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Recently, within the hyperradial-adiabatic approach, Igarashi and Lin discussed a possibility of using the universal J(J + 1)/2MR² term in close-coupling transfer reaction calculations. Here, we demonstrate that accurate analytic diagonalization of the Coriolis coupling terms results in the J(J + 4)/2MR² centrifugal potential.

Charge transfer in the collision system He²⁺ + N⁴⁺ has been investigated both theoretically and experimentally for centre-of-mass energies between 8 and 200 keV. The theoretical calculations of the collision process have been carried out in the semi-classical impact parameter eikonal approach expanding the electronic wavefunction in a multi-state molecular-orbital basis with translation factors. The measurements of the charge-transfer cross sections were performed at an ion-ion crossed-beams experiment. Good agreement between the calculations and the experimental results for both total and differential cross sections is obtained.

Measurements of the angular distribution of photoelectrons and the associated Auger electrons, together with their angular correlation, have allowed the relative amplitudes and phases of the s and d matrix elements for 4p_3/2 photoionization of Sr to be determined. The results indicate that there is destructive interference between the s and d channels and that the photoionization parameters show little dependence on energy.

We identify the elementary normal modes of the collective motion of three electrons in intrashell triply excited states of atoms. By modelling the atom as consisting of three electrons on the surface of a sphere with the nucleus at the centre, we analysed the calculated wavefunctions in the body-fixed frame. Using contour surfaces in appropriate coordinates, the nodal surfaces and the normal modes of the collective motion of the three electrons can be visualized. Three elementary normal modes and their excited modes are identified, thus allowing the new classification of intrashell triply excited states.

We propose a simple stirring experiment to generate quantized ring currents and solitary excitations in Bose-Einstein condensates in a toroidal trap geometry. Simulations of the three-dimensional Gross-Pitaevskii equation show that pure ring current states can be generated efficiently by adiabatic manipulation of the condensate, which can be realized on experimental timescales. This is illustrated by simulated generation of a ring current with winding number two. While solitons can be generated in quasi-one-dimensional tori, we show the even more robust generation of hybrid, solitonic vortices (svortices) in a regime of wider confinement. Svortices are vortices confined to essentially one-dimensional dynamics, which obey a similar phase-offset-velocity relationship as solitons. Marking the transition between solitons and vortices, svortices are a distinct class of symmetry-breaking stationary and uniformly rotating excited solutions of the two-dimensional and three-dimensional Gross-Pitaevskii equation in a toroidal trapping potential. Svortices should be observable in dilute-gas experiments.

Various total cross sections for the impact of electrons on free radicals CH_x, NH_x and OH are calculated at intermediate to high energies. The known ionization data on the parent molecules CH₄, NH₃ and H₂O are employed to extract total ionization cross sections of the respective radicals using total inelastic cross sections, which we have calculated through a complex optical potential. The results give an upper bound of the total ionization cross section in the energy region around 100 eV, and are in suitable harmony with available experimental data at higher energies.

An experimental study of the two-photon spectra of high members of the Sr I 5snd ^1,3D₂ Rydberg series is described. We use transverse excitation of an atomic beam with a narrow-bandwidth tuneable dye laser within a heat pipe equipped for field ionization detection. The presence of weak collisions has revealed several anomalies. Strong spin-orbit interaction and the breakdown of parity and selection rules combined with `l' mixing are observed. The breakdown is due to the Stark field present in the interaction region. Single-photon transitions originating from the 5snp ¹P₁ and 5snp³P_J states populated by electron impact excitations are also observed. They create highly compact doubly excited 4d²³P_J, 5p²¹D₂ and 4d5p ³P_J states, which act as dominant intruders in the observed spectrum. Such states with highly localized orbitals survive collision and field effects, whereas Rydberg states are highly sensitive to external perturbations. Also, we have observed l-changing collisional processes like Ar + Sr (5s}nd ^1,3D₂) → Sr (5snl,l⩾ 3). Several members of a new series which begin to appear at large n are observed and are tentatively designated as the 5snf ¹F₃ Rydberg series. Furthermore, we have observed and identified several members of the Sr I 5snp ¹P₁ and 5snp ³P₂ Rydberg series by two-photon absorption from the ground state. They are excited via the breakdown of parity and other selection rules, owing to the presence of external fields. New values of energy levels of the Sr I 5snd ^1,3D₂ Rydberg series are presented: n=25-73 for singlet and n=25-47 for triplet members, obtained from two-photon spectra. Comparison of these values with those given by other authors who used different excitation schemes shows good agreement. The spectrum of fluorescence emission from a low-pressure discharge around the field ionization probe was obtained using a monochromator-photomultiplier detection system. The emission lines confirm the collisional excitations observed in the spectra of the Rydberg series. An energy level diagram summarizing the various transitions observed in our study is also presented.

We examine the most important processes leading to the creation of excited states from the ground configurations of Ar⁸⁺ to Ar¹⁶⁺ ions in an electron-cyclotron resonance ion source, which lead to the emission of K x-ray lines. Theoretical values for inner-shell excitation and ionization cross sections, including double KL ionization, transition probabilities and energies for the de-excitation processes, are calculated in the framework of the multi-configuration Dirac-Fock method. With reasonable assumptions about the electron energy distribution, a theoretical Kα x-ray spectrum is obtained, which reproduces very closely a recent experimental result.

We report on ab initio calculations of two-photon, single- and double-electron ejection cross sections in helium, using multichannel wavefunctions constructed within the framework of discretized bases involving linear combinations of B-splines. Within the lowest non-vanishing order of perturbation theory, two-photon double ionization, for photon energies from 42.5 to 46.5 eV, is found to be about 10% that of single ionization. It is also found that electron correlation in the final state tends to lower the amount of double ionization in comparison to predictions involving incomplete or no correlation. The potential of the approach in the context of further open problems is also discussed.

Time of flight mass spectrometry in combination with laser ablation (LA) can be successfully applied to low-isotope-ratio measurements of metallic samples. In this connection, the non-selective quasi-resonant photoionization by Nd-YAG pulse laser third-harmonic radiation of uranium atoms created by either LA on a metallic target or by radiative heating in an oven was studied in a wide range of laser fluence both experimentally and with computer simulation. In the latter case both the level structure of the uranium atom and the experimentally obtained profiles of spectral and spatial distributions of the third-harmonic radiation of the Nd-YAG laser in the photoionization zone were taken into account. Comparison of experimental and simulated results was made. Experimental and simulated results for the LA case were found to be in agreement only with the assumption that the ionization yield from the metastable level of the uranium atom is strongly suppressed. This could be explained by low population of the metastable level in the photoionization volume due to the expansion and subsequent population cooling of uranium atom vapour created by LA. The unknown parameters of uranium transitions (lifetimes and ionization cross sections) involved in the quasi-resonant photoionization process under study are estimated.

The cross section, σ_n, for electron capture by Na⁺ ions from coherent elliptic states (CES) of Li was studied experimentally. The dependences on the principal quantum number, n, the eccentricity, e, and the spatial orientation of the CES were determined at the two reduced ion velocities v_r = 1.20 and 1.68 (v_r≡v/v_e, where v is the ion speed and v_e = 1/n the mean electron speed of the CES in atomic units). The ion velocity, , was perpendicular to the minor axis of the CES throughout the experiment, so the orientation is fully specified by the angle, φ, between and the electric dipole moment, , of the CES. According to a generalized correspondence principle, the reduced cross section, = lim _n→∞σ_n/n⁴, is given accurately by classical mechanics, and it is a universal function of the scaled parameters of the collision, which in the present experiment are v_r, e and φ. It is assumed that this result can be extrapolated to the range of n-values, 20-35, covered in this paper, i.e. σ_n = ·n^p where and p≃4 depend only weakly on n. The experimental data support the assumed existence of a universal cross section function and the expected n⁴-dependence is also confirmed at v_r = 1.20, but at v_r = 1.68 the n-dependence is closer to n³.

An analysis of the spectra of calcium-like Mn VI, Fe VII and Co VIII in the 550-1375 Å wavelength region is reported following observations involving collision-based spectroscopy of ion-rare-gas collisions in an ion beam from an ECR ion source. The strongest lines were found to originate from 3d4s→3d4p and 3d4d→3d4f transitions in the spectra of Mn⁵⁺ and from 3d4p→3d4d and 3d4d→3d4f transitions in the spectra of Fe⁶⁺ and Co⁷⁺ and were classified using multiconfiguration Hartree-Fock calculations and isoelectronic comparison. This permitted the location of the 3d4f³H levels in these ions and established the energies of 14 of the levels in Fe VII and 13 of the levels in Co VIII of the 3d4d configuration for the first time.

Absolute grand total cross sections (TCSs) for electron-disilane (Si₂H₆) scattering have been measured over the energy range from 1 to 370 eV in a linear transmission experiment. The low-energy TCS is dominated by a broad resonant-like enhancement. In the region of the maximum the present grand TCS values appeared to be distinctly lower than previously reported integral elastic cross section data. A comparison of total electron scattering cross sections for the two simplest silicon hydrides and relevant hydrocarbons is given.

The L x-ray production (XRP) differential cross sections in Th and U have been measured at the 17.8 keV incident photon energy (E_L₃<E_inc<E_L₂, E_Li is the L_i subshell ionization threshold) in an angular range 90°-160°, and at the 25.8 and 46.9 keV incident photon energies (E_L₁<E_inc<E_K) at an angle of 130°. The measurements were performed using the energy dispersive x-ray fluorescence set-up in secondary excitation mode. In contrast to the findings of an earlier experiment (Sharma and Allawadhi 1999 J. Phys. B: At. Mol. Opt. Phys.32 2343), the present measurements rule out the possibility of a strong angular dependence of differential cross sections for various L₃ subshell x-rays following selective photoionization of the L₃ subshell. Integral L XRP cross sections at the 17.8, 25.8 and 46.9 keV photon energies, deduced assuming isotropic emission of the L x-rays, are found to be in good agreement with those evaluated using the most reliable theoretical values of L_i (i = 1,2,3) subshell photoionization cross sections, fluorescence yields, x-ray emission rates and Coster-Kronig transition probabilities.

A relativistic model of f⟷f transitions introduced previously is completed here by the third-order contributions caused by electron correlation. The approach is based on the transformation of all tensor operators to their relativistic effective form; the final effective operators that act within the 4f shell are derived by means of the so-called partial closure. The tensorial structure of the new effective operators, that are in general two-particle objects, is discussed and their reduced form is analysed in the light of the standard single-particle parametrization scheme of f⟷f transition spectra.

Non-radiative transitions cause changes in the generation of the intensity of the L lines. In order to investigate the physical quantities relevant to the L lines affected by the non-radiative transitions, experimental measurements were carried out using a Si(Li) x-ray spectrometer. Atomic L shell Coster-Kronig yields (f₁₂, f₁₃ and f₂₃) for some elements in the atomic number range 59⩽Z⩽90 were determined. These selected measured semi-empirical values were also fitted by least squares to polynomials in Z of the form ∑_na_nZⁿ (except for f₁₃) and compared with theoretical and with earlier fitted values.

Recent R-matrix calculations of electron impact excitation rates in Ni XII are used to derive the emission line ratios R₁ = I(154.17 Å)/I(152.15 Å), R₂ = I(152.95 Å)/I(152.15 Å) and R₃ = I(160.55 Å)/I(152.15 Å). This is the first time (to our knowledge) that theoretical emission line ratios have been calculated for this ion. The ratios are found to be insensitive to changes in the adopted electron density (N_e) when N_e⩾5×10¹¹ cm^-3, typical of laboratory plasmas. However, they do vary with electron temperature (T_e), with for example R₁ and R₃ changing by factors of 1.3 and 1.8, respectively, between T_e = 10⁵ and 10⁶ K. A comparison of the theoretical line ratios with measurements from the Joint European Torus (JET) tokamak reveals very good agreement between theory and observation for R₁, with an average discrepancy of only 7%. Agreement between the calculated and experimental ratios for R₂ and R₃ is less satisfactory, with average differences of 30 and 33%, respectively. These probably arise from errors in the JET instrument calibration curve. However, the discrepancies are smaller than the uncertainties in the R₂ and R₃ measurements. Our results, in particular for R₁, provide experimental support for the accuracy of the Ni XII line ratio calculations, and hence for the atomic data adopted in their derivation.

The correlation function for the process of double ionization of the He atom by ion impact is calculated. A simple quantum mechanical approach is developed to take into account the dynamics of two-electron ejection as well as the effects of correlated motion of electrons in the final state and electron exchange. The reasonable agreement with the available experimental data shows that our approach can describe the most essential features of electron correlation in the correlation function.

Spin-orbit effects in electric field gradients (EFG) of alkali atoms in the ²P_3/2 state are investigated by comparison of correlated (CCSD(T)) four-component Dirac-Fock results with scalar relativistic Douglas-Kroll calculations. Since the Douglas-Kroll method is based on a unitary transformation of the full four-component Dirac Hamiltonian to a two-component form the use of the untransformed EFG operator leads to the well known picture change error. We therefore use the point charge nuclear quadrupole moment method in combination with the Douglas-Kroll transformation which removes picture-change effects since only Coulomb-type operators are to be transformed. We find that spin-orbit effects partially compensate scalar relativistic effects and are only important for the heavier elements Rb and Cs. A comparison to EFGs derived from valence-only ⟨r^-3⟩ values shows that Sternheimer corrections are roughly proportional to the dipole polarizability of the positively charged atom and can become very large for the heaviest atom studied, Cs, amounting to 46% of the total EFG.

We study a three-level atomic system of the vee type, but driven on only one transition by a monochromatic laser. It is shown that the gain of a probe beam, recently predicted for this system by Menon and Agarwal (Menon S and Agarwal G 2000 Phys. Rev. A 61 13 807), is due to an unexpected amplification on a completely inverted, nondecaying (dark) transition. This prediction violates the well known balance condition between the population inversion and the coupling strength of the probe field to the inverted transition, which requires that the coupling strength reduces with increasing population inversion. We show that the condition may be violated only if the probe field selectively couples to just one of the atomic transitions: when it couples to both transitions, the balance condition is satisfied and the system is transparent for the probe field coupled to the dark transitions. No amplification is possible in the latter case.

We have carried out a 200 level close-coupling calculation for C-like Fe using the R-matrix method in conjunction with the intermediate coupling frame transformation method. We have generated effective collision strengths over T = 8×10⁴-8×10⁷ K for all 19 900 inelastic transitions, which is an order of magnitude larger than has been generated hitherto. We provide illustrative comparisons with the results of previous workers, where possible, and find a broad accord. The consistent and comprehensive set of data that we have generated (energy levels, radiative rates and effective collision strengths) is necessary for the collisional-radiative modelling of metallic impurities that will arise in the next generation of magnetic fusion reactors as well as being of relevance to studies utilizing observations from the high-resolution x-ray satellites Chandra and XMM-Newton.

An alternative method is presented for the evaluation of the two-photon ionization transition amplitude and transition rates of atomic hydrogen in the ground state above the one-photon ionization threshold. In this approach it is straightforward to calculate the angular distribution of the emitted electrons. These angular distributions are plotted and calculated transition rates are compared with the previously reported results.