Fine-structure Cross Sections Research Articles

Collisional radiative model for Ar-O2 mixture plasma with fully relativistic fine structure cross sections

A collisional radiative (C-R) model has been developed to diagnose the rf generated Ar-O2 (0%–5%) mixture plasma at low temperatures. Since in such plasmas the most dominant process is an electron impact excitation process, we considered several electron impact fine structure transitions in an argon atom from its ground as well as excited states. The cross-sections for these transitions have been obtained using the reliable fully relativistic distorted wave theory. Processes which account for the coupling of argon with the oxygen molecules have been further added to the model. We couple our model to the optical spectroscopic measurements reported by Jogi et al. [J. Phys. D: Appl. Phys. 47, 335206 (2014)]. The plasma parameters, viz. the electron density (ne) and the electron temperature (Te) as a function of O2 concentration have been obtained using thirteen intense emission lines out of 3p54p → 3p54s transitions observed in their spectroscopic measurements. It is found that as the content of O2 in Ar increases from 0%–5%, Te increases in the range 0.85–1.7 eV, while the electron density decreases from 2.76 × 1012–2.34 × 1011 cm−3. The Ar-3p54s (1si) fine-structure level populations at our extracted plasma parameters are found to be in very good agreement with those obtained from the measurements. Furthermore, we have estimated the individual contributions coming from the ground state, 1si manifolds and cascade contributions to the population of the radiating Ar-3p54p (2pi) states as a function of a trace amount of O2. Such information is very useful to understand the importance of various processes occurring in the plasma.

Collisional-radiative model for non-Maxwellian inductively coupled argon plasmas using detailed fine-structure relativistic distorted-wave cross sections

Our recently developed collisional-radiative model which included fine-structure cross sections calculated with a fully relativistic distorted-wave method [R.K. Gangwar, L. Sharma, R. Srivastava, A.D. Stauffer, J. Appl. Phys. 111, 053307 (2012)] has been extended to study non-Maxwellian inductively coupled argon plasmas. We have added more processes to our earlier collisional-radiative model by further incorporating relativistic distorted-wave electron impact cross sections from the 3p54sJ = 0, 2 metastable states, (1s3, 1s5 in Paschen’s notation) to the 3p55p (3pi) excited states. The population of various excited levels at different pressures in the range of 1–25 mTorr for an inductively coupled argon plasma have been calculated and compared with the recent optical absorption spectroscopy measurements as well as emission model results of Boffard et al. [Plasma Sources Sci. Technol. 19, 065001 (2010)]. We have also calculated the intensities of two emission lines, 420.1 nm (3p9 → 1s5) and 419.8 nm (3p5 → 1s4) and compared with measured intensities reported by Boffard et al. [J. Phys. D 45, 045201 (2012)]. Our results are in good agreement with the measurements.

Argon plasma modeling with detailed fine-structure cross sections

Our recently reported fully relativistic distorted-wave electron-impact cross sections from the ground and metastable states of argon to various excited fine-structure levels are incorporated in a collisional-radiative model to obtain the population densities for the 3p54s and 3p54p (1s and 2p) fine-structure manifolds for low temperature argon plasmas. Excitation cross sections from the two 3p54s J = 1 resonance levels, 1s2 and 1s4, to the higher lying 2p fine-structure manifold as well as for transitions among individual levels of the 1s and 2p manifolds are also calculated and included in the present model which were not fully considered in any earlier model. Our results for the population densities of the 1s and 2p levels show good agreement with recent measurements. The variation of population densities of all the 1s and 2p levels with electron temperature and density are presented. We have also calculated and compared the intensities for the 750.38 nm (2p1 → 1s2) and 696.54 nm (2p2 → 1s5) lines with recently reported experimental results. The present work suggests that the inclusion of a complete fine-structure description of the electronic processes occurring in the plasma is important for a collisional radiative model, which includes separate 1s and 2p levels.

Investigation of sodium-helium interactionpotentials

We present a detailed theoretical investigation of thesodium-helium interaction and report results of quantumclose-coupled calculations for the pressure-broadened widthsand shifts of the sodium D lines and the 819.48 nm line overthe temperature range 70-500 K, and for the total and fine-structure cross sections in the 3p 2P level. Wealso present the first theoretical investigation of thehyperfine pressure shifts using a semiclassical scatteringformalism and report results for the hyperfine transition inthe 3s 2S1/2 ground state over the temperaturerange 70-1000 K. Finally, we calculate the refractive index ofhelium for sodium matter waves with velocities up to 2000 m s-1.Several of the calculations were performed using two versions ofthe adiabatic molecular potentials of Peach (Peach G 1996unpublished) and the results compared with those obtainedusing the l-dependent pseudo-potentials of Pascale (Pascale J1985 XIII Summer School on Quantum Optics ed J Fiutak andJ Mizerski pp 39-91) and Peach (Peach G 1996 unpublished).Detailed comparisons with other theoretical calculations andexperimental results are also made.

Polarization study of the extreme-ultraviolet emission from helium following electron impact

Experimental results are presented on the degree of linear polarization of the extreme-ultraviolet emission of neutral and ionized helium following electron-impact excitation and ionization excitation of helium. The polarization of the photon emission from the decay of He $(1snp){}^{1}{P}^{o}$ states with wavelengths of 517 to 584 \AA{} has been extended to electron-impact energies of 1500 eV, and compared with theory for electron-impact excitation of neutral helium. In addition, the polarization of a number of the ${\mathrm{He}}^{+}$ decays with wavelengths between 256 and 1640 \AA{} have been measured. Particular attention has been paid to the $(2p){}^{2}{P}^{o}\ensuremath{\rightarrow}(1s){}^{2}S$ and $(3p){}^{2}{P}^{o}\ensuremath{\rightarrow}(1s){}^{2}S$ decays with wavelengths of 304 and 256 \AA{}, respectively. These have been measured from threshold (66 and 73 eV, respectively) to 1500 eV using a characterized molybdenum/silicon multilayer mirror polarimeter whose reflection and polarization characteristics have been optimized at 304 \AA{} ${\mathrm{He}}^{+}(2p){}^{2}{P}^{o}$ results are compared with a recent threshold alignment measurement and distorted-wave Born-approximation calculation for the ${(e}^{\ensuremath{-}}{,2e}^{\ensuremath{-}})$ reaction of ionization excitation. Very good agreement with the threshold theoretical predictions is obtained. These results indicate that near threshold, partial waves with $L>0$ contribute substantially to the two-electron wave function of the escaping electrons. Measurements on the unresolved multiplets at 1215 and 1640 \AA{}, made using a more conventional reflection-type polarization analyzer, indicate that relative fine-structure cross sections are quite different than predicted by some theories.

Single-frequency laser measurements of two-photon cross sections and Doppler-free spectra for atomic oxygen.

Absolute bound-bound atomic two-photon cross sections have been measured using a single-frequency laser for the first time, eliminating the usual uncertainties about unresolved temporal fluctuations. For the $3p^{3}P_{{J}^{\ensuremath{'}}}\ensuremath{\leftarrow}2p^{3}P_{2}$ transition in atomic oxygen at 226 nm the integrated cross section is $\ensuremath{\Sigma}{{J}^{\ensuremath{'}}}^{}{\ensuremath{\sigma}}_{0}^{(2)}({J}^{\ensuremath{'}}\ensuremath{\rightarrow}2)=1.87\ifmmode\pm\else\textpm\fi{}0.60\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}35}$ ${\mathrm{cm}}^{4}$. Doppler-free spectra have been used to measure relative fine-structure cross sections and energy spacings. Absolute and relative two-photon cross sections agree well with perturbation theory, validating this approach.

Fluorescence ratios in non-resonant excitation of alkali-helium pairs

The fluorescence intensities for the D lines in non-resonant excitation of the alkali-helium pairs under single-collision conditions were obtained from calculations of the optical collision spectra by the quantum coupled-channels method. The close relation of the non-adiabatic transition probability from a molecular level to an atomic state during the final stage of an optical collision and the spectral dependence of the relative fluorescence intensities is investigated. An analysis of the impact region in terms of line-broadening and fine-structure cross sections calculated separately for all systems confirms the factorisation approximation.

Energy average of the scattering matrix in picket-fence models

The optical-model potential at low energy is defined by the requirement that it yields a suitably defined energy average of the scattering function. It is argued that the quantities which appear in the expression for this energy average derived by Lane and Thomas in the framework of R-matrix theory can be identified with those which appear in a convenient parametrization of the fine structure cross section. The accuracy of this identification is illustrated with the help of various picket-fence models. We discuss the independence of the results with respect to the size of the energy domain covered by the experimental data and to the nature of the R-matrix boundary parameters. Also we discuss terms that can be included if one wishes to discuss fluctuations about a smooth average.

Photodissociation of homonuclear diatomics: Fine structure cross sections for Na 2(X 1Σ g+) → Na( 2S 1/2) + Na( 2P 1/2,3/2

Exact quantum calculations are presented for fine structure cross sections in photodissociation of Na 2. Resonance peaks in the Na( 2P 3/2) cr

Fine structure cross sections from reactance matrices

Fine structure cross sections from reactance matrices

Fine structure cross sections from reactance matrices — A more versatile development of the program JAJOM

Title of program: JAJOMPRE Nature of physical problem Results from calculations with LS-coupled wave functions Catalogue number: ACYG (reactance matrices and term coupling coefficients) are used to compute collision strengths for transitions between fine Program obtainable from: CPC Program Library, Queen’s structure levels. University of Belfast, N. Ireland (see application form in this issue) Method of solution Computer: IBM 360/65, IBM 360/195, CDC 6400; InstallaThe input is carefully checked. The algebraic transformation tion: University College London, SRC/Rutherford Laboratory, uses Racahalgebra, the coefficients are computed as required. UK, JILA, University of Colorado An optional further transformation to allow for intermediate coupling in the target requires term coupling coefficients to Operating system: OS 360, scope 34 be supplied by the user. The collision strengths are computed using either of two formulae, corresponding to weak or strong coupling respectively. Programming language used: FORTRAN IV

Fine structure cross sections from reactance matrices — a more versatile development of the program jajom

We present in this work new quantum Stark broadening data 8 neon-like ions from Mg III to Cl VIII and separately Fe XVII and Br XXVI: twenty spectral lines for each of the mentioned 8 ions have been considered. To the best of our knowledge, among the 160 Stark widths provided for these ions, only three lines of Mg III and six of Si V were calculated in precedent works, the rest Stark widths are new, and no results have been found to compare with. The only comparisons have been performed with the available data. We present some of the collision data (collision strengths, cross sections, ...) used in our broadening calculations, and we compare them to other results. The comparison shows an acceptable agreement which can help us to decide about the accuracy of our Stark broadening calculations. We use also our quantum calculations to provide systematic trends of Stark widths with the effective charge Z of all the Ne-like ions from Mg III to Br XXVI. We find that for most of these lines, there is a regularity with the charge Z in this iso-electronic sequence. We hope that our calculations can fill the lack of line broadening data used in stellar spectroscopy and plasma diagnostics, and we hope also that the provided relation between Stark broadening and the charge Z can be useful for the deduction of Stark widths of all the neon-like ions between Mg III and Br XXVI without labour calculations.

Fine structure cross sections from reactance matrices

To calculate fine-structure electron–atom collision cross-section, a set including 3 codes was developed at University College London, presented in Computer Physics Communications: SUPERSTRUCTURE (1974), UCL Distorted wave (1998), JAJOM (1972, 1978). Due to improvements in computers, more ambitious calculations of atomic data were performed. Unfortunately, unexpected problems have appeared which required an update version of these codes, presented here.