Table of contents

We report on (e, 2e) experiments for excitation-ionization of helium made in asymmetric geometry at intermediate incident energy and low momentum transfer. The results are compared with Born 1 calculations. Some degree of agreement is obtained and seems to be improved by correction for post-collision interactions. But it cannot be expected that such simple models will correctly reproduce processes that depend so strongly on correlations.

Resonant Auger electron spectra following Kr:2p → 5s photoexcitation have been measured for the first time using monochromatized undulator radiation and a cylindrical mirror electron energy analyser. It is found that the kinetic energy of the resonant Auger electron is higher than that of the corresponding normal Auger electron. The angular distribution of the resonant Auger electrons is nearly isotropic relative to the polarization direction of the incident light.

The inverted perturbation approach (IPA) method is used to construct the potential energy curve of the double minimum 6 ¹Σ⁺ state of NaK. The IPA potential makes it possible to reproduce the experimental energies of 1032 rovibrational levels situated both in the inner well and above the internal maximum of the potential.

The vibrational distribution of H₂⁺(X ²Σ⁺_g; ν) after charge transfer in C²⁺(2s2p; ³P) + H₂ collisions is obtained ab initio using the semiclassical eikonal method and the sudden approximation. Results are in good agreement with experiment (Leputsch et al 1997 J. Phys. B: At. Mol. Opt. Phys.30 5009).

A more accurate variant of the semiclassical quantum-mechanical treatment of Crothers (1986 J. Phys. B: At. Mol. Phys.19463-83) has been used to reinvestigate the ionization cross sections of He by electron impact at an excess energy of 2 eV above the threshold. All partial wave contributions for singlets and triplets are accounted for up to L = 6. It is found that within the coplanar geometry, both the symmetric and asymmetric triple differential cross sections, peaking at and near the Wannier ridge, are greatly improved when compared to the measurements (Rösel et al 1992 Phys. Rev. A 46 2539-52). However, far away from the Wannier ridge the triple differential cross sections tend to show qualitative differences from the measurements of Rösel et al.

We have measured the projectile momentum distributions for single ionization in 50 to 150 keV proton-helium collisions as a function of both the longitudinal and the transverse momentum component. Furthermore, we calculated these distributions using the classical trajectory Monte Carlo (CTMC) method, and found good agreement with the experimental data. Our results are consistent with experimental data for the momentum distribution of the ionized electron.

We report on the study of heavy-ion-induced Si KL x-ray satellite spectra of several 3d, 4d and 5d silicides, with the aim of elucidating the trends in bonding properties of transition metal (TM) silicides. Measurements of the 43 MeV Ne³⁺-beam-induced Si KL spectra of elemental silicon and compound targets were performed at the Paul Scherrer Institute, Villigen, Switzerland, with a high-resolution von Hamos crystal spectrometer. The interpretation of the data was based on the importance of the effect of the solid and chemical environment on the KL x-ray satellite relative intensities due to the L-vacancy refilling by intra- and inter-atomic electron transitions. Within the framework of a statistical procedure for L-shell vacancy refilling the average L-shell rearrangement probability for each silicide was extracted so that calculated x-ray intensity distributions matched the experimental ones. The results are discussed in terms of the interplay of Si-Si, Si-TM and TM-TM bonding interactions in the studied TM silicides.

In this paper, we extend the Coulomb-Glauber approximation to charged particles scattering processes in the presence of a laser field. With this method, laser-assisted electron scattering by hydrogen-like ions is studied. Numerical results of the elastic and 1s→2s, 1s→2p excitation scattering differential cross sections are obtained. Based on these results we discuss and compare with the Born approximation, the selection rules, the dressing effects and the use of the closure approximation in the calculation of the S-matrix. The cross section of the elastic scattering as a function of the charge number Z has also been shown.

Li(n = 2) to H*(n = 2,3) and Li*(2p, n = 3) charge transfer and excitation processes are studied in the medium-energy range 0.1-10 keV. The static model includes a basis of 26 molecular states. The lithium atom is described using a model potential and the semiclassical impact parameter method with straight trajectories is used for the dynamical simulation. A common translation factor is included to account correctly for the electron translation movement.

For this collision system, we calculate the total cross sections and probabilities for the different capture and excitation processes. The calculation reflects a Lyα radiation enhancement for the capture to H*(2p) from Li*(2p). A strong alignment effect in the capture processes to H*(2) is also shown and evaluated through the alignment parameter A(n = 2,3). Finally, we provide a mechanism through the non-adiabatic radial and rotational couplings, showing the importance of the H*(3) and Li*(3) states in this collision.

In this paper we present a non-destructive Bell discrimination scheme on photons maximally entangled in polarization with the help of a nonlinear device, a Fock filter. Moreover, the scheme is generalized to 2n-partite Greenberger-Horne-Zeilinger bases.

Absolute partial cross sections for electron-impact ionization of CO are reported for electron energies from threshold to 1000 eV. The product ions are mass analysed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output unequivocally demonstrates that the various product ions are collected with equal efficiency irrespective of their initial kinetic energies. Data are presented for the production of CO⁺, C⁺, O⁺ and CO²⁺, and for the total cross section which is obtained as the sum of these partial cross sections. The overall uncertainty in the absolute cross section values is ±5% for singly charged parent ions and ±6% for fragment ions. Comparison is made with prior experiments and calculations.

Photoionization of the ground state 1s²2s²2p⁶3s^2 1S of atomic magnesium is studied theoretically in the energy region between the Mg⁺(3s) and Mg⁺(4p) thresholds using the non-iterative variational R-matrix method combined with multichannel quantum defect theory at the R-matrix surface. The initial and final states are represented by configuration-interaction wavefunctions. We calculated total and partial photoionization cross sections in both length and velocity formulations. Photoionization cross sections are dominated by Rydberg series of autoionization resonances converging to Mg⁺(3p, 4s, 3d and 4p) thresholds. We also calculated the photoelectron angular distribution asymmetry parameter β for the process leaving the ion in the Mg⁺(3p) state. The present photoionization cross sections are in good agreement with available experiments and previous calculations. There is excellent agreement between length and velocity gauges in our calculation.

A strong orientation effect is discovered theoretically upon the change of the scheme of a suggested experiment on the anomalous elastic x-ray scattering of linearly polarized x-ray radiation by linear diatomics near the ionization thresholds of inner molecular orbitals. The effects on the shape of theoretical scattering spectra of vibronic phenomena (for the HF and HCl molecules) are studied, as well as the additional excitation/ionization of outer-shell electrons (the CO molecule). Within the one-centre approximation for the molecular orbital wavefunctions the analytical structure of the linear molecule's form factor is obtained. The calculation of one-electron wavefunctions is carried out with the inclusion of the effect of core relaxation in the field of the inner-shell vacancy within the one-centre method in the Hartree-Fock approximation. The dipole transition matrix elements and photoabsorption cross sections are calculated by the methods of the theory of non-orthogonal orbitals.

We set out aspects of a numerical algorithm used in solving the full-dimensionality time-dependent Schrödinger equation describing the electronic motion of the hydrogen molecular ion driven by an intense, linearly polarized laser pulse aligned along the molecular axis. This algorithm has been implemented within the fixed inter-nuclear separation approximation in a parallel computer code, a brief summary of which is given. Ionization rates are calculated and compared with results from other methods, notably the time-independent Floquet method. Our results compare very favourably with the precise predictions of the Floquet method, although there is some disagreement with other wavepacket calculations. Visualizations of the electron dynamics are also presented in which electron rescattering is observed.

Astrophysically relevant transitions in the Mg-like ions Si III, S V and Fe XV have been studied. A J-dependent CIV3 calculation, more extensive than other, earlier ones, has led to improved accuracy in the calculated energy levels and oscillator strengths. Independent relativistic quantum defect orbital and multiconfigurational Dirac-Fock calculations have also been performed. An analysis of the results is given.

In this paper, we report a joint theoretical-experimental study on electron-OCS collisions in the low- and intermediate-energy ranges. More specifically, elastic differential and integral cross sections, as well as grand total (elastic + inelastic) cross sections in the 0.4-600 eV energy range, are reported. A complex optical potential consisting of static, exchange, correlation-polarization plus absorption contributions, derived from a fully molecular wavefunction, is used for the electron-molecule interaction. The Schwinger variational iterative method, combined with the distorted-wave approximation, is applied to calculate the scattering amplitudes. Additionally, we also report measured elastic differential and integral cross sections in the 100-600 eV energy range determined using the relative-flow technique. Comparison between calculated results and present and existing experimental data, as well as with other theoretical results, is encouraging.

Theoretical calculations of the electron capture cross sections in collisions of beryllium ions with helium atoms are performed in a close-coupling method based on molecular-state expansion for the Be^{q +} (q = 2,4) ion impacts below 35 keV amu^-1, and in the continuum distorted-wave method for the Be^{q +} (q = 1-4) ion impacts above 100 keV amu^-1. The present results are discussed in comparison with other available theoretical calculations.

Charge exchange and crossings of corresponding energy levels that enhance charge exchange are strongly connected with problems of energy loss and diagnostics in high-temperature plasmas. Charge exchange has also been proposed as one of the most effective mechanisms for population inversion in the soft x-ray and VUV ranges. One area of the most fundamental theoretical importance in the study of charge exchange is the problem of electron terms in the field of two stationary Coulomb centres (TCC) of charges Z and Z' separated by a distance R. This involves fascinating atomic physics: the terms can have crossings and quasicrossings. These rich features of the TCC problem are also manifest in other areas of physics such as plasma spectroscopy: a quasicrossing of the TCC terms, by enhancing charge exchange, can result in an unusual structure (a dip) in the spectral line profile emitted by a Z-ion from a plasma consisting of both Z- and Z'-ions, as has been shown theoretically and experimentally. The paradigm is that these sophisticated features of the TCC problem and their flourishing applications are inherently quantum phenomena. In this paper we disprove this paradigm. We present a purely classical description of both the crossings of energy levels in the TCC problem and the dips in the corresponding spectral line profiles caused by the crossing (via enhanced charge exchange). Our classical description is based on first principles and does not use any model assumptions.

The dynamics of singly ionizing proton-helium collisions have been studied experimentally for several energies of the projectile (0.2, 0.5, 1.0 and 1.3 MeV) with the technique of cold target recoil-ion momentum spectroscopy (COLTRIMS). The complete final-state distribution in momentum space of all three particles was determined by measuring the three momentum components of the emitted electron and the coincident recoiling target ion. The momentum transfer and energy loss of the outgoing projectile was determined by momentum and energy conservation laws. Doubly differential cross sections of the kinematically complete experimental investigation are presented. The present data are compared with results from fast highly charged heavy-ion impact experiments.

We have investigated multiple ionization of noble gases by proton and hydrogen impact at low and intermediate projectile energies; 50, 120 and 210 keV. The recoil-ion-charge-state-selected yields were measured in coincidence with electrons emitted at 90° with respect to the beam direction. Differential yields were obtained for recoil-ion charges up to five. For single ionization a comparison with previous data gives good agreement for the electron energy range of interest here. Significant differences were observed for the case of charged and neutral projectiles. Comparisons between multiple and single ionization were made. For the targets investigated, Kr, Ne, Ar, we observed a characteristic structure, which depends on projectile charge and velocity. For proton impact, the ratio of multiply to singly charged recoils presents a distinctive enhancement at an electron velocity close to that of the projectile. This structure disappears for H impact, probably due to the dominance of simultaneous ionization of target and projectile electrons.

The complex eigenenergy surfaces of quasistationary states for the hydrogenic Stark problem as a function of complex field strength have been calculated. It is shown that the location of `hidden crossings' of energy surfaces connected with different states can be obtained from a semiclassical quantization of `top of the barrier' motion. A numerical calculation of the excitation of hydrogenic states by an electric field pulse shows an unexpected threshold-like behaviour in the adiabatic regime. In addition, the lower the peak-field strength, the higher is the state populated maximally. It is found that these effects are correlated to the position of the `top of the barrier' hidden crossings as a function of field strength.

The feasibility of using periodic amplitude-modulated radio-frequency (RF) irradiation under `magic-angle spinning' for heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance is addressed. The RF waveforms used are tailored to satisfy the theoretical criterion for decoupling which calls for an irradiated spin propagator which is cyclic in the sense that it equals the identity matrix irrespective of the strength and orientation of the chemical shift and dipolar coupling tensors. This requirement is met by using the Floquet formalism to provide insight into the influence of an arbitrary waveform on the dynamics of the irradiated spin-½ nuclei and invoking perturbation methods to design particular modulation functions which impose the required cyclicity on the propagator. Simple RF modulations which are synchronized with the sample spinning are thus derived analytically. Finally, the validity of the scheme is explored in simple test experiments and the decoupling performance is compared with the traditional `continuous-wave' method and the recently developed technique of `two-pulse phase modulation'.

We present a model for laser cooling of a single ion in the Penning trap. The model solves the equations of motion in the presence of the damping caused by the interaction with the laser beam, and predicts for the first time the dependence of the cooling rates for the two radial degrees of freedom in the trap as a function of the laser detuning, laser beam offset and saturation parameter. The conditions derived for both radial degrees of freedom to be cooled simultaneously agree with those found in earlier studies. The results indicate that under conditions where both motions are cooled simultaneously, the cooling rates are both significantly smaller than the maximum rate for either motion considered in isolation. Furthermore, the magnetron cooling rate is typically much lower than the cyclotron cooling rate, as has been found in experiments. The model indicates how the cooling rate for a single ion may be optimized.

Photon-induced K x-ray spectra of Ca, Ti, Fe, Zn and Ge have been investigated. The measurements have been made using a crystal spectrometer combined with a thin scintillation detector. Excited by the collimated photon beam from an Rh-anode x-ray tube, the spectra of all these elements reveal the existence of radiative Auger emission (RAE) structure and the satellite and hypersatellite lines along with the diagram lines. The energies and intensities of the Kα₂, Kα₁, Kβ_1,3 and Kβ₅ diagram lines and the Kα satellites and hypersatellite transitions are presented. The intensity of the RAE structure corresponding to the Kβ_1,3 x-ray transition and the energy of the RAE edge for each element is also reported. The measured results have been compared with the values from other sources such as electron/heavy-ion excitation and theoretical values. From the intensities of the satellite lines of these elements, the average L-vacancy fraction P_L has been deduced in each case.

Electronic and structural properties of energetically low-lying isomers of isolated Ti_xO_y (x = 1-6, y = 1-12) molecular systems have been investigated by density functional theoretical methods. A variety of stationary points are thoroughly characterized. We report total cluster energies, equilibrium geometries and harmonic vibrational wavenumbers.

The projectile-electron excitation and loss processes are considered in collisions with heavy atomic targets in the intermediate-to-high-velocity regime. In order to describe these processes we use the sudden and Glauber approximations. We show that these approximations lead to the same results for electronic cross sections. We estimated cross sections for the total loss from H and He⁺ projectiles. Our results favour the interpretation of the total electron loss as a process where the projectile-electron interaction with the screened heavy target nucleus dominates over the antiscreening mode.

We employ an efficient procedure based on second-order perturbation theory to obtain the asymmetry parameters of isolated resonances without computing the spectra. We show that this method gives values of the asymmetry parameters of the predissociating resonances corresponding to the vibrational levels of the Cs₂ molecule that are in good agreement with those obtained by fitting the spectra. We also show the onset of overlapping between the resonances by computing the asymmetry parameters at several increasing values of the spin-orbit coupling between the discrete and continuum states.

The Dirac R-matrix theory is used to calculate the collision strengths for electron impact excitation of He-like ions (S¹⁴⁺, Ca¹⁸⁺ and Fe²⁴⁺) using the Dirac atomic R-matrix code, which has been developed mainly by Norrington and Grant (Norrington P H and Grant I P 1987 J. Phys. B: At. Mol. Phys.20 4869). The lowest 31 target levels are included in the calculation. Rate coefficients are obtained for transitions from the ground 1s^2 1S₀ state to the fine-structure levels of all excited states of 1s2ℓ (ℓ = 0,1) and 1s3ℓ (ℓ = 0,1,2) configurations. We compare our results for rate coefficients with the results of previous calculations, and indicate where significant differences arise because of our more accurate representation of resonance and relativistic effects.

We developed a theoretical model to investigate the compressibility of atoms. Atoms are confined inside a spherical cavity, simulated numerically by a finite repulsive potential barrier. The energy levels and wavefunctions of confined atoms are determined by solving, for different cavity radii, the relativistic Dirac-Fock equations, including formally the repulsive barrier. The changes in the atomic size and in the ground-state energy level allow one to define a positive isotropic pressure exerted on the confined atom. The model is applied to atomic caesium and it is demonstrated quantitatively that the remarkable compressibility of caesium originates from a purely atomic mechanism, namely the pressure-induced collapse of the 5d orbital. We propose that this mechanism can also drive, at an atomic level, a reversible insertion of atoms into solids. Applications to lithium-ion batteries are briefly discussed at the end of this paper.

We have derived non-Coulomb phase shifts from measured differential cross sections for electron scattering by the ions Na⁺, Cs⁺, N³⁺, Ar⁸⁺ and Xe⁶⁺ at energies below the inelastic threshold. Values of the scaled squared deviation between the observed and fitted differential cross sections, χ², for the best-fit phase shifts were typically in the range 3-6 per degree of freedom. Generally good agreement with experiment is obtained, except for wide-angle scattering by Ar⁸⁺ and Xe⁶⁺. Current measurements do not define phase shifts to better than ≈ 0.1 rad even in the most favourable circumstances and uncertainties can be much larger.