Narayan C. Deb

Calculation of Intensity Ratios of Observed Infrared [Fe II] Lines

Two recent observational studies of the [Fe II] {lambda}12567/{lambda}16435 line ratio by Smith and Hartigan and Rodriguez-Ardila et al. have suggested that the available theoretical A-values could be incorrect to 10%-40%. We have carried out an extensive configuration interaction calculation of [Fe II] lines to investigate this claim, as well as the variability in observed line ratios for {lambda}8617/{lambda}9052 and {lambda}8892/{lambda}9227 of Dennefeld. For these transitions, we are generally in good agreement with the results of Nussbaumer and Storey, less so with those of Quinet et al. In comparison, the ratios derived from observations appear either to be less secure, or other factors influence those results.

Double ionization of helium by alpha-particle impact

The authors present cross sections for double ionization of helium by alpha-particle impact within the independent-event model. Within this model the probability of ionizing the first electron (P1) from neutral helium is calculated in the continuum distorted-wave (CDW) approximation using an explicitly correlated wavefunction of Pluvinage (1950), whereas the probability of ionizing the second electron (P2) from the He+ ion is calculated in the CDW approximation with an eikonal initial state (CDW-EIS). The total probability for double ionization is then P1P2 rather than the P2 which is usually assumed in the independent-electron model. The calculated cross sections in the present model show excellent agreement with the measurement of Shah and Gilbody (1985), at all energies above 200 keV amu-1.

Electric quadrupole and magnetic dipole transitions among 3d6levels of Fe III

Radiative rates for electric quadrapole (E2) and magnetic dipole (M1) transitions among the lowest 32 levels belonging to the 3d6 configuration of doubly charged iron (Fe III) are calculated using our recently generated (Deb and Hibbert 2007 J. Phys. B: At. Mol. Opt. Phys. 40 F251) J-dependent configuration interaction (CI) wavefunctions. These results are then compared with other available theoretical calculations. It is found that there are large disagreements among various calculations for many of the transitions, with the present A-values for E2 and M1 transitions often significantly higher than the corresponding values in other calculations.

[Ti II] lines observed in η Carinae Sr-filament and lifetimes of the metastable states of Ti+

Forbidden (E2 and M1) transitions among the lowest 37 fine-structure even parity levels of Ti II belonging to 3d24s, 3d3 and 3d4s2 configurations are evaluated in a configuration interaction (CI) calculation in which relativistic effects are accounted for through the full Breit–Pauli interaction in the Hamiltonian matrix. To our knowledge this is the first complete calculation of E2 and M1 transition probabilities among all 37 even parity levels. Comparison of the present gA values with the only other calculated values by Palmeri et al (2008 J. Phys. B: At. Mol. Opt. Phys. 41 125703) for the 29 [Ti II] lines observed in the η Carinae Sr-filament show varying agreement of between 5% → 50%. Agreements among the calculated and observed lifetimes of a few metastable states are mostly good. It is found that single and double core–valence correlation between the n = 3 and 4 complexes are needed for well-converged results.

Double ionization of helium by fully stripped ions in the independent-event model

Double ionization of He by fully stripped ions are studied within the independent-event model. Ionization of each of the two electrons in He are considered to be independent events and probabilities for these events are calculated individually by using the continuum-distorted-wave approximation. Electron-electron correlation has been accounted for through the explicitly correlated two-electron wavefunction of Pluvinage (1950). Good agreement between our results and measurements are attributed to this static correlation as well as to the dynamical correlation of events as against particles.

Positronium formation in positron-helium collision

The authors apply the first-order target continuum distorted wave (TCDW1) approximation to electron capture by positrons from helium into the ground state of positronium. Total cross sections for positronium formation are found to be in good agreement with the measurement of Fromme et al. (1986) above 160 eV. This is the energy range in which most theoretical results disagree with the measurements and indeed in previous classical trajectory Monte Carlo (CTMC) calculations it has been suggested that the experiments incorrectly describe large-angle ionization as charge transfer. The authors elaborate on this matter.

Electron capture into excited s states in the target continuum distorted-wave approximation

Total cross sections for electron capture by protons into the ground and excited s states of a hydrogen atom are calculated employing the target continuum distorted-wave approximation. This apparently high-energy approximation, which is much simpler than any other calculation of its kind, is found to be capable of giving good results at energies as low as 50 keV by comparison with other calculations. The authors calculated total cross sections for capture into 3s and 4s states provide a fair comparison with the experiment of Brower and Pipkin (1989) around 100 keV. Even for the more probable 1s-1s capture the results are in close agreement with existing results. They also present total cross sections for electron capture by protons into various s states from the inner shells of Ne and Ar.

Electron impact ionization of He above the threshold

Near-threshold ionization of He has been studied by using a uniform semiclassical wavefunction for the two outgoing electrons in the final channel. The quantum mechanical transition amplitude for the direct and exchange scattering derived earlier by using the Kohn variational principle has been used to calculate the triple differential cross sections. Contributions from singlets and triplets are critically examined near the threshold for coplanar asymmetric geometry with equal energy sharing by the two outgoing electrons. It is found that in general the triplet contribution is much smaller compared to its singlet counterpart. However, at unequal scattering angles such as ?1 = 60?, ?2 = 120? the smaller peaks in the triplet contribution enhance both primary and secondary TDCS peaks. Significant improvements of the primary peak in the TDCS are obtained for the singlet results both in symmetric and asymmetric geometry indicating the need to treat the classical action variables without any approximation. Convergence of these cross sections are also achieved against the higher partial waves. Present results are compared with absolute and relative measurements of R?sel et al (1992 Phys. Rev. A 46 2539) and Selles et al (1987 J. Phys. B. At. Mol. Phys. 20 5195) respectively.

Evaluation of two-centre one-electron four-orbital overlap integrals involving Coulomb waves

The authors present a very efficient and powerful numerical technique to evaluate two-centre one-electron four-orbital integrals for the overlap of two component wavefunctions, each involving a hydrogenic bound state on one centre and either an outgoing or an ingoing Coulomb wave on the other. Using a double complex inverse Laplace transform for the two Coulomb waves they invert the order of integration. Addressing the space integral as the inner integral they evaluate its angular part in prolate spheroidal coordinates. The radial integral and the two outer complex integrals arising from the double inverse Laplace transform are then evaluated numerically with suitable precision. The accuracy of this technique is checked for some special limiting values, and as a function of nested tolerances. These overlap integrals appear in the distorted-wave theories especially in the impact parameter treatment for elastic as well as rearrangement collisions.

Positronium formation in the 2p state in e++H collisions

Constructive interference between two important second-order matrix elements is found for positronium (Ps) formation in the 2p state for the reaction e++H(1s)Ps(2p)+H+ with high positron velocity. As a result a shoulder-like structure is formed in the differential cross section near 45 if one considers only these two matrix elements for the 1s-2p amplitude. The corresponding result for Ps formation in the ground state has a sharp minimum at the same angle as shown by McGuire et al (1986). This minimum occurs due to the destructive interference between the corresponding two matrix elements. The author also calculates total cross sections for positronium formation in the 2p state and compares his results with calculations in various approximations. The large discrepancies between second Born results and his earlier results for 1s-1s capture are discussed.