R. H. G. Reid

Electron-impact ionization of Ne6+

We have calculated total and single differential cross sections for electron impact ionization of using a method that combines the distorted-wave Born approximation for the incident/scattered electron with an R-matrix treatment of the system. Our calculation included eight states of the final ion, namely , and six states with the configuration , and up to the -pole component of the interaction between the ionizing electron and the target. The single differential cross sections exhibit considerable structure due to autoionizing resonances, including a large resonance due to the quasi-bound state . In the calculation of the total cross section, a modification to the usual half-range approximation is proposed, which ensures that the contributions from autoionizing resonances have the correct thresholds. Our theoretical results for the total cross section are in good agreement with the experimental results.

Fine-structure transitions in Li(2p)-He collisions

Cross sections for intermultiplet transitions in Li(2p2P)-He(11S) collisions are calculated for impact energies E between 6*10-5 eV and 6*10-2 eV by a close-coupled wave treatment. A special study is made of the onset of spin uncoupling as E is increased. The total cross sections indicate that the spin-uncoupled limit (in which the spin of the valence electron is unaffected by the collision) is reached at 3*10-4 eV. However partial-wave analysis indicates that there are significant departures from the limit up to 3*10-3 eV. A correlation between peaks in the calculated cross sections and resonances in elastic scattering by the 2 Pi molecular potential is studied, and is interpreted as showing the essentially molecular nature of the collision.

Resonance charge transfer between protons and excited hydrogen atoms I. Quantal two-state approximation

Using the quantal two-state approximation the cross sections for symmetrical resonance charge transfer between protons and excited hydrogen atoms (with principal quantum number 5 or less) are computed. Allowance is made for the effect of the change in the velocity of relative motion during the encounter. In most cases this correction is appreciable if the impact energy is less than about 25 ev.

Differential cross sections for excited alkali-inert gas collisions

A simple expression for the differential cross section, based on the elastic approximation, is tested by a numerical comparison with a close-coupled quantal expression. When spin is uncoupled, the agreement is good, but spin coupling gives oscillations in the close-coupled results which are absent from the elastic results.

Electron-impact excitation of the levels of carbon

The R-matrix method is used to investigate electron-impact excitation of neutral carbon over an energy range of interest for fusion plasma modelling. Two independent calculations are carried out. In the first, all levels of carbon with are included in the expansion of the total wavefunction and cross sections including exchange are calculated for collision energies from threshold to 150 eV. In the second, all levels with are included in the expansion and spin-allowed cross sections excluding exchange are calculated from threshold to 60 eV. Cross sections and effective collision strengths are presented for important transitions.

CROSS SECTIONS AND RATE COEFFICIENTS FOR EXCITATION OF THE 1s22s2p3PoJ→ 1s22s2p3PoJ′FINE-STRUCTURE TRANSITIONS IN BERYLLIUM-LIKE IONS BY HEAVY PARTICLE IMPACT

Cross sections for excitation of the 1s{sup 2}2s2p {sup 3}P{sub J}{sup 0} {r_arrow} 1s{sup 2}2s2p {sup 3}P{sub J}{sup 0}, fine-structure transitions in beryllium-like ions by proton, deuteron, triton, and {alpha}-particle impact have been calculated using a close-coupled impact-parameter method. This technique includes the effects of dipole coupling to the nearby triplet 2p{sup 2}, 2s3s, and 2s3d configurations by means of a polarization potential. The authors consider the ions C III, N IV, O V, Ne VII, Mg IX, Al X, Si XI, S XIII, Ar XV, CaXVII, Ti XIX, Cr XXI, Fe XXIII, and Ni XXV. Excitation rate coefficients have also been calculated from the cross sections for a range of temperatures.

Electron impact ionization of Cr (3d54s) 7S

The R-matrix method of Bartschat and Burke (1987) has been used to calculate total and single differential cross sections for electron impact ionization of Cr I in its ground 7S state for impact energies from near threshold to 100 eV. The total cross section for ionization has a peak value of 30.3 a20 at an impact energy of 31 eV. For energies above 30 eV, the total cross section for the production of Cr+ (3d44s) 6D is significantly greater than that for production of Cr+ (3d5) 6S, while for energies below 30 eV the reverse is true. The single differential cross section exhibits pronounced resonance structure for energy losses of the incident electron that are below the threshold for Cr+ (3d44s) 6D production. These resonances have been analysed in detail.

Extreme-Ultraviolet Emission Lines of S XII in Solar Active Region and Flare Spectra

New -matrix calculations of electron impact excitation rates for transitions among the 2s22p, 2s2p2 and 2p3 levels of S XII are presented. These data are subsequently used to derive the theoretical electron density diagnostic emission line intensity ratios R1 = I(215.16 A)/I(299.50 A), R2 = I(218.19 A)/I(299.50 A), R3 = I(288.40 A)/I(299.50 A), and R4 = I(221.41 A)/I(299.50 A). A comparison of these with observational data for solar active regions and flares, obtained with the Naval Research Laboratorys S082A spectrograph on board Skylab and the Solar EUV Rocket Telescope and Spectrograph (SERTS), reveals that the electron densities determined from R1, R3, and R4 are consistent with each other. In addition, the densities deduced from these ratios are in good agreement with those estimated from diagnostic lines in Fe XIV or Fe XV, which are formed at similar electron temperatures to S XII. However, the R2 ratios in the flare observations imply densities smaller than those from Fe XIV/Fe XV, although the active region measurements do not show such discrepancies, suggesting that the 218.19 A line may be blended with a transition from a high-temperature ion. An emission feature in the SERTS active region spectrum at 215.29 A, previously identified as the 2s2p 3P2-2s3s 3S1 transition in O V, may be due primarily to the S XII 215.16 A line.

Electron-impact ionization of Ar9+

We have calculated total and single differential cross sections for electron-impact ionization of Ar9+ in its ground (1s22s22p5)2p0 state by using a method that combines the distorted-wave Born approximation and the R-matrix theory. The incident and the scattered electrons are described by distorted waves, while the wavefunctions of the initial ground state of the Ar9+ ion and its final continuum state (Ar10++e-) are calculated by using the R-matrix approach. This allows us to take into account the excitation-autoionization process. Five states of the final Ar10+ ion, namely 1s22s22p43P, 1D, 1S and 1s22s2p53P0, 1P0, have been included in our calculation. Up to the 24-pole components of the interaction with the ionizing electron were included, exciting ten distinct Ar9+ continuum symmetries. The single differential cross sections exhibit considerable structure due to autoionizing resonances. Total cross sections for production of Ar10+ in each of the five states are presented for impact energies from the threshold energy at 17.6 to 100 au. Our theoretical values for the total cross section are in fair agreement with the experimental results.

Mg VIII DIAGNOSTIC LINE RATIOS IN SKYLAB SOLAR OBSERVATIONS

Recent calculations of Mgviii electron and proton impact excitations rates are used to derive theoretical electron temperature (Te)- and density (Ne)-sensitive emission line ratios involving transitions in the 315–782 Å wavelength range. Some of these ratios are presented in the form of ratio–ratio diagrams, which should in principle allow both Ne and Te to be deduced. These results are compared with solar observational data from Skylab, but agreement between theory and observation is very poor, probably due to blending.