Results 1 - 10 of 11418
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[en] Absolute cross-section measurements for resonant double photoexcitation of Li+ ions followed by autoionization have been performed at high resolution in the photon energy range from 148 eV, just below the (2s2p, 2(0,1)n+) resonance, to 198 eV (the region of the double ionization threshold). The measurements have been made using the photon-ion merged-beam endstation at the Advanced Light Source, Lawrence Berkeley National Laboratory, USA. The absolute cross-section measurements show excellent agreement with theoretical results from the R-matrix plus pseudo-state (RMPS) method. Comparisons between theory and experiment for the Auger resonance energies, autoionization linewidth (?)and the Fano line profile index q for several members of the principal(2snp, 2(0, 1)n+) and (3snp, 3(1,1)n+) Rydberg series found in the photoionization spectra for the 1Po symmetry show satisfactory accord
[en] The depopulation of excited states in beams of negatively charged carbon and silicon ions was demonstrated using collisional detachment and laser photodetachment in a radio-frequency quadrupole ion guide filled with helium. The high-lying, loosely bound 2D excited state in C- was completely depleted through collisional detachment alone, which was quantitatively determined within 6%. For Si- the combined signal from the population in the 2P and 2D excited states was only partly depleted through collisions in the cooler. The loosely bound 2P state was likely to be completely depopulated, and the more tightly bound 2D state was partly depopulated through collisions. 98(2)% of the remaining 2D population was removed by photodetachment in the cooler using less than 2 W laser power. The total reduction of the excited population in Si-, including collisional detachment and photodetachment, was estimated to be 99(1)%. Employing this novel technique to produce a pure ground state negative ion beam offers possibilities of enhancing selectivity, as well as accuracy, in high-precision experiments on atomic as well as molecular negative ions.
[en] In this study, electron collisional broadening of observed spectral lines depends on plasma electron temperature and density. Including this effect in models of measured spectra is necessary to determine plasma conditions; however, computational limits make accurate line broadening treatments difficult to implement in large-scale plasma modeling efforts. In this paper, we report on improvements to the treatment of electron collisional line broadening and illustrate this with calculations using the Los Alamos ATOMIC code. We implement the Dimitrijevic and Konjevic modified semi-empirical model Dimitrijevic and Konjevic (1986 Astron. and Astrophy. 163 297 and 1987 Astron. Astrophys. 172 345), which we amend by employing oscillator strengths from Hartree-Fock calculations. This line broadening model applies to near-neutral plasmas with electron temperatures of Te ~ 1 eV and electron densities of N_e ~10"1"7 cm"-"3. We evaluate the D.K.-inspired model against the previous hydrogenic approach in ATOMIC through comparison to NIST-rated measurements for selected neutral and singly-ionized Ca, O, Fe, and Sn lines using both fine-structure and configuration-averaged oscillator strengths. The new D.K.-inspired model is significantly more accurate than the previous hydrogenic model and we find the use of configuration-averaged oscillator strengths a good approximation for applications such as LIBS (laser induced breakdown spectroscopy), for which we demonstrate the use of the D.K.-inspired model
[en] Energy levels, radiative transition probabilities, and autoionization rates for [Ni]4s24p6nl, [Ni]4s24p54l′nl (l′=d,f,n = 4–7), [Ni]4s4p64l′nl, (l′=d,f,n = 4–7), [Ni]4s24p55l′nl (n = 5–7), and [Ni]4s4p66l′nl (n = 6–7) states in Rb-like tungsten (W37+) are calculated using the relativistic many-body perturbation theory method (RMBPT code) and the Hartree–Fock-relativistic method (COWAN code). Autoionizing levels above the [Ni]4s24p6 threshold are considered. It is found that configuration mixing among [Ni]4s24p54l′nl and [Ni]4s4p64l′nl plays an important role for all atomic characteristics. Branching ratios relative to the first threshold and intensity factors are calculated for satellite lines, and dielectronic recombination (DR) rate coefficients are determined for the [Ni]4s24p6nl (n = 4–7) singly excited states, as well as the [Ni]4s24p54dnl, [Ni]4s24p54fnl, [Ni]4s4p64dnl, [Ni]4s24p64fnl, (n = 4–6), and [Ni]4s24p55l′5l doubly excited nonautoionizing states in Rb-like W37+ ion. Contributions from the [Ni]4s24p64fnl (n = 6–7), [Ni]4s24p55l′nl (n = 5–6), and [Ni]4s24p56l′nl (n = 6–7) doubly excited autoionizing states are evaluated numerically. The high-n state (with n up to 500) contributions are very important for high temperatures. These contributions are determined by using a scaling procedure. Synthetic dielectronic satellite spectra from Rb-like W are simulated in a broad spectral range from 8 to 70 Å. Here, these calculations provide highly accurate values for a number of W37+ properties useful for a variety of applications including for fusion applications.
[en] Energy and angle differential cross sections for the electron-impact double ionization of helium are calculated using a non-perturbative time-dependent close-coupling method. Collision probabilities are found by projection of a time evolved nine dimensional coordinate space wave function onto fully antisymmetric products of spatial and spin functions representing three outgoing Coulomb waves. At an incident energy of 106 eV, we present double energy differential cross sections and pentuple energy and angle differential cross sections. The pentuple energy and angle differential cross sections are found to be in relative agreement with the shapes observed in recent (e,3e) reaction microscope experiments. Integration of the differential cross sections over all energies and angles yields a total ionization cross section that is also in reasonable agreement with absolute crossed-beams experiments.
[en] The x-ray free electron lasers can enable diffractive structural determination of protein nanocrystals and single molecules that are too small and radiation-sensitive for conventional x-ray diffraction. However the electronic form factor may be modified during the ultrashort x-ray pulse due to photoionization and electron cascade caused by the intense x-ray pulse. For general x-ray imaging techniques, the minimization of the effects of radiation damage is of major concern to ensure reliable reconstruction of molecular structure. Here in this paper, we show that radiation damage free diffraction can be achieved with atomic spatial resolution by using x-ray parametric down-conversion and ghost diffraction with entangled photons of x-ray and optical frequencies. We show that the formation of the diffraction patterns satisfies a condition analogous to the Bragg equation, with a resolution that can be as fine as the crystal lattice length scale of several Ångstrom. Since the samples are illuminated by low energy optical photons, they can be free of radiation damage.
[en] Time-resolved K_α spectroscopy measurements from high-intensity laser interactions with thin-foil solid targets are reviewed. Thin Cu foils were irradiated with 1- to 10-J, 1-ps pulses at focused intensities from 10"1"8 to 10"1"9 W/cm"2. The experimental data show K_α-emission pulse widths from 3 to 6 ps, increasing with laser intensity. The time-resolved K_α-emission data are compared to a hot-electron transport and K_α-production model that includes collisional electron-energy coupling, resistive heating, and electromagnetic field effects. The experimental data show good agreement with the model when a reduced ponderomotive scaling is used to describe the initial mean hot-electron energy over the relevant intensity range
[en] Here, we measured the fluorescence photon yield of neon upon soft x-ray ionization (~1200 eV) from the x-ray free-electron laser at Linac Coherent Light Source, and demonstrated the usage of a grazing incidence spectrometer with a variable line spacing grating to perform x-ray fluorescence spectroscopy on a gas phase system. Our measurements also allowed us to estimate the focal size of the beam from the theoretical description developed, in terms of the rate equation approximation accounting for photoionization shake off of neutral neon and double auger decay of single core holes.