K L Bell

Penning ionization by metastable helium atoms

Values of the van der Waals interaction between the 2 1S and 2 3S metastable states of helium and the systems Ne, Ar, Kr, Xe, H2, N2, O2, CH4, Li, Na, K, Rb, Cs and O are computed. The values are used to determine Penning ionization probabilities from the experimental data with results differing significantly from an earlier analysis. Estimates are made of Penning ionization cross sections for He (2 3S) in the alkali metals, in atomic oxygen and in methane, for which no experimental data exist.

The scattering of low-energy electrons by argon atoms

Phaseshifts, differential, total and momentum transfer cross sections are calculated using an R-matrix approach for the elastic scattering of electrons by argon atoms in the impact energy range 0-19 eV. The coupled-state calculation is based upon a single-configuration atomic ground-state wavefunction coupled to a 1P pseudostate. A critical assessment of earlier theoretical and experimental data is made and the conclusion is reached that the present results are the most satisfactory over the entire energy range considered.

Free-free absorption coefficient of the negative hydrogen ion

The free-free absorption coefficient of the negative ion of hydrogen is calculated within the framework of the R-matrix method. The 1s, 2s and 2p states of hydrogen together with three pseudostates have been employed in the eigenfunction expansion. The results have been found to be in good agreement with other calculations and are believed to be currently the most accurate available.

Nebular and Auroral Emission Lines of [Ar IV] in the Optical Spectra of Planetary Nebulae

Recent R-matrix calculations of electron impact excitation rates in Ar IV are used to calculate the emission-line ratio: ratio diagrams (R1, R2), (R1, R3), and (R1, R4), where R1 = I(4711 ?)/I(4740 ?), R2 = I(7238 ?)/I(4711 + 4740 ?), R3 = I(7263 ?)/I(4711 + 4740 ?), and R4 = I(7171 ?)/I(4711 + 4740 ?), for a range of electron temperatures (Te = 5000-20,000 K) and electron densities (Ne = 10-106 cm-3) appropriate to gaseous nebulae. These diagrams should, in principle, allow the simultaneous determination of Te and Ne from measurements of the [Ar IV] lines in a spectrum. Plasma parameters deduced for a sample of planetary nebulae from (R1, R3) and (R1, R4), using observational date obtained with the Hamilton echelle spectrograph on the 3 m Shane Telescope at the Lick Observatory, are found to show excellent internal consistency and to be in generally good agreement with the values of Te and Ne estimated from other line ratios in the echelle spectra. These results provide observational support for the accuracy of the theoretical ratios and, hence, the atomic data adopted in their derivation. In addition, they imply that the 7171 ? line is not as seriously affected by telluric absorption as previously thought. However, the observed values of R2 are mostly larger than the theoretical high-temperature and density limit, which is due to blending of the Ar IV 7237.54 ? line with the strong C II transition at 7236 ?.

Transition probabilities for some spectral lines of singly ionised nitrogen

Sophisticated configuration interaction wave functions have been used to obtain transition probabilities for transitions between the fine structure levels of the thirty-two lowest lying 2s22p2, 2s2p3, 2s22p3s, 2s22p3p, 2s22p3d, 2s22p4s, 2s22p4p states together with the 2s2p23s state of singly ionised nitrogen. The calculations were performed using the CIV3 code and small empirical adjustments were introduced to the diagonal Hamiltonian matrix elements in order to achieve accurate energy splittings between the levels and thereby the CI mixing coefficients. The present results are believed to be the most accurate available. Significant differences are found when the present lifetimes of the more highly excited states are compared with recent experimental data.

Electron-impact excitation rates for transitions involving the n=2 and n=3 levels of beryllium-like N IV

An R-matrix calculation for the electron impact excitation of the berylliumlike ion N IV is described, in which the first 12 eigenstates formed from the 2s2, 2s2p, 2p2 and 2s3l (l = s, p, d) configurations are included in the wavefunction expansion. These 12 ionic states correspond to 20 fine-structure levels, leading to a total of 190 possible independent transitions. Excitation rates, obtained by averaging the electron collision strengths over a Maxwellian distribution of electron velocities, are tabulated for all fine-structure transitions between the lowest n = 2 states together with those between the n = 2 and n = 3 levels, for electron temperatures ranging from 1000 to 1 000 000 K. Comparisons are made with other available theoretical data and significant deviations are found to occur. In particular the present results indicate that the excitation rates, for most of the transitions considered, are significantly affected by resonances converging to the 2s3l thresholds.

Systematic Studies of N IV Transitions of Astrophysical Importance

E1, M2, M1 and E2 rates of transitions between n = 2 levels of N IV have been calculated using the tw independent codes CIV3 and MCHF. Convergence of each of the approaches has been studied and comparisons made as the complexity of the calculations increases to include valence, core-valence and core-core correlation. The agreement between the two methods is sufficiently good to allow us to set quite narrow uncertainty bars. For the S-1-P-1 degrees resonance line, our recommended f-value is 0.609 with an estimated uncertainty of 0.002, while our recommended A-value for the S-1(0)-P-3(1) degrees intercombination line is 580 s(-1) with an estimated uncertainty of 10 s(-1).

Accurate transition probabilities for some spectral lines of singly ionized oxygen

Transition probabilities based on sophisticated configuration interaction wave functions have been obtained for transitions between the fine structure levels of the 26 lowest lying 2s22p3, 2s2p4, 2s22p23s, 2s22p23p, 2s22p23d states of singly ionized oxygen. The calculations were performed using the CIV3 code of Hibbert and small empirical adjustments were introduced to the diagonal Hamiltonian matrix elements in order to achieve accurate energy splittings between the levels. The present results are believed to be the most accurate available and, for several allowed transitions, a significant departure from LS coupling, due to relativistic effects, is found.

Penning ionization: He(23S)-H(12S) collisions

Quantal calculations are carried out on the rate at which the electronic transitions involved in He(23S)+H(12S) to He(11S)+H++e occur. An impact parameter formulation is then employed to determine the rate coefficient for the process. At thermal temperatures the value obtained is 7.5(+or-20%)*10-10 cm3s-1.

First Born approximation cross sections for electron loss from fast hydrogen atoms passing through atomic helium and fast helium atoms passing through atomic hydrogen and helium

The first Born approximation is used to calculate cross sections for one-electron loss from fast atoms A passing through atomic targets B for the following colliding partners (A, B): (H, He), (He, H) and (He, He). Full account is taken of double transitions and the calculations are performed using length and velocity formulation values of the helium matrix elements. Comparison is made with available experimental data, and for the case (H, He) excellent agreement is obtained at high impact energies. Further experimental measurements are desirable for (He, H) and (He, He).