical expression for ionization rate as a function of instantaneous laser phase $\ensuremath{\varphi}(t),$ for arbitrary values of the Keldysh parameter $\ensuremath{\gamma},$ within the usual strong-field approximation. Our analysis allows us to explicitly distinguish multiphoton and tunneling contributions to the total ionization probability. The range of intermediate $\ensuremath{\gamma}\ensuremath{\sim}1,$ which is typical for most current intense field experiments, is the regime of nonadiabatic tunneling. In this regime, the instantaneous laser phase dependence differs dramatically from both quasistatic tunneling and multiphoton limits. For cycle-averaged rates, our results reproduce standard Keldysh-like expressions.

From molecular clusters to nanoparticles: Role of ambient ionization in tropospheric aerosol formation

Abstract: We investigate the role of background ionization, associated mainly with galactic cosmic radiation, in the generation and evolution of ultrafine particles in the marine boundary layer. We follow the entire course of aerosol evolution, from the initial buildup of molecular clusters (charged and uncharged) through their growth into stable nanoparticles. The model used for this purpose is based on a unified collisional (kinetic) mechanism that treats the interactions between vapors, neutral and charged clusters, and particles at all sizes. We show that air ions are likely to play a central role in the formation of new ultrafine particles. The nucleation of aerosols under atmospheric conditions involves a series of competing processes, including molecular aggregation, evaporation, and scavenging by preexisting particles. In this highly sensitive nonlinear system, electrically charged embryos have a competitive advantage over similar neutral embryos. The charged clusters experience enhanced growth and stability as a consequence of electrostatic interactions. Simulations of a major nucleation event observed during the Pacific Exploratory Mission (PEM) Tropics-A can explain most of the observed features in the ultrafine particle behavior. The key parameters controlling this behavior are the concentrations of precursor vapors and the surface area of preexisting particles, as well as the background ionization rate. We find that systematic variations in ionization levels due to the modulation of galactic cosmic radiation by the solar cycle are sufficient to cause a notable variation in aerosol production. This effect is greatest when the ambient nucleation rate is limited principally by the availability of ions. Hence we conclude that the greatest influence of such ionization is likely to occur in and above the marine boundary layer. While a systematic change in the ultrafine particle production rate is likely to affect the population of cloud condensation nuclei and hence cloud optical properties, the magnitude of the effect cannot be directly inferred from the present analysis, and requires additional analysis based on specific aerosol-cloud interactions.

DNA Excited-State Dynamics: Ultrafast Internal Conversion and Vibrational Cooling in a Series of Nucleosides

Abstract: To better understand DNA photodamage, several nucleosides were studied by femtosecond transient absorption spectroscopy. A 263-nm, 150-fs ultraviolet pump pulse excited each nucleoside in aqueous solution, and the subsequent dynamics were followed by transient absorption of a femtosecond continuum pulse at wavelengths between 270 and 700 nm. A transient absorption band with maximum amplitude near 600 nm was detected in protonated guanosine at pH 2. This band decayed in 191 ± 4 ps in excellent agreement with the known fluorescence lifetime, indicating that it arises from absorption by the lowest excited singlet state. Excited state absorption for guanosine and the other nucleosides at pH 7 was observed in the same spectral region, but decayed on a subpicosecond time scale by internal conversion to the electronic ground state. The cross section for excited state absorption is very weak for all nucleosides studied, making some amount of two-photon ionization of the solvent unavoidable. The excited state life...

Line ratios for helium-like ions: Applications to collision-dominated plasmas ?

Abstract: The line ratios R and G of the three main lines of He-like ions (triplet: resonance, intercombination, forbidden lines) are calculated for C v ,N vi ,O vii ,N eix ,M gxi, and Si xiii. These ratios can be used to derive electron density ne and temperatureTe of hot late-type stellar coronae and O, B stars from high-resolution spectra obtained with Chandra (LETGS, HETGS) and XMM-Newton (RGS). All excitation and radiative processes between the levels and the eect of upper-level cascades from collisional electronic excitation and from dielectronic and radiative recombination have been considered. When possible the best experimental values for radiative transition probabilities are used. For the higher-Z ions (i.e. Ne ix ,M gxi ,S ixiii) possible contributions from blended dielectronic satellite lines to each line of the triplets were included in the calculations of the line ratios R and G for four specic spectral resolutions: RGS, LETGS, HETGS-MEG, HETGS-HEG. The influence of an external stellar radiation eld on the coupling of the 2 3 S (upper level of the forbidden line) and 2 3 P levels (upper levels of the intercombination lines) is taken into account. This process is mainly important for the lower-Z ions (i.e. C v ,N vi ,O vii) at moderate radiation temperature (Trad). These improved calculations were done for plasmas in collisional ionization equilibrium, but will be later extended to photo-ionized plasmas and to transient ionization plasmas. The values for R and G are given in extensive tables, for a large range of parameters, which could be used directly to compare to the observations.

Detection of Multiple Protein Conformational Ensembles in Solution via Deconvolution of Charge-State Distributions in ESI MS

Abstract: Monitoring the changes in charge-state distributions of protein ions in electrospray ionization (ESI) mass spectra has become one of the commonly accepted tools to detect large-scale conformational changes of proteins in solution. However, these experiments produce only qualitative, low-resolution information. Our goal is to develop a procedure that would produce quantitative data on protein conformational isomers coexisting in solution at equilibrium. To that end, we have examined the evolution of positive ion charge-state distributions in the

The ionization fraction in alpha-models of protoplanetary disks

Abstract: We calculate the ionization fraction of protostellar alpha disks, taking into account vertical temperature structure, and the possible presence of trace metal atoms. Both thermal and X-ray ionization are considered. Previous investigations of layered disks used radial power-law models with isothermal vertical structure. But alpha models are used to model accretion, and the present work is a step towards a self-consistent treatment. The extent of the magnetically uncoupled (``dead'') zone depends sensitively on alpha, on the assumed accretion rate, and on the critical magnetic Reynolds number, below which MHD turbulence cannot be self-sustained. Its extent is extremely model-dependent. It is also shown that a tiny fraction of the cosmic abundance of metal atoms can dramatically affect the ionization balance. Gravitational instabilities are an unpromising source of transport, except in the early stages of disk formation.

The XSTAR Atomic Database

Abstract: We report on the XSTAR atomic database that contains a large quantity of atomic rates for use in spectral modeling of astrophysical plasmas. The database includes atomic energy levels, line wavelengths, radiative transition probabilities, electron impact excitation rates, photoionization cross section, recombination rate coefficients, electron impact ionization rates, and fluorescence and Auger yields. The species considered are all the ions of H, He, C, N, O, Ne, Mg, Si, S, Ar, Ca, Fe, and Ni. The database collects recent data from many sources including CHIANTI, TOPbase, ADAS, NIST, and the IRON Project. Two particular features of the database are the incorporation of photoionization cross sections from the Opacity Project for all levels of every ion, and state-specific recombination rates. State-specific collisional ionization and three-body recombination are also included. The collection of data is used to build excitation-ionization models of each ion for calculation of their spectra for electron densities of up to 1018 cm-3 and temperatures between 100 and 109 K. In addition, every ion model is designed to converge to LTE under appropriate conditions. These models and data are implemented in the photoionization modeling code XSTAR v.2.

Separation of Recollision Mechanisms in Nonsequential Strong Field Double Ionization of Ar: The Role of Excitation Tunneling

Abstract: Vector momentum distributions of two electrons created in double ionization of Ar by 25 fs, $0.25\mathrm{PW}/{\mathrm{cm}}^{2}$ laser pulses at 795 nm have been measured using a ``reaction microscope.'' At this intensity, where nonsequential ionization dominates, distinct correlation patterns are observed in the two-electron momentum distributions. A kinematical analysis of these spectra within the classical ``recollision model'' revealed an $(e,2e)$-like process and excitation with subsequent tunneling of the second electron as two different ionization mechanisms. This allows a qualitative separation of the two mechanisms demonstrating that excitation-tunneling is the dominant contribution to the total double ionization yield.

Detailed study of pyridine at the C 1s and N 1s ionization thresholds: The influence of the vibrational fine structure

Abstract: High resolution, vibrationally resolved, near-edge x-ray absorption fine structure (NEXAFS) spectra at the C 1s and N 1s ionization thresholds of pyridine and deuterated d5-pyridine in the gas phase have been recorded. The high resolution of 65 meV (150 meV) at the C s (N 1s) ionization thresholds reveals vibrational structures in the spectra. Detailed ab initio and density functional theory (DFT) calculations were performed to interpret the experimental spectra and to assign the observed peaks. In particular we focused on the previously unexplained intensity ratio for the two components of the C 1s→1π* transition. For this transition the vibrational structure is included through a linear coupling model in the DFT calculations and leads to the experimentally observed ∼2:3 intensity ratio between the two π* components in the C 1s spectrum rather than the ∼3:2 ratio obtained without vibrational effects. After inclusion of relaxation effects in the excited states, in addition to the vibrational effects, both theoretical methods yield almost perfect agreement with experiment.

Practical Improvements to the Lee‐More Conductivity Near the Metal‐Insulator Transition

Abstract: The wide-range conductivity model of Lee and More [1] is modified to allow better agreement with recent experimental data and theories for dense plasmas in the metal-insulator transition regime. Modifications primarily include a new ionization equilibrium model, consisting of a smooth blend between single ionization Saha (with a pressure ionization correction) and the generic Thomas-Fermi ionization equilibrium, a more accurate treatment of electron-neutral collisions using a polarization potential, and an empirical modification to the minimum allowed collision time. These simple modifications to the Lee-More algorithm permit a more accurate modeling of the physics near the metal-insulator transition, while preserving the generic Lee-More results elsewhere.

Self-guided propagation of ultrashort IR laser pulses in fused silica.

Abstract: We report self-guided propagation of ultrashort IR laser pulses in fused silica over several Rayleigh lengths. Self-guiding is accompanied by pulse splitting and time compression. Numerical simulations involving pulse self-focusing, temporal dispersion, and multiphoton ionization are found to be in good agreement with the experimental results. They show that a quasidynamic equilibrium between multiphoton ionization and self-focusing drives the filamentation process, while temporal dispersion plays a negligible role.

Supercharged protein and peptide ions formed by electrospray ionization.

Abstract: The multiple charging of large molecules in electrospray ionization provides key advantages for obtaining accurate molecular weights by mass spectrometry and for obtaining structural information by tandem mass spectrometry and MS(n) experiments. Addition of glycerol or m-nitrobenzyl alcohol into the electrospray solutions dramatically increases both the maximum observed charge state and the abundances of the high charge states of protein and peptide ions. Adding glycerol to acidified aqueous solutions of cytochrome c shifts the most abundant charge state from 17+ to 21+, shifts the maximum charge state from 20+ to 23+, and shifts the average charge state from 16.6+ to 20.9+. Much less m-nitrobenzyl alcohol (<1%) is required to produce similar results. With just 0.7% m-nitrobenzyl alcohol, even the 24+ charge state of cytochrome c is readily observed. Similar results are obtained with myoglobin and (Lys)4. For the latter molecule, the 5+ charge state is observed in the electrospray mass spectrum obtained from solutions containing 6.7% m-nitrobenzyl alcohol. This charge state corresponds to protonation of all basic sites in this peptide. Although the mechanism for enhanced charging is unclear, it does not appear to be a consequence of conformational changes of the analyte molecules. This method of producing highly charged protein ions should be useful for improving the performance of mass measurements on mass spectrometers with performances that decrease with increasing m/z. This should also be particularly useful for tandem mass spectrometry experiments, such as electron capture dissociation, for which highly charged ions are desired.

Intense-field laser ionization rates in atoms and molecules

Abstract: We discuss experiments on the ionization of atoms and molecules in intense laser fields, designed to facilitate a comparison of theory and experiment especially in the case of complex molecules. We concentrate on ionization in the regime where single ionization dominates. We describe an approach which allows a direct measurement of ionization efficiencies, absolute ionization rates, and well-defined saturation intensities. In addition, intensities where multiple ionization and Coulomb explosion set in are determined. We also analyze how ion yields are sensitive to any dependence of ionization rates on molecular alignment. Application of these procedures to molecular ionization is demonstrated in detail for the organic molecules benzene and cyclohexane.

A MEMS radio-frequency ion mobility spectrometer for chemical vapor detection

Abstract: A first-of-a-kind micro-electro-mechanical systems (MEMS) radio-frequency ion mobility spectrometer (rf-IMS) with a miniature drift tube of total volume 0.6 cm 3 has been fabricated and tested. The spectrometer has detection limits in the parts per billion (ppb) and the ability to identify chemicals such as isomers of xylene not resolved in conventional time-of-flight ion mobility spectrometry. Spectrometer operation with a miniature 10.6 eV (l ¼ 116:5 nm) UV photodischarge lamp and a 1 mCi radioactive ionization source has been demonstrated. The resultant spectra with both these ionization sources are similar, with several additional peaks evident for the radioactive source. The effect of varying the carrier gas flow rate on the resultant spectra has been investigated and optimal flow conditions are found at flow rates between 2 and 3 l/min. The rf-IMS has been interfaced to a mass spectrometer (MS) and rf-IMS spectral peaks have been confirmed. The rf-IMS/MS configuration illustrates another use for the rf-IMS as a pre-filter for atmospheric pressure chemical ionization (APCI) mass spectrometry applications. # 2001 Elsevier Science B.V. All rights reserved.

Electrospray and tandem mass spectrometry in biochemistry.

Abstract: Over the last 20 years, biological MS has changed out of all recognition. This is primarily due to the development in the 1980s of 'soft ionization' methods that permit the ionization and vaporization of large, polar, and thermally labile biomolecules. These developments in ionization mode have driven the design and manufacture of smaller and cheaper mass analysers, making the mass spectrometer a routine instrument in the biochemistry laboratory today. In the present review the revolutionary 'soft ionization' methods will be discussed with particular reference to electrospray. The mass analysis of ions will be described, and the concept of tandem MS introduced. Where appropriate, examples of the application of MS in biochemistry will be provided. Although the present review will concentrate on the MS of peptides/proteins and lipids, all classes of biomolecules can be analysed, and much excellent work has been done in the fields of carbohydrate and nucleic acid biochemistry.

Choosing between atmospheric pressure chemical ionization and electrospray ionization interfaces for the HPLC/MS analysis of pesticides.

Abstract: An evaluation of over 75 pesticides by high-performance liquid chromatography/mass spectrometry (HPLC/MS) clearly shows that different classes of pesticides are more sensitive using either atmospheric pressure chemical ionization (APCI) or electrospray ionization (ESI) For example, neutral and basic pesticides (phenylureas, triazines) are more sensitive using APCI (especially positive ion) While cationic and anionic herbicides (bipyridylium ions, sulfonic acids) are more sensitive using ESI (especially negative ion) These data are expressed graphically in a figure called an ionization-continuum diagram, which shows that protonation in the gas phase (proton affinity) and polarity in solution, expressed as proton addition or subtraction (pKa), is useful in selecting APCI or ESI Furthermore, sodium adduct formation commonly occurs using positive ion ESI but not using positive ion APCI, which reflects the different mechanisms of ionization and strengthens the usefulness of the ionization-continuum diagram

Three-Body Coulomb Problem Probed by Mapping the Bethe Surface in Ionizing Ion-Atom Collisions

Abstract: The three-body Coulomb problem has been explored in kinematically complete experiments on single ionization of helium by 100 MeV/u C(6+) and 3.6 MeV/u Au(53+) impact. Low-energy electron emission ( E(e)<150 eV) as a function of the projectile deflection theta(p) (momentum transfer), i.e., the Bethe surface [15], has been mapped with Delta theta(p)+/-25 nanoradian resolution at extremely large perturbations ( 3.6 MeV/u Au(53+)) where single ionization occurs at impact parameters of typically 10 times the He K-shell radius. The experimental data are not in agreement with state-of-the-art continuum distorted wave-eikonal initial state theory.

X-ray reflection by photoionized accretion discs

Abstract: We present the results of reflection calculations that treat the relevant physics with a minimum of assumptions The temperature and ionization structure of the top five Thomson depths of an illuminated disc are calculated while also demanding that the atmosphere is in hydrostatic equilibrium In agreement with Nayakshin, Kazanas & Kallman, we find that there is a rapid transition from hot to cold material in the illuminated layer However, the transition is usually not sharp so that often we find a small but finite region in Thomson depth where there is a stable temperature zone at T \sim 2 x 10^{6} K due to photoelectric heating from recombining ions As a result, the reflection spectra often exhibit strong features from partially-ionized material, including helium-like Fe K lines and edges We find that due to the highly ionized features in the spectra these models have difficulty correctly parameterizing the new reflection spectra There is evidence for a spurious $R-\Gamma$ correlation in the ASCA energy range, where $R$ is the reflection fraction for a power-law continuum of index $\Gamma$, confirming the suggestion of Done & Nayakshin that at least part of the R-Gamma correlation reported by Zdziarski, Lubinski & Smith for Seyfert galaxies and X-ray binaries might be due to ionization effects Although many of the reflection spectra show strong ionized features, these are not typically observed in most Seyfert and quasar X-ray spectra

High-resolution X-ray spectroscopy and modeling of the absorbing and emitting outflow in NGC 3783

Abstract: The high-resolution X-ray spectrum of NGC 3783 shows several dozen absorption lines and a few emission lines from the H-like and He-like ions of O, Ne, Mg, Si, and S, as well as from Fe XVII-Fe XXIII L-shell transitions. We have reanalyzed the Chandra HETGS spectrum using better flux and wavelength calibrations, along with more robust methods. Combining several lines from each element, we clearly demonstrate the existence of the absorption lines and determine that they are blueshifted relative to the systemic velocity by -610 ? 130 km s-1. We find the Ne absorption lines in the High-Energy Grating spectrum to be resolved with FWHM = 840 km s-1; no other lines are resolved. The emission lines are consistent with being at the systemic velocity. We have used regions in the spectrum where no lines are expected to determine the X-ray continuum, and we model the absorption and emission lines using photoionized-plasma calculations. The model consists of two absorption components, with different covering factors, which have an order-of-magnitude difference in their ionization parameters. The two components are spherically outflowing from the active galactic nucleus, and thus contribute to both the absorption and the emission via P Cygni profiles. The model also clearly requires O VII and O VIII absorption edges. The low-ionization component of our model can plausibly produce UV absorption lines with equivalent widths consistent with those observed from NGC 3783. However, we note that this result is highly sensitive to the unobservable UV to X-ray continuum, and the available UV and X-ray observations cannot firmly establish the relationship between the UV and X-ray absorbers. We find good agreement between the Chandra spectrum and simultaneous ASCA and RXTE observations. The 1 keV deficit previously found when modeling ASCA data probably arises from iron L-shell absorption lines not included in previous models. We also set an upper limit on the FWHM of the narrow Fe K? emission line of 3250 km s-1. This is consistent with this line originating outside the broad-line region, possibly from a torus.

Heating and Ionization of the Intergalactic Medium by an Early X-Ray Background

Abstract: Observational studies indicate that the intergalactic medium (IGM) is highly ionized up to redshifts just over 6. A number of models have been developed to describe the process of reionization and the effects of the ionizing photons from the first luminous objects. In this paper we study the impact of an X-ray background, such as high-energy photons from early quasars, on the temperature and ionization of the IGM prior to reionization, before the fully ionized bubbles associated with individual sources have overlapped. X-rays have large mean free paths relative to EUV photons, and their photoelectrons can have significant effects on the thermal and ionization balance. We find that hydrogen ionization is dominated by the X-ray photoionization of neutral helium and the resulting secondary electrons. Thus, the IGM may have been warm and weakly ionized prior to full reionization. We examine several related consequences, including the filtering of the baryonic Jeans mass scale, signatures in the cosmic microwave background, and the H--catalyzed production of molecular hydrogen.

Highly charged ions

Abstract: This paper reviews some of the fundamental properties of highly charged ions, the methods of producing them (with particular emphasis on table-top devices), and their use as a tool for both basic science and applied technology. Topics discussed include: charge dependence and scaling laws along isoelectronic or isonuclear sequences (for wavefunction size or Bohr radius, ionization energy, dipole transition energy, relativistic fine structure, hyperfine structure, Zeeman effect, Stark effect, line intensities, linewidths, strength of parity violation, etc), changes in angular momentum coupling schemes, selection rules, interactions with surfaces, electron-impact ionization, the electron beam ion trap (EBIT), ion accelerators, atomic reference data, cosmic chronometers, laboratory x-ray astrophysics, vacuum polarization, solar flares, ion implantation, ion lithography, ion microprobes (SIMS and x-ray microscope), nuclear fusion diagnostics, nanotechnology, quantum computing, cancer therapy and biotechnology.

First Principles Calculation of pKa Values for 5-Substituted Uracils

Abstract: Oxidation of uracil (U) and thymine (5-Me-U) are believed to play a role in genetic instability because of the changes these oxidations cause in the ionization constants (pK_a values), which in turn affects the base pairing and hence coding. However, interpretation of the experimental evidence for the changes of pK_a with substitution at U has been complicated by the presence of two sites (N1 and N3) for ionization. We show that a procedure using first principles quantum mechanics (density functional theory with generalized gradient approximation, B3LYP, in combination with the Poisson−Boltzmann continuum-solvation model) predicts such pK_a values for a series of 5-substituted uracil derivatives in excellent correlation with experiment. In particular, this successfully resolves which cases prefer ionization at N1 and N3. Such first principles predictions of ionization constant should be useful for predicting and interpreting pK_a for other systems.

Nonadiabatic Multielectron Dynamics in Strong Field Molecular Ionization.

Abstract: We report the observation of a general strong field ionization mechanism due to highly nonadiabatic multielectron excitation dynamics in polyatomic molecules. We observe that such excitation mechanisms greatly affect molecular ionization, fragmentation, and energetics. We characterized this phenomenon as a function of optical frequency, intensity, and molecular properties.

Elemental Composition Analysis of Processed and Unprocessed Diesel Fuel by Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Abstract: High-resolution (100 000 < m/Δm50% < 250 000, in which Δm50% denotes mass spectral peak full width at half-maximum height) electrospray ionization Fourier transform ion cyclotron resonance positive-ion mass spectra of unprocessed (and processed) diesel fuels resolves approximately 500 (and 200) chemically different constituents over a mass range from 200 to 452 Da, with as many as 6 resolved elemental compositions at a given nominal mass. Molecular formulas were assigned from accurate mass measurement to within ±1 ppm. Compound types were identified by Kendrick mass analysis. On the basis of the experimental behavior of model compounds, electrospray ionization was found to ionize selectively basic pyridine homologuescompounds responsible for deactivation of hydrotreatment catalysts and instability of fuels during storage. Compound classes identified in the unprocessed diesel fuel include those containing N, N2, NS, NO, N2O, O2, and SO and, in the processed diesel, N, N2, NO2, and SO. Comparison of unproce...

Physical Diagnostics from a Narrow Fe Kα Emission Line Detected by Chandra in the Seyfert 1 Galaxy NGC 5548

Abstract: We report the detection of a narrow Fe K? emission line in the Seyfert 1 galaxy NGC 5548 with the Chandra High-Energy Transmission Grating. In the galaxy frame we measure a center energy of 6.402 keV, a FWHM of 4515 km s-1, an intensity of 3.6 ? 10-5 photons cm-2 s-1, with an equivalent width of 133 eV (errors are 90% confidence for one parameter). The line is only marginally resolved at the 90% confidence level. The line energy is consistent with an origin in cold, neutral matter, but ionization states up to ~Fe XVIII are not ruled out. We cannot constrain the detailed dynamics but, assuming Keplerian motion, the velocity width is consistent with the line being produced in the outer optical/UV broad-line region (BLR) at about a light month from the central X-ray source. We cannot rule out some contribution to the narrow Fe K? line from a putative, parsec-scale, obscuring torus that is postulated to be a key component of active galactic nuclei (AGNs) unification models. The continuum intensity during the Chandra observation was a factor ~2 less than typical historical levels. If the X-ray continuum was at least a factor of 2 higher in the recent past before the Chandra observation and the narrow Fe K? intensity had not yet responded to such a change, then the predicted line intensity and equivalent width for an origin in the BLR is within the 90% measurement errors. Anisotropic X-ray continuum illumination of the BLR and/or additional line emission from a torus structure would improve the agreement with observation. Two out of three archival ASCA data sets are consistent with the narrow line being present with the same intensity as in the Chandra observation. However, there is strong evidence that the narrow-line intensity varied and was unusually low during one of the ASCA campaigns. In general, inclusion of the narrow line to model the overall broad Fe K? line profile in terms of a rotating disk plus black-hole model can have a non-negligible effect on the disk line intensity and variability properties. Variability of the broad disk line in NGC 5548 is difficult to reconcile with the expectations of the simple disk model, even when the narrow-line component is accounted for. It will be critical to ascertain the importance of a similar nondisk Fe K? line in other Seyfert 1 galaxies. Future monitoring of the narrow Fe K? component with large collecting area and high spectral resolution will enable reverberation mapping the BLR region, complementary to similar studies using the optical/UV lines, and therefore provide independent constraints on the black-hole mass.