He-like Oxygen Ions Research Articles

Cusp-electron production in collisions of open-shell He-like oxygen ions with atomic targets

We report on double differential cross sections of cusp electrons obtained in MeV/u collisions of open-shell He-like oxygen ions with helium. We use zero-degree electron spectroscopy and our double measurement technique, involving ${\mathrm{O}}^{6+}(1{s}^{2})$ ground-state and ${\mathrm{O}}^{6+}(1{s}^{2},1s2s)$ mixed-state beams, to extract the cusp contribution from only the excited ${\mathrm{O}}^{6+}(1s2s)$ beam. Theoretical calculations based on the continuum distorted wave eikonal initial state are in very good agreement with the measurements. The roles of the processes of electron capture to the continuum, electron loss to the continuum, and electron loss to the continuum with simultaneous target ionization, which contribute to the cusp peak, are discussed in detail.

Diagnostics of plasma produced by femtosecond laser pulse impact upon a target with an internal nanostructure

X-ray diagnostics of the interaction of femtosecond laser pulses with intensities of 1016–1018 W/cm2 with CO2 clusters and frozen nanosize water particles is carried out. The stage of cluster expansion and the formation of a plasma channel, which governs the parameters of the formed X-ray radiation source and accelerated ion flows, is studied. The measurements are based on recording spatially resolved X-ray spectra of H- and He-like oxygen ions. Utilization of Rydberg transitions for spectra diagnostics makes it possible to determine plasma parameters on a time scale of t ∼ 10 ps after the beginning of a femtosecond pulse. The role of the rear edge of the laser pulse in sustaining the plasma temperature at a level of ∼100 eV in the stage of a nonadiabatic cluster expansion is shown. The analysis of the profiles and relative intensities of spectral lines allows one to determine the temperature and density of plasma electrons and distinguish the populations of “thermal” ions and ions that are accelerated up to energies of a few tens of kiloelectronvolts. It is shown that the use of solid clusters made of frozen nanoscale water droplets as targets leads to a substantial increase in the number of fast He-like ions. In this case, however, the efficiency of acceleration of H-like ions does not increase, because the time of their ionization in plasma exceeds the time of cluster expansion.

Application of snow nanograin targets for the generation of fast ions in femtosecond laser plasma

We have measured the energy of the directed motion of multiply charged ions produced when solid targets are exposed to low-contrast (10−3–10−2) femtosecond laser pulses with intensities 1015–1016 W cm−2. The measurements are based on the recording of spatially resolved X-ray spectra for H-and He-like oxygen ions in the target plane. Analysis of the Heβ and Lyα line profiles has revealed fractions of accelerated ions in plasma with energies from several to several tens of kiloelectronvolts. We show that using a layer of frozen nanometer-size water droplets as the targets leads to an effective absorption of laser pulses and a twofold rise in the energy (to 0.1 MeV) of He-like oxygen ions compared to the use of solid targets.

Chandra and XMM–Newton observations of Tololo 0109-383

We present and discuss Chandra and XMM-Newton observations of the Seyfert 2 galaxy and Compton-thick absorbed source, Tololo 0109-383. The hard X-ray emission (i.e. above ∼2 keV), is dominated by a spatially unresolved reflection component, as already discovered by previous ASCA and BeppoSAX observations. The soft X-ray emission is partly (∼15%) extended over about 1 kpc. Below 2 keV, the spectrum is very steep and two emission lines, probably due to recombination to He-like ions of oxygen and neon, are clearly present. Combining X-rays and optical information taken from the literature, we propose an overall scenario for the nuclear regions of this source.

X-ray spectra of fast ions generated from clusters by ultrashort laser pulses

The high precision X-ray spectroscopy studies of plasma created from the CO2 clusters in gas jet targets by the ultrashort laser pulses (35 and 60 fs duration) were performed at the intensities IL ∼ 1017–1018 W cm−2. The spectral line shape of the H-like and He-like oxygen ions gains an asymmetry with increasing the laser pulse intensity. Theoretical modeling of the line shape shows that the asymmetry can be explained by absorption of the Doppler-shifted line radiation from the essential fraction of ions (over 10−3) with energies above 1 MeV due to photoionization of inner shells of carbon ions. The results obtained demonstrate measurement capabilities of the X-ray spectral measurements of multicharged ions accelerated during the interaction with a laser radiation.

High Resolution Spectroscopy and Plasma Diagnostics of Supernova Remnants

AbstractThe MIT group has used data from the Focal Plane Crystal Spectrometer on the Einstein Observatory to perform plasma diagnostics of four supernova remnants (SNRs), the Cygnus Loop, Puppis A, N132D, and Cas A. The ratio of luminosities of the forbidden line to resonance line of He-like ions of oxygen and neon allow us to show that all four SNRs depart from ionization equilibrium in that they are under-ionized for their electron temperatures. Thus despite the fact that their ages range from 300 yr to 20,000 yr, all four SNRs are still ionizing and, in that sense, are still young. We derive values of ionization time and electron temperature for one or more components in each remnant. The agreement between these values and those deduced by others using entirely different means (e.g. broad-band spectroscopy or imaging) gives us confidence in the reliability of the diagnostics. Two of the SNRs, Puppis A and N132D, show evidence for an overabundance of oxygen by factors of three or more. These results, based on a handful of weak lines, show the great promise of the much more powerful future spectroscopy missions for revealing detailed information about astrophysical plasmas.

State-selective dielectronic-recombination measurements for He-like oxygen ions in an electron cooler.

We report on high-resolution, state-selective dielectronic-recombination measurements for metastable, He-like ${\mathrm{O}}^{6+}$ ions. The measurements were performed with a merged-beams configuration in an electron cooler. The energy resolution was found to be as low as 0.135 eV. Dielectronic recombination associated with excitations of the 1s2s${(}^{1}$S) and 1s2s${(}^{3}$S) cores are reported. The data demonstrate strong effects of configuration interaction between different intermediate states with almost identical energy.

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Ratio of intercombination to resonance line intensities for He-like oxygen ions in TFR tokamak plasmas

The ratio of intercombination to resonance line intensities has been measured in TFR tokamak plasmas, using space-and time-resolved soft X-ray spectroscopy. The experimental ratio values are compared to those obtained with available theoretical atomic models.

Mapping the Solar Corona in X-Ray Lines of OVII and NeIX

Some preliminary results of a rocket experiment, which utilized collimated Bragg Crystal spectrometers to map solar X-ray line emissions from He-like ions of oxygen and neon are presented. The two spectrometers used KAP crystals with a common field of view 1.7’ FWHM in the narrow dimension. The spectrometers were scanned in wavelength once each second to observe the resonance, intersystem and forbidden lines of OVII and NeIX.