Neon-like Selenium Research Articles

Stimulated Raman scattering in helium with soft-x-ray laser radiation.

We report calculations for stimulated Raman scattering (SRS) with pump photons from a soft-x-ray laser. The Raman transitions from the 1{sup 1}{ital S}{sub 0} ground state to the 2{sup 1}{ital S}{sub 0} and 3{sup 1}{ital S}{sub 0} metastable states of helium at 20.6 and 22.9 eV, respectively, are considered. We calculate {chi}{sup (3)} for pump photons with an energy close to the autoionizing resonances (2{ital s}2{ital p}){sup 1}{ital P} at 60.15 eV and ({ital sp},23+){sup 1}{ital P} at 63.66 eV. SRS gain coefficients are derived for x-ray laser lines in close resonance with the autoionizing levels, viz., the 3{ital p}(3/2,3/2){sub 2}{r_arrow}3{ital s}(3/2,1/2){sub 1} transition in neonlike selenium ({lambda}=20.64 nm) and the analogous transition in neonlike bromine ({lambda}=19.47 nm). It is shown that the Raman gain coefficient with experimentally realistic parameters is significant and that a high gain length product can be achieved even with pump intensities below 10{sup 11} W/cm{sup 2}. {copyright} {ital 1996 The American Physical Society.}

Plasma fluctuations and x-ray laser transverse coherence.

The effect of plasma fluctuations on transverse spatial coherence of x-ray lasers is investigated. Hose type (random) transverse displacements of the x-ray lasing medium induced by pump-laser nonuniformities are considered in detail. Such displacements lead to decreased transverse coherence via reduced gain discrimination from mode coupling. This effect may be related to a previously reported insensitivity of transverse coherence to laser length in neonlike selenium at 206 and 210 A [Trebes {ital et} {ital al}., Phys. Rev. Lett. {bold 68}, 588 (1992)].

Nearly Monochromatic Lasing at 182 Å in Neonlike Selenium[Phys. Rev. Lett. 74, 3376 (1995)

Nearly Monochromatic Lasing at 182 Å in Neonlike Selenium[Phys. Rev. Lett. 74, 3376 (1995)

Nearly Monochromatic Lasing at 182 Å in Neonlike Selenium.

The $J\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0\ensuremath{\rightarrow}1,3p\ensuremath{\rightarrow}3s$ transition at 182 \AA{} in neonlike selenium is observed to completely dominate the lasing output, as predicted but never observed before this work. Instead of a single long pulse we use a series of short 100 ps pulses from the Nova laser to illuminate slab targets of selenium. The 182 \AA{} line is more than an order of magnitude brighter than the usual 206 and 209 \AA{} laser lines. Similar behavior is observed for neonlike ions of germanium, zinc, and nickel.

Use of the prepulse technique to enhance the weak 182-nm laser line in neonlike selenium

We report strong lasing in selenium on the neonlike 3p ? 3s, J = 0 ? 1 transition at 18.2 mn when the prepulse technique of using a low-intensity prepulse before the main optical drive pulse is used to illuminate the selenium target. Without the use of the prepulse, this line is 2 orders of magnitude weaker. The usual J = 2 ? 1 lines at 20.6 and 20.9 nm are observed to dominate the spectra both with and without the use of the prepulse. These experiments may provide important clues to help our understanding of why the 18.2-nm line, which was predicted to dominate the laser output, was always observed weakly in previous experiments.

Experimental and theoretical investigation of neonlike selenium x-ray laser spectral linewidths and their variation with amplification.

This paper describes our recent x-ray laser spectral linewidth measurements and discusses the consequences of these measurements. The experiments observed the 206.38- and 182.45-\AA{} laser transitions in neonlike selenium using an extremely high-resolution soft-x-ray spectrometer, and measured the spectral linewidths and relative spectral intensities of the laser lines from amplifiers of varying lengths. The data allow the intrinsic (unamplified) linewidths of the 206- and 182-\AA{} lasers to be extrapolated as 50 and 35 m\AA{}, respectively. The intrinsic spectral width of the 206-\AA{} laser is somewhat greater than the predicted 43-m\AA{} Voigt-profile width based on calculated transition rates and on the expected ion temperature in the plasma, and while the number of data points obtained with the 182-\AA{} laser is small and a conclusive comparison with calculations is not possible, the extrapolated intrinsic linewidth of this laser is clearly consistent with the predicted 37-m\AA{} Voigt-profile width and is not significantly broader or narrower than expected. The data also show significant gain narrowing of both lines with increasing amplifier length and show no significant rebroadening of the 206-\AA{} laser in long, saturated amplifiers. We discuss line broadening and line transfer calculations we have performed which show that the saturated 206-\AA{} laser line transfer behavior can be treated homogeneously due to non-negligible homogeneous lifetime broadening contributions, which significantly reduce inhomogeneous Doppler saturation rebroadening, and due to collisional redistribution rates which are large enough in any case to homogenize the otherwise inhomogeneous Doppler contributions. We also discuss the implications of these results with regard to earlier suggestions that Dicke narrowing effects could play a significant role in determining the intrinsic line profiles of these laser transitions, and we conclude that Dicke narrowing effects are not likely to be significant for either laser.

Precision measurements and identification of laser plasma emission spectra due to transitions from doubly excited levels of the sodium-like selenium ion

The spectrum of dielectronic satellites of resonance lines of the neon-like selenium ion was analyzed experimentally and theoretically. The satellite line spectrum resulting from stabilizing radiative transitions from doubly excited states of the sodium-like selenium ion to nonautoionizing states was obtained using a laser plasma and a record spectral resolution of λ/Δλ ≈ 5000–8000. A detailed analysis is given of the laser plasma x-ray emission spectrum in the range 7.2–8.8 A, more than 300 lines or compact groups of spectral lines were identified, and their wavelengths were measured with an error of ±5−10−4 A.The good agreement between the modeled and experimental spectra indicates that the emission spectrum of Na-like ions shows promise for use in high-temperature plasma diagnostics.

Ionization balance and gain calculations for neon-like selenium x-ray laser plasmas

We have developed compact collisional-radiative models to describe the ionization balance and excitation mechanisms in neon-like selenium x-ray laser plasmas. These models can be used for calculations of the ionization dynamics, detailed emission spectra, and gain coefficients. Careful attention has been paid to indirect processes such as dielectronic recombination, excitation-autoionization, and resonant excitation. We discuss the importance of different atomic processes and model approximations in the ionization balance and gain calculations. These results will be compared to experimental measurements and to previous calculations.

Analysis of pumping mechanisms affecting the gain of the J=0-1 and J=2-1 lines in neonlike selenium.

We present collisional-radiative-equilibrium (CRE) gain calculations for the important J=2-1 and J=0-1 3p-3s transitions of neonlike selenium using an extensive set of accurate excited-state couplings and self-consistently calculated ionization and excited-state abundances. An atomic model was put together containing all the most significant processes impacting the gain calculation of these transitions in order to determine the sensitivity of the gain ratio of the J=0-1 to the J=2-1 lines to the monopole excitation cross sections in CRE. It is seen that variations of a factor of 2 or 3 of these cross sections are all that are needed to reverse the calculated J=0-1 to J=0-2 gain ratios. For example, a decrease in the direct excitation rate from the ground to the (1/2,1/2${)}_{0}$, 3p level by a factor of 3 is sufficient to reverse the gain ratio for the J=0-1 line at 182 \AA{} to the J=2-1 line at 210 \AA{} and move it into agreement with the experimentally observed ratio for these transitions. Experimental observations would suggest that for low-Z elements (Cu, Zn) this ratio is larger than 1, while for high-Z elements (Mo, Sr) it is less than 1. At issue is where the crossover occurs. We draw attention to another instance where the discrepancy between theoretically calculated behavior and experimental behavior can be attributed to the large size of the calculated monopole excitation rate. We also present the calculated temperature and density behavior of the J=0-1 and J=2-1 gains and discuss how this behavior might be used to determine the adequacy of present atomic models to explain experimental neonlike x-ray-laser observations.

Measurement of the spatial coherence of a soft-x-ray laser

The spatial coherence of a neonlike selenium x-ray laser operating at 206 and 210 \AA{} has been measured using a technique based on partially coherent x-ray diffraction. The time-integrated spatial coherence of the selenium x-ray laser was determined to be equivalent to that of a quasimonochromatic spatially incoherent disk source whose diameter is comparable to the line focus of the visible-light laser pumping the x-ray laser. The spatial coherence was improved by narrowing the line focus width.

Angular spectrum of plasma laser radiation influenced by features of the optical properties of the active medium

Calculations were made of the divergence of stimulated radiation in a laser plasma for various electron density profiles. A qualitative agreement was achieved between the calculated and experimental angular distributions of radiation due to 3p–3s transitions in neon-like selenium assuming a homogeneous gain distribution in the active medium.

Electron scattering by Se XXV (neon-like selenium) using the Dirac R-matrix method

The Dirac R-matrix method has been used to calculate collision strengths for the scattering of electrons with kinetic energy 100-300 Ryd by a neon-like selenium (Se24+) target. In these calculations the 27 jj-coupled levels arising from the configurations 1s22s22p6, 1s22s22p53s, 1s22s22p53p and 1s22s220p53d are included. Collision strengths converged to within 5% are obtained with twenty-four (N+1)-particle collision symmetries (partial waves). The rates for radiative transitions between the 27 levels have also been calculated. The authors' results are compared with previous calculations.

Effects of autoionizing resonances on electron-impact excitation rates for transitions among n=3 levels in neonlike selenium.

Direct and resonance excitation rate coefficients have been calculated for transitions between states with principal quantum number {ital n}=3 in Ne-like selenium. A relativistic distorted-wave method was used to compute the direct excitation rate coefficients and the resonance excitation rate coefficients were obtained using a multiconfiguration Dirac-Fock method. The effect of autoionizing resonances has been found to be significant for most transitions, especially at low temperatures. The resonances enhance the rate coefficients for some electric-dipole-allowed transitions by as much as a factor of 2. For strongly forbidden transitions, resonances can increase the rate coefficients by more than an order of magnitude.

An R-matrix calculation of the electron impact excitation of the fine-structure levels in neon-like selenium (Se XXV)

The authors present calculations based on the R-matrix method for electron impact excitation of Ne-like selenium. In these elaborate calculations, the 27 fine-structure levels arising from the four lowest configurations 2s22p6, 2s22p53s, 2s22p53p and 2s22p53d are included. Terms of the Breit-Pauli Hamiltonian are explicitly included in determining both the target and the scattering wavefunction. All target levels included in the calculations are represented by configuration interaction wavefunctions using the 1s, 2s, 2p 3s, 3p and 3d orbitals. The collision strengths are compared with previous theoretical results. For many transitions they found complicated resonance structures in the excitation cross sections. The excitation cross sections are integrated over a Maxwellian distribution of electron energies to give electron excitation rate coefficients; the rates for transitions from the ground state to each fine-structure level are tabulated and are compared with results from distorted-wave calculations. At low temperatures (Tc=500 eV) some of their rates are six times larger than earlier calculations. They also present estimates of the relative populations of the (2p53p)1D2 and 1S0 levels of Se XXV at Tc=1000 eV and at various electron densities.

The analysis of resonance structures in electron impact ionisation cross sections by many-channel quantum defect theory

Techniques for analysing resonance structures in photoionisation cross sections by means of many-channel quantum defect theory developed by Dubau and Seaton (1984) are extended to distorted-wave electron impact ionisation calculations. The theory is applied to the analysis of resonances in electron impact ionisation of neon-like selenium ions.

Electron-impact ionization of Se24+

Cross sections for electron-impact ionization of neonlike selenium have been calculated using close-coupling wave functions. We report cross sections for ionization of the 2${s}^{2}$2${p}^{6}$ $^{1}S$ ground state and the 2${s}^{2}$2${p}^{5}$3s $^{3}P$ excited state of the ion. We found numerous resonances in the ejected electron channels due to excitation of 2${s}^{2}$2${p}^{4}$3snl autoionizing states. The resonances are very narrow, but some have peaks that are several orders of magnitude above the nonresonant background cross section. Even with radiative decay taken into account, the resonances enhance the total integrated cross section by nearly a factor of 4.

Multipass amplification of soft x rays in a laser cavity

We report the first demonstration to our knowledge of multipass amplification of soft x rays. A gain medium of neonlike selenium ions was placed within a resonant cavity composed of a multilayer mirror and a beam splitter designed for normal-incidence use at the 20.63- and 20.96-nm laser lines of the neonlike selenium. The laser-cavity output was time resolved and exhibited three distinct temporal components identifiable as the single-, double-, and triple-pass amplified emission. In these experiments, multipass amplification was limited by the finite duration of the gain medium.

Dielectronic capture into excited levels of Ne-like selenium

Dielectronic recombination into the singly excited M-shell states of neon-like selenium is studied using results from a multiconfigurational relativistic distorted-wave Auger model. The relativistic calculations are based on the use of a continuous radial grid which is logarithmic at small r and linear at large r. Detailed results are presented for recombination coefficients on a state by state basis, in a manner compatible with detailed level population kinetics modelling of highly-stripped ions in a plasma. The theoretical partial recombination coefficients for recombination through the 3l3l' manifold yield peak recombination rates of 1.58*10-11, 1.37*10-11 and 1.56*10-11 cm3 s-1 for recombination from the fluorine-like 2P3/2, 2P1/2 and 2S1/2 states respectively.

Mechanisms for creating population inversions in Ne-like ions.

We investigate the effects of inner-shell ionization of sodiumlike ions on the kinetics of neonlike selenium, using a simple model that separates the calculation of the excited-state kinetics from that of the ionization balance. Our model includes collisional and radiative transitions between the neonlike states, inner-shell ionization of sodiumlike states, and dielectronic recombination from fluorinelike ions. By examining the relative importance of these various population mechanisms, we demonstrate that inner-shell ionization of sodiumlike ions has a significant effect on the inversion kinetics of neonlike selenium, although it is not sufficient alone to account for the discrepancy with experimental observation.

Recombination of Neon-Like and Adjacent Ions in Plasmas

The neon-like and sodium-like ions of medium Z elements, chromium and nickel are identified as abundant or strongly radiating species in the transport dominated region of the JET plasma. Also in plasma produced by laser ablation of multilayer targets stimulated emission has been identified from neon-like selenium. For both observations accurate theoretical electronic recombination rate coefficients contribute to spectral interpretation, to unambiguous separation of diffusive and atomic processes in impurity transport and to derivation of metal concentrations from VUV lines.A new calculation is presented of recombination of fluorine-like, neon-like and sodium-like ions of chromium, iron, nickel and selenium. Radiative recombination and dielectronic recombination rates to angularly resolved energy levels of both low and high principal quantum number are determined using combination of multiconfiguration perturbation theory and Correspondence Principle techniques. The modifying effects of collisional redistribution in finite density plasma are evaluated and effective net recombination coefficients and effective recombination coefficients to excited levels leading to radiative emission obtained.The theoretical radial distribution of nickel ions across the JET torus is obtained by combining the recombination results with a transport model.