Triply differential single ionization cross sections in coplanar and non-coplanar geometry for fast heavy ion-atom collisions

We have performed a kinematically complete experiment and calculations on single ionization in 100 MeV/amu C6+ + He collisions. For electrons ejected into the scattering plane (defined by the initial and final projectile momentum vectors) our first- and higher-order calculations are in good agreement with the data. In the plane perpendicular to the scattering plane and containing the initial projectile axis a strong forward-backward asymmetry is observed. In this plane both the first-order and the higher-order calculations do not provide good agreement neither with the data nor amongst each other.

Ionization of atomic hydrogen and He+ by slow antiprotons

We study the ionization of H(1s), He+(1s) and He+(2s) by antiprotons in the energy range from 0.1 to 500 keV. We adopt a semiclassical single centre close-coupling approach in which the wavefunction for the electron is expanded in a B-spline basis centred on the nucleus of the atom/ion. Comparison is made with existing theoretical calculations and available experimental data. The results are encouraging.

Effective collision strengths for transitions among the 3s(2), 3s3p and 3p(2) configurations of Fe xv

Effective collision strengths for transitions among the ten energetically lowest fine-structure levels belonging to the (1s(2)2s(2)2p(6))3s(2), 3s3p and 3p(2) configurations of Fe xv have been calculated in the electron temperature range of 10(5)-10(7) K, using the recent Dirac atomic R-matrix code of Norrington and Grant. The results are compared with the other recently available independent Breit-Pauli R-matrix calculations of Eissner et al (Eissner W, Galavis M E, Mendoza C and Zeippen C J 1999 Astron. Astrophys. Suppl. 137 165) and Griffin et al (Griffin DC, Badnell N R, Pindzola M S and Shaw J A 1999 J. Phys. B: At. Mol. Opt. Phys. 32 2139, 4129). Large differences are observed for many transitions over almost the entire temperature range. These differences are analysed and discussed, and the accuracy of the calculations is assessed.

Experimental observation of dissociative electron attachment to S2O and S2O2 with a new spectrometer for unstable molecules

A new spectrometer, electron radical interaction chamber, has been developed to study dissociative electron attachment to unstable molecules such as free radicals. It includes a trochoidal electron monochromator and a time-of-flight mass spectrometer. Radicals are generated with a microwave discharge at 2.45 GHz. Preliminary data are presented for radicals formed when a mixture of helium and sulphur dioxide was passed through the microwave discharge. Several new resonances are observed with the discharge on. Resonances at 0 eV (S-), 0.8, 1.2, 3.0 eV (SO-) and 3.7 eV (SO- and S2O-) are assigned to the radical S2O2 and a resonance at 1.6 eV (S-) is assigned to S2O. No new resonances have been assigned to SO, which was also generated in the microwave discharge.

Strong parallel magnetic field effects on the hydrogen molecular ion

Equilibrium distances, binding energies and dissociation energies for the ground and low-lying states of the hydrogen molecular ion in a strong magnetic field parallel to the internuclear axis are calculated and refined, by using the two- dimensional pseudospectral method. High-precision results are presented for the binding energies over a wider field regime than already given in the literature (Kravchenko and Liberman 1997 Phys. Rev. A 55 2701). The present work removes a long- standing discrepancy for the R-eq value in the 1sigma(u) state at a field strength of 1.0 x 10(6) T. The dissociation energies of the antibonding 1pi(g) state induced by magnetic fields are determined accurately. We have also observed that the antibonding 1pi(g) potential energy curve develops a minimum if the field is sufficiently strong. Some unreliable results in the literature are pointed out and discussed. A way to efficiently treat vibrational processes and coupling between the nuclear and the electronic motions in magnetic fields is also suggested within a three-dimensional pseudospectral scheme.

The closed orbits and the photo-excitation scaled spectrum of the hydrogen atom in crossed fields

In a recent Letter to the Editor (J Rao, D Delande and K T Taylor 2001 J. Phys. B: At. Mol. Opt. Phys. 34 L391-9) we made a brief first report of our quantal and classical calculations for the hydrogen atom in crossed electric and magnetic fields at constant scaled energy and constant scaled electric field strength. A principal point of that communication was our statement that each and every peak in the Fourier transform of the scaled quantum photo-excitation spectrum for scaled energy value epsilon = -0.586 538 871028 43 and scaled electric value (f) over tilde = 0.068 537 846 207 618 71 could be identified with a scaled action value of a found and mapped-out closed orbit up to a scaled action of 20. In this follow-up paper, besides presenting full details of our quantum and classical methods, we set out the scaled action values of all 317 closed orbits involved, together with the geometries of many.

Single- and multiphoton detachment of K

Single- and multiphoton detachment rates have been calculated for K- using the R-matrix Floquet approach. Single-photon detachment rates, obtained at a laser field peak intensity of 10(9) W cm(-2), are discussed and compared with other theoretical work. Two-photon detachment rates at the same intensity have also been obtained, and similarities with results from earlier calculations for Li- and Na- are discussed. Three-photon rates are also presented at this laser intensity, and are compared and contrasted with those arising in the single-photon case, since both involve resonance structure with P-1(o) symmetry. The influence of resonances such as the 5s(2) S-1(e) doubly excited state and excitations of the residual atom are also considered.

Multi-ionization of helium and lithium using the independent electron and independent event models with intrinsic CDW

Cross sections for the multi-ionization of He and Li are presented for impact energies in the range of 50 to 1000 keV/amu. These are calculated using the eikonal initial state approximation to represent the input and exit channels of the active electrons. The ionization process is simulated in a variety of ways, most notably an attempt to account for the effects of electron correlation via the inclusion of a continuum density of states (CDS) term. Inadequacies, of the CDW formulation at small impact parameters, and of the models themselves, are discussed and conclusions are drawn on what repercussions this has for the cross sections calculated.

Collision mechanisms in one-electron capture by slow H and He- like ions of C, N and O in H and H-2

Translational energy spectroscopy (TES) has been used to study state-selective one-electron capture by H and He-like ions of C, N and O in both H and H-2 within the range 250-900 eV amu(- 1). The main collision mechanisms leading to state-selective electron capture have been identified, their relative importance assessed and compared, where possible, with theoretical predictions and with any previous measurements based on photon emission spectroscopy. For one-electron capture in H-2, the relative importance of contributions from non- dissociative and dissociative capture as well as from two- electron capture into autoionizing states is found to be strikingly different for the cases considered. Our TES measurements in atomic hydrogen provide an important extension of previous measurements to energies below 1000 eV amu(-1) and show that, as the impact energy decreases, electron capture becomes more selective until only a single n product channel is significant. These product main channels are well described by reaction windows calculated using a Landau-Zener approach. However, the same approach applied to the more complex energy- change spectra observed in H-2 is found to be less successful.

High harmonic generation by halogen anions and noble gas atoms in a laser field

A comparative study of high harmonic generation (HHG) by atoms and ions with active p-electrons is carried out in the theoretical framework of the rescattering mechanism. The substate with m(l) = 0, i.e. zero orbital momentum projection along the electric vector of a linearly polarized laser wave, is found to give the major contribution to the HHG rate. Our calculations for HHG by an H atom in an excited 2p-state demonstrate that the rate for recombination into a final state with a different value of m(l) (= +/- 1), is higher for lower harmonic orders N, while for higher N (beyond the plateau domain) the difference vanishes. For species with closed electron shells, the m(l)-changing transitions are forbidden by the Pauli exclusion principle. We report absolute HHG rates for halogen ions and noble gas atoms at various intensities. These results demonstrate that the Coulomb binding potential of the atoms considerably enhances both the ionization and recombination steps in the rescattering process. However, the weak binding energy of the anions allows lower orders of HHG to be efficiently produced at relatively low intensities, from which we conclude that observation of HHG by an anion is experimentally feasible.