High-energy cutoff in the spectrum of strong-field nonsequential double ionization

Electron energy distributions of singly and doubly ionized helium in an intense 390 nm laser field have been measured at two intensities (0.8 PW/cm(2) and 1.1 PW/cm(2), where PW equivalent to 10(15) W/cm(2)). Numerical solutions of the full-dimensional time-dependent helium Schrodinger equation show excellent agreement with the experimental measurements. The high-energy portion of the two-electron energy distributions reveals an unexpected 5U(p) cutoff for the double ionization (DI) process and leads to a proposed model for DI below the quasiclassical threshold.

Electron-impact ionization of hydrogen-like iron ions

Electron-impact ionization cross sections have been determined for hydrogen like iron ions at selected electron energies between 1.45 and 4.3 times the threshold energy. The cross sections were obtained by measuring the equilibrium ionization balance in an electron beam ion trap. This ionization balance is obtained from x-ray measurements of radiative recombination into the K-shell of hydrogen-like and bare iron ions. The measured cross sections are compared with distorted-wave calculations and several semiempirical formulations.

State-Selective Photodissociation Dynamics of Formaldehyde: Near Threshold Studies of the H+HCO Product Channel

The laser-induced photodissociation of formaldehyde in the wavelength range 309<λ<330nm 309<λ<330nm has been investigated using H (Rydberg) atom photofragment translational spectroscopy. Photolysis wavelengths corresponding to specific rovibronic transitions in the A ˜ A 2 1 ←X ˜ A 1 1 ÃA21←X̃A11 2 1 0 4 3 0 201403 , 2 2 0 4 1 0 202401 , 2 2 0 4 3 0 202403 , 2 3 0 4 1 0 203401 , and 2 1 0 5 1 0 201501 bands of H 2 CO H2CO were studied. The total kinetic energy release spectra so derived can be used to determine partial rotational state population distributions of the HCO cofragment. HCO product state distributions have been derived following the population of various different N K a NKa levels in the A ˜ A 2 1 ÃA21 2 2 4 3 2243 and 2 3 4 1 2341 states. Two distinct spectral signatures are identified, suggesting competition between dissociation pathways involving the X ˜ A 1 1 X̃A11 and the a ˜ A 2 3 ãA23 potential energy surfaces. Most rovibrational states of H 2 CO(A ˜ A 2 1 ) H2CO(ÃA21) investigated in this work produceH+HCO(X ˜ A ′ 2 ) H+HCO(X̃A′2) photofragments with a broad kinetic energy distribution and significant population in high energy rotational states of HCO. Photodissociation via the A ˜ A 2 1 ÃA21 2 2 4 3 2243 1 1,1 11,1 (and 1 1,0 11,0 ) rovibronic states yields predominantly HCO fragments with low internal energy, a signature that these rovibronic levels are perturbed by the a ˜ A 2 3 ãA23 state. The results also suggest the need for further careful measurements of the H+HCO H+HCO quantum yield from H 2 CO H2CO photolysis at energies approaching, and above, the barrier to C–H bond fission on the a ˜ A 2 3 ãA23 potential energy surface.

Observations of collimated ionization channels in aluminum-coated glass targets irradiated by ultraintense laser pulses

Filamentary ionization tracks have been observed via optical probing inside Al-coated glass targets after the interaction of a picosecond 20-TW laser pulse at intensities above 10(19) W/cm(2). The tracks, up to 700 mu m in length and between 10 and 20 mu m in width, originate from the focal spot region of the laser beam. Simulations performed with 3D particle-in-cell and 2D Fokker-Planck hybrid codes indicate that the observations are consistent with ionization induced in the glass target by magnetized, collimated beams of high-energy electrons produced during the laser interaction.

Quantitative study of the ionization-induced refraction of picosecond laser pulses in gas-jet targets

A quantitative study of refractive whole beam defocusing and small scale breakup induced by optical ionization of subpicosecond and picosecond, 0.25 and 1 mu m, laser pulses in gas-jet targets at densities above 1 x 10(19) cm(-3) has been carried out. A significant reduction of the incident laser intensity was observed due to refraction from ionization-induced density gradients. The level of refraction measured with optical probing correlated well with the fraction of energy transmitted through the plasma. The numerical and analytical models were found to agree well with experimental observations.

Influence of multiple ionization thresholds on harmonic generation from Ar+

We apply time-dependent R-matrix theory to investigate harmonic generation from ground-state Ar+ with M = 0 at a wavelength of 390 nm. Contributions associated with the different 3s(2)3p(4) ionization thresholds are assessed, including the interference between these. The dominant contribution originates from the second ionization threshold, 3s(2)3p(4 1)D. Changes to the harmonic yields arising from the higher 3s3p(5) thresholds are also assessed. We further confirm that Ar+ has a higher harmonic yield than He for the same laser pulse, despite having a higher ionization threshold. 

Supra-treatment threshold neonatal jaundice: Incidence in HIV-exposed compared to non-exposed neonates at Queen Elizabeth Central Hospital in Blantyre, Malawi

INTRODUCTION: Jaundice is the yellowish pigmentation of the skin, sclera, and mucous membranes resulting from bilirubin deposition. Children born to mothers with HIV are more likely to be born premature, with low birth weight, and to become septic-all risk factors for neonatal jaundice. Further, there has been a change in the prevention of mother-to-child transmission (PMTCT) of HIV guidelines from single-dose nevirapine to a six-week course, all of which theoretically put HIV-exposed newborns at greater risk of developing neonatal jaundice.

AIM: We carried out a study to determine the incidence of severe and clinical neonatal jaundice in HIV-exposed neonates admitted to the Chatinkha Nursery (CN) neonatal unit at Queen Elizabeth Central Hospital (QECH) in Blantyre.

METHODS: Over a period of four weeks, the incidence among non-exposed neonates was also determined for comparison between the two groups of infants. Clinical jaundice was defined as transcutaneous bilirubin levels greater than 5 mg/dL and severe jaundice as bilirubin levels above the age-specific treatment threshold according the QECH guidelines. Case notes of babies admitted were retrieved and information on birth date, gestational age, birth weight, HIV status of mother, type of feeding, mode of delivery, VDRL status of mother, serum bilirubin, duration of stay in CN, and outcome were extracted.

RESULTS: Of the 149 neonates who were recruited, 17 (11.4%) were HIV-exposed. One (5.88%) of the 17 HIV-exposed and 19 (14.4%) of 132 HIV-non-exposed infants developed severe jaundice requiring therapeutic intervention (p = 0.378). Eight (47%) of the HIV-exposed and 107 (81%) of the non-exposed neonates had clinical jaundice of bilirubin levels greater than 5 mg/dL (p < 0.001).

CONCLUSIONS: The study showed a significant difference in the incidence of clinical jaundice between the HIV-exposed and HIV-non-exposed neonates. Contrary to our hypothesis, however, the incidence was greater in HIV-non-exposed than in HIV-exposed infants.

Electron-impact ionization of Xe 24 +

Configuration-average distorted-wave calculations are carried out for the electron-impact single ionization of Xe 24 + . Contributions are included from direct ionization of the 3s, 3p, 3d and 4s subshells and from indirect ionization via 3s → nl , 3p → nl and 3d → nl excitations followed by autoionization. Branching ratios are found for single versus double ionization of the 3s and 3p subshells and for autoionization versus radiative decay of all 3 l → nl excitations. Additional distorted-wave and R -matrix calculations find resonant-capture double-autoionization contributions to be quite small. The total ionization cross section for Xe 24 + is found to be dominated by indirect excitation–autoionization contributions, especially near the single-ionization threshold. An approximate 15% reduction in the total ionization cross section is found due to the radiative decays included in the branching ratios.

Electron-impact ionization of the metastable excited states of Li +

Electron-impact ionization cross sections for the 1s2s 1S and 1s2s 3S metastable states of Li+ are calculated using both perturbative distorted-wave and non-perturbative close-coupling methods. Term-resolved distorted-wave calculations are found to be approximately 15% above term-resolved R-matrix with pseudostates calculations. On the other hand, configuration-average time-dependent close-coupling calculations are found to be in excellent agreement with the configuration-average R-matrix with pseudostates calculations. The non-perturbative R-matrix and close-coupling calculations provide a benchmark for experimental studies of electron-impact ionization of metastable states along the He isoelectronic sequence.

Contribution of near threshold states to recombination in plasmas

Resonance states in atoms or ions at low energies can control the rates of important plasma processes (e.g., dielectronic recombination). We examine the role of states at negative energies just below the ionization threshold of the recombined system and find that they can contribute as much, or more, to recombination as positive energy states. In plasmas, negative energy states can be populated by three body recombination, photorecombination, or continuum lowering. Properly including these negative energy states in a theoretical treatment of plasma processes can change the thermally averaged rate coefficients and, in some cases, removes much of the sensitivity to the energy of a state.

Electron impact fragmentation of thymine:partial ionization cross sections for positive fragments

We have measured mass spectra for positive ions for low-energy electron impact on thymine using a reflectron time-of-flight mass spectrometer. Using computer controlled data acquisition, mass spectra have been acquired for electron impact energies up to 100 eV in steps of 0.5 eV. Ion yield curves for most of the fragment ions have been determined by fitting groups of adjacent peaks in the mass spectra with sequences of normalized Gaussians. The ion yield curves have been normalized by comparing the sum of the ion yields to the average of calculated total ionization cross sections. Appearance energies have been determined. The nearly equal appearance energies of 83 u and 55 u observed in the present work strongly indicate that near threshold the 55 u ion is formed directly by the breakage of two bonds in the ring, rather than from a successive loss of HNCO and CO from the parent ion. Likewise 54 u is not formed by CO loss from 82 u. The appearance energies are in a number of cases consistent with the loss of one or more hydrogen atoms from a heavier fragment, but 70 u is not formed by hydrogen loss from 71 u.

Gas chromatographic electron capture negative ionization mass spectrometric (GC/ECNI/MS) determination of unique fluorinated compounds in the sediments of Lake Ontario and the effect of high-boiling alcohols (as injection solvents) on chromatographic behaviour of polycyclic aromatic hydrocarbons in gas chromatography

Part I - Fluorinated Compounds A method has been developed for the extraction, concentration, and determination of two unique fluorinated compounds from the sediments of Lake Ontario. These compounds originated from a common industrial landfill, and have been carried to Lake Ontario by the Niagara River. Sediment samples from the Mississauga basin of Lake Ontario have been evaluated for these compounds and a depositional trend was established. The sediments were extracted by accelerated solvent extraction (ASE) and then underwent clean-up, fractionation, solvent exchange, and were concentrated by reduction under nitrogen gas. The concentrated extracts were analyzed by gas chromatography - electron capture negative ionization - mass spectrometry. The depositional profile determined here is reflective of the operation of the landfill and shows that these compounds are still found at concentrations well above background levels. These increased levels have been attributed to physical disturbances of previously deposited contaminated sediments, and probable continued leaching from the dumpsite. Part II - Polycyclic Aromatic Hydrocarbons Gas chromatography/mass spectrometry is the most common method for the determination of polycyclic aromatic hydrocarbons (PAHs) from various matrices. Mass discrimination of high-boiling compounds in gas chromatographic methods is well known. The use of high-boiling injection solvents shows substantial increase in the response of late-eluting peaks. These solvents have an increased efficiently in the transfer of solutes from the injector to the analytical column. The effect of I-butanol, I-pentanol, cyclopentanol, I-hexanol, toluene and n-octane, as injection solvents, was studied. Higher-boiling solvents yield increased response for all PAHs. I -Hexanol is the best solvent, in terms of P AH response, but in this solvent P AHs were more susceptible to chromatographic problems such as peak splitting and tailing. Toluene was found to be the most forgiving solvent in terms of peak symmetry and response. It offered the smallest discrepancies in response, and symmetry over a wide range of initial column temperatures.

Measurement of laser ablation threshold on doped BiSrCaCuO high temperature superconductors by the pulsed photothermal deflection technique

Laser‐induced damage and ablation thresholds of bulk superconducting samples of Bi2(SrCa)xCu3Oy(x=2, 2.2, 2.6, 2.8, 3) and Bi1.6 (Pb)xSr2Ca2Cu3 Oy (x=0, 0.1, 0.2, 0.3, 0.4) for irradiation with a 1.06 μm beam from a Nd‐YAG laser have been determined as a function of x by the pulsed photothermal deflection technique. The threshold values of power density for ablation as well as damage are found to increase with increasing values of x in both systems while in the Pb‐doped system the threshold values decrease above a specific value of x, coinciding with the point at which the Tc also begins to fall.  

Photoionization of Kr 4s: III. Detailed and extended measurements of the Kr 4s-electron ionization cross section

Absolute Kr 4s-electron photoionization cross sections as a function of the exciting-photon energy were measured by photon-induced fluorescence spectroscopy (PIFS) at improved primary-energy resolution. The cross sections were determined from threshold to 33.5 eV and to 90 eV with primary-photon bandwidths of 25 meV and 50 meV, respectively. The measurements were compared with experimental data and selected theoretical calculations for the direct Kr 4s-electron photoionization cross sections.

Photoionization of Kr near the 4s threshold: IV. Photoionization through the autoionization of doubly-excited states

The photoionization cross sections for the production of the Kr II 4s state and Kr II satellite states were studied in the 4s ionization threshold region. The interference of direct photoionization and ionization through the autoionization decay of doubly-excited states was considered. In the calculations of doubly-excited state energies, performed by a configuration interaction technique, the 4p spin-orbit interaction and the (Kr II core)-(excited electron) Coulomb interaction were included. The theoretical cross sections are in many cases in good agreement with the measured values. Strong resonant features in the satellite spectra with threshold energies greater than 30 eV are predicted.