Helium-like Carbon Research Articles

Dipole transition‐matrix elements and oscillator strengths for the C4+doubly excited states with Coulomb and screened Coulomb (Debye–Hückel) potentials

AbstractThe dipole transition‐matrix elements (DTMEs) and oscillator strengths (OSs) for the doubly excited states (DESs) in heliumlike carbon are investigated using the correlated exponential wavefunctions (CEW). The interactions between the charged particles are taken care of by using both Coulomb and screened Coulomb (Debye–Hückel) potentials. The singly excited 1s3p1P state and doubly excited 2p2 1Destate are also calculated using CEW. The singly excited states of C4+in terms of Debye length (DL) are calculated using the Ritz variational principle. The DESs for different DLs are determined using the stabilization method. The DTMEs and OSs for the transitions between the 2p2 1Destate and the 1s2p1P, 1s3p1P, 2s2p+1Po, 2s3p−1Po, 2s3p+1Po, 2p3d1Postates, along with the 2p2 1Deresonance parameters are reported for various DLs. The benchmark data for DTMEs and OSs for the DESs in the C4+ion in pure Coulomb and screened Coulomb environments, obtained by employing highly CEW functions and utilizing the stabilization method, are reported for the first time.

Oscillator strengths for 1s2 1S0–1s2p 3P1,2 transitions in helium-like carbon, nitrogen and oxygen including the effects of a finite nuclear mass

We have calculated the electric dipole (E1) and magnetic quadrupole (M2) oscillator strengths and spontaneous decay rates for the spin-changing transitions of helium-like C v, N vi and O vii. We added the effects of the finite nuclear mass and the anomalous magnetic moment of the electron including an extra term derived by Pachucki. For the E1 calculations we used the Breit approximation and pseudostate expansions to perform the perturbation sums over intermediate states in both the length and velocity gauge as a check on numerical accuracy and the validity of the transition operators. There is some cancellation in the corrections for the nuclear mass and the electron anomaly so that if one is included the other should not be ignored

Plasma screening effects on the atomic structure and radiative opacity of dense carbon plasmas based on the DLA model

A study of plasma screening effects using a Debye-Hückel model is presented. The effects on the energy levels, oscillator strengths, ionization potential and photoionization cross sections of different ionization stages of carbon and on the radiative opacity of hot dense carbon plasmas are presented. The treatment of the atomic structure and radiative opacity is based on the detailed fine-structure level accounting formalism. For the basic atomic data, we compare our calculated energy levels and oscillator strengths with other theoretical results available in the literature and good agreement is found. The calculated ionization potential depressions of helium-like carbon, C V, are compared with the experimental and other theoretical results for a variety of plasma condition. Since the Saha–Boltzmann equation, which determines the ion stage populations, is dependent on the energy levels and ionization potentials of all ionization stages of carbon, the plasma screening effects on these quantities were included to obtain the population distribution. The screening effects play an important role on the spectrally resolved opacity by affecting the position and intensity of the absorption peaks and the K-shell ionization thresholds. With the decrease of Debye screening length, the number and intensity of the absorption peaks are reduced. In particular, the continuum opacity near the K-edge will be so sufficiently affected by the screening effects as to be experimentally observed. Finally, the plasma screening effects on the Rosseland and Planck mean opacities are investigated.

Dissociation and Multiply Charged Silicon Ejection in High Abundance from Hexamethyldisilane

Quadruply charged, neon-like silicon and helium-like carbon were generated by the exposure of hexamethyldisilane to intense femtosecond laser pulses. Dissociation of the silicon-silicon bond, the formation of highly charged silicons, as well as the saturation intensity of their formation were studied by mass spectroscopy. The production of these ions in high abundance, but also with lower laser intensity than theoretically expected for the element, was accomplished by using organosilicon compounds. Multiply charged silicon was generated at low laser intensity because stripping electrons from organosilicon compounds is much easier than from pure silicon due to the loose binding of electrons belonging to molecular orbitals. Femtosecond laser ionization is a valuable methodology for producing highly charged ions in high abundance and is useful in many fields of interest.

Spectra of heliumlike carbon, aluminium and argon under strongly coupled plasma

Spectral line positions for the highly stripped helium like carbon, aluminium and argon embedded in intense plasma environments have been calculated theoretically to compare with the existing data available from laser plasma experiments. The changes in the ionization potentials for such ions have been determined and the excitation energies, oscillator strengths and transition probabilities for the transitions 1s2 1S → 1snp 1P (n=2-5) have been evaluated for the diagnostic determination of such plasmas. The ion sphere (IS) model was used for estimating the effect of strongly coupled plasma on the ions within the non-relativistic as well as the relativistic framework. Time dependent perturbation theory has been applied for obtaining the linear response properties of the ions in the non-relativistic approximation. The effects of the plasma environment on such properties of the ions under the Debye screening model with suitable cut off radii have also been considered for comparing the data with those obtained from the IS model of the plasma.

Multipole (E1, M1, E2, M2) transition wavelengths and rates between states with n⩽6 in helium-like carbon, nitrogen, oxygen, neon, silicon, and argon

Transition wavelengths and rates are given for E1, E2, M1, and M2 transitions between singlet and triplet S, P, D, and F states in heliumlike ions of astrophysical interest: carbon, nitrogen, oxygen, neon, silicon, and argon. All possible transitions between states with n⩽6 are considered. Wave functions and energies are calculated using the relativistic configuration-interaction (CI) method including both Coulomb and Breit interactions. For transitions to the ground state, the present theoretical wavelengths agree to five digits with precise measurements.

E1 Transitions between States with n = 1–6 in Helium‐like Carbon, Nitrogen, Oxygen, Neon, Silicon, and Argon

Wavelengths and transition rates are given for E1 transitions between singlet 1S, 1P, 1D, and 1F states, between triplet 3S, 3P, and 3D states, and between triplet 3P1 and singlet 1S0 states in ions of astrophysical interest: helium-like carbon, nitrogen, oxygen, neon, silicon, and argon. All possible E1 transitions between states with J ≤ 3 and n ≤ 6 are considered. Energy levels and wave functions used in calculations of the transition rates are obtained from relativistic configuration-interaction calculations that include both Coulomb and Breit interactions.

Complex resonance absorption structure in the X-ray spectrum of IRAS 13349+2438

The luminous infrared-loud quasar IRAS 13349+2438 was observed with the XMM - Newton Observatory as part of the Performance Verification program. The spectrum obtained by the Reflection Grating Spectrometer (RGS) exhibits broad (FWHM - 1400 km/s) absorption lines from highly ionized elements including hydrogen- and helium-like carbon, nitrogen, oxygen, and neon, and several iron L - shell ions (Fe XVII - XX). Also shown in the spectrum is the first astrophysical detection of a broad absorption feature around lambda = 16 - 17 A identified as an unresolved transition array (UTA) of 2p - 3d inner-shell absorption by iron M-shell ions in a much cooler medium; a feature that might be misidentified as an O VII edge when observed with moderate resolution spectrometers. No absorption edges are clearly detected in the spectrum. We demonstrate that the RGS spectrum of IRAS 13349+2438 exhibits absorption lines from two distinct regions, one of which is tentatively associated with the medium that produces the optical/UV reddening.

Theoretical study of dielectronic recombination between electrons and heliumlike carbon ions

A revised simplified relativistic configuration-interaction method is used to study the dielectronic recombination processes of He-like carbon ions. Every resonance of type $1s2Lnl$ has been determined for $n=2,3.$ The total integrated cross sections of the dielectronic recombination are in agreement with the experimental measurements within 10%. It is found that the strong resonance peak around 283 eV consists of three important doubly excited resonance states with the dominant contribution due to two $(1s2p{}^{1}P)3d{}^{2}{F}_{5/2,7/2}$ states, in contrast to the previous assignment of $(1s2p{}^{1}P)3p{}^{2}{S}_{1/2}$ as the dominant resonance state.

Calculation of dielectronic recombination cross sections and rate coefficients for heliumlike carbon

A simplified relativistic configuration interaction method is used to calculate the dielectronic recombination cross sections and rate coefficients for heliumlike carbon. In this method, the infinite resonant doubly excited states can be treated conveniently in the frame of Quantum Defect Theory. Our calculated cross sections are in agreements with the experimental measurements except for the 1s2lnl'(n=6,7) resonances. The total energy-integrated cross sections and rate coefficients over all dielectronic resonances are in agreements with the experimental measurements within percent.

Collisional-Radiative models for hydrogen-like and helium-like carbon and oxygen ions and applications to experimental data from the TS tokamak and the reversed field pinch RFX

Collisional radiative models (CRM) are needed to simulate experimental line brightnesses and emissivities from fusion devices. CRM are built for H-like and He-like carbon and oxygen ions, following a detailed review of the published literature on the required atomic data (including both collision rate coefficients and atomic transition probabilities). The impurity ion radial distribution is obtained using a transport code with two radius dependent transport parameters: a diffusion coefficient D and an inward convection velocity V. Reliable atomic data are a prerequisite to these simulations, since they are supposed to be well known; adjustement of the simulations to the experimental line and/or continuum brightnesses and emissivities is done by varying only D and V. Examples are given of the quantitative interpretation of experimental spectroscopic data from two fusion devices: the Tore Supra Tokamak and the Reversed Field Pinch RFX. As a consequence of the order of magnitude difference in the central electron temperatures in the two devices, the sensitivity of the H-like and He-like ion emissions to the transport parameters is different. These differences are highlighted in the examples given.

Study of neutral-hydrogen-sensitive variations in C V line ratios of the TJ-I tokamak

It has been shown previously that charge exchange that involves slow collisions between hydrogen-like carbon and neutral hydrogen are responsible for variations in the intensity ratio between the helium-like carbon triplet lines, 1s2s 3S 1−1s2p 3P 2 (2271.59 Å) and 1s2s 3S 1-1s2p 3P 0,1 (227.841 nn), which were seen in line-of-sight measurements across the TJ-I tokamak. We compare how this process may cause anomalies in the associated intercombination to the resonance line ratio G, and we shown how line-of-sight measurements across a plasma radius and the equations developed here may be used to estimate the plasma neutral concentration. Finally, we estimate the sensitivities of these line ratios to the neutral density for tokamak plasmas.

Polarization of impurity emission lines from a tokamak plasma.

The visible-uv spectrometer was equipped with a double refracting calcite plate for the purpose of resolving a spectral line into the two linearly polarized components and observing these components simultaneously. Relative intensities of these components have been measured for heliumlike carbon and berylliumlike oxygen impurity lines from the WT-3 tokamak plasma at Kyoto University. In the joule heating mode, for transition from the joule heating to the lower-hybrid current drive mode, and for application of the electron cyclotron heating, changes in the relative intensities were found, which indicated that these lines were polarized. Implications of the polarization of the emission lines from plasma are discussed.

Absolute calibration of a vacuum Czerny–Turner spectrometer in the range 1200–7000 Å

The vacuum Czerny–Turner high-resolution spectrometer observing the plasma of the reversed-field-pinch device RFX has been absolutely calibrated in the spectral range 1215–7000 Å, in order to measure the emission spectrum in absolute units, with particular reference to the heliumlike carbon and oxygen ion lines. The source used to calibrate the spectrometer in the vacuum and near-ultraviolet region is a deuterium lamp; its spectral radiance was determined by comparison with the synchrotron radiation of the electron storage ring BESSY, in Berlin. A typical error of 35% has been estimated. In the visible a standard tungsten lamp has been used. The calibration has been transferred to an ultragrazing incidence XUV spectrometer by means of the branching-ratio technique, using the intensity ratio between the intercombination 1s2 1S–1s2p 3P0 and the 1s2s 3S–1s2p 3P0 transitions of carbon, nitrogen, and oxygen heliumlike ions.

High-precision measurement of the magnetic-dipole decay rate of metastable heliumlike carbon ions in a storage ring.

High-precision measurement of the magnetic-dipole decay rate of metastable heliumlike carbon ions in a storage ring.

Dielectronic recombination from the ground state of heliumlike carbon ions.

The cross section for dielectronic recombination of ${\mathrm{C}}^{4+}$ ions in the 1${\mathit{s}}^{2}$ ground state has been measured with an energy resolution of \ensuremath{\approxeq}2 eV at the heavy-ion storage ring TSR (at Heidelberg) up to the 1s2s${(}^{1,3}$S) and 1s2p${(}^{1,3}$P) excitation thresholds just below 310 eV. Resonance energies and absolute cross sections are found to be well reproduced by theoretical calculations except in a region extending \ensuremath{\approxeq}5 eV below the almost degenerate excitation thresholds 1s2s${(}^{1}$S) and 1s2p${(}^{3}$P). These discrepancies imply that theory overestimates the cross section, energy-integrated over all dielectronic resonances, by \ensuremath{\approxeq}10%.

( n, l)−Subshell electron capture cross sections in collisions of C 4+, N 5+ and O 6+ with atomic hydrogen

By means of VUV-photon spectroscopy we have determined absolute Subshell selective electron capture cross sections σ nl for collisions of helium-like carbon, nitrogen and oxygen ions with atomic hydrogen, in the velocity range 0.1–0.5 a.u. The atomic hydrogen beam target was produced by means of a radio-frequency discharge source, and absolutely calibrated by measuring atomic and molecular radiation in the visible region produced by electron impacts. For all systems studied we find that the total capture cross section ( σ t = Σ nl σ nl depends only weakly on the impact velocity, and is in good agreement with published results from other authors. In contrast to σ t, the Subshell selective cross sections σ nl are strongly velocity dependent. Our results are compared with recent calculations by Fritsch and Lin; the agreement is quite satisfactory.

Measurement of Collisional Rate Coefficients for Heliumlike Carbon Ions in a Plasma

Measurements of the emitted line intensities from a $\ensuremath{\theta}$ pinch discharge are interpreted in terms of a modified corona model for heliumlike ions. The relative intensities of the resonance and intercombination lines have been measured for C V and O VII as a function of time, as has the absolute intensity of the $C \mathrm{v} 2^{3}S\ensuremath{-}2^{3}P$ multiplet. Electron temperatures and densities are obtained as functions of radius and time from the spectrum of Thomson-scattered laser light, and the densities also from Abel-inverted absolute continuum intensities. The derived model allows for population of the $n=2$ levels by direct impact excitation and by cascading, for collision-induced (exchange) transitions from $n=2$ triplets to singlets, and for collisional ionization from both the ground and the $n=2$ triplet levels. Collisional de-excitation of the triplets is shown to be negligible compared with exchange collisions and radiative transitions. Singlet excitation rates (near threshold) are derived with an accuracy of 40% which agree with Seaton and Van Regemorter's semiempirical formulas, triplet excitation rates which are smaller than the singlet rates by a factor 1.8\ifmmode\pm\else\textpm\fi{}0.2, and ground-state ionization rates to an accuracy of 25% agreeing with those estimated from the Born-exchange approximation for hydrogenic ions, suitably modified for two-electron systems. The measured value for the triplet-singlet exchange rate for C V is equal to ⅕ of the value calculated for He I using the close-coupling approximation, and it has an estimated accuracy of 25%.