Table of contents

Evidence for the presence of the ⁴Σ_u⁻ resonance in dissociative electron attachment to O₂ is obtained for the first time from the angular distribution measurement of O⁻ ions in the entire 2π angles using a novel experimental technique employing velocity map imaging. This observation, while settling the question of the presence of this state observed in inelastic vibrational excitation of O₂, calls for fresh calculations on the lifetime of the resonance. It may also impact the interpretations of the negative ion formation from O₂ in clusters and in a condensed state.

Fully differential cross sections for single ionization of helium induced by impact of 100 MeV/u C⁶⁺ ions are examined within a quantum-mechanical distorted wave model. The experimental uncertainties are included in the theoretical calculations, leading to a quantitative description of the experimental cross sections. In particular, the ionization cross section for the electron emitted in the plane perpendicular to the scattering plane is reproduced for the first time by a full quantum-mechanical model. The role of the internuclear interaction in this perturbative-regime collision is discussed and found to be unimportant for the present kinematical conditions.

It is shown that the line strength for the transition n' ↔ n of a hydrogenic atom with nuclear charge Z is (in atomic units) where the 0s in the matrix elements are the values of the quantum number l. This agrees with the expression for the hydrogen intensities originally given by McLean (1932 Nature129 25). Expressions for the general l-diagonal matrix elements of r and r² are given in terms of hypergeometric functions, and their asymptotic approximations for large n and relatively small c = n' − n are shown to agree with the asymptotic formula for the oscillator strength.

At the new free-electron laser (FEL) for vacuum ultraviolet (VUV) and soft x-ray radiation FLASH of the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, multi-photon double ionization of molecular nitrogen has been observed and studied by ion time-of-flight spectroscopy. The experiments have been performed at the microfocus beamline BL2 with photon pulses of 25 fs duration and irradiance levels up to 2 × 10¹³ W cm⁻² at a photon energy of 38 eV, i.e. well above the first ionization/dissociation thresholds of the target. A new and important aspect of our experiments has been the reliable determination of absolute photon numbers per pulse with the help of a calibrated setup allowing the investigation of nonlinear effects by quantitative measurements. Results obtained are discussed in terms of a sequential two-photon ionization scheme.

The theoretical description and the experimental methods and results for above-threshold ionization (ATI) by few-cycle pulses are reviewed. A pulse is referred to as a few-cycle pulse if its detailed shape, parametrized by its carrier-envelope phase, affects its interaction with matter. Angular-resolved ATI spectra are analysed with the customary strong-field approximation (SFA) as well as the numerical solution of the time-dependent Schrödinger equation (TDSE). After a general discussion of the characteristics and the description of few-cycle pulses, the behaviour of the ATI spectrum under spatial inversion is related to the shape of the laser field. The ATI spectrum both for the direct and for the rescattered electrons in the context of the SFA is evaluated by numerical integration and by the method of steepest descent (saddle-point integration), and the results are compared. The saddle-point method is modified to avoid the singularity of the dipole transition matrix element at the steepest-descent times. With the help of the saddle-point method and its classical limit, namely the simple-man model, the various features of the ATI spectrum, their behaviour under inversion, the cut-offs and the presence or absence of ATI peaks are analysed as a function of the carrier-envelope phase of the few-cycle laser field. All features observed in the spectra can be explained in terms of a few quantum orbits and their superposition. The validity of the SFA and the concept of quantum orbits are established by comparing the ATI spectra with those obtained numerically from the ab initio solution of the TDSE.

Energy levels, radiative transition probabilities and autoionization rates for 1s²2s²2p⁶3l'nl (n = 3–12, l ⩽ n − 1) and 1s²2s²2p⁶4l'nl (n = 4–7, l ⩽ n − 1) states in Mg-like iron (Fe¹⁴⁺) are calculated by the Hartree–Fock-relativistic method (Cowan code) and the relativistic many-body perturbation theory method (RMBPT code). Autoionizing levels above three thresholds 1s²2s²2p⁶3s, 1s²2s²2p⁶3p and 1s²2s²2p⁶3d are considered. It is found that configuration mixings [3sns + 3pnp + 3dnd] and [3snp + 3pns + 3pnd + 3dnp] play an important role for all atomic characteristics. Branching ratios relative to the first threshold and intensity factors are calculated for satellite lines, and dielectronic recombination (DR) rate coefficients are determined for the excited 444 odd-parity and 419 even-parity states. It is shown that the contribution of the highly-excited states is very important for calculation of total DR rates. Contributions from the excited 1s²2s²2p⁶3l'nl states with n ⩾ 12 and 1s²2s²2p⁶4l'nl states with n ⩾ 7 to DR rate coefficients are estimated by extrapolation of all atomic parameters. The total DR rate coefficient is derived as a function of electron temperature.

Absolute differential elastic and vibrational excitation cross sections have been measured for formic acid at 135° from threshold to 5 eV. Most vibrationally inelastic cross sections have a narrow peak at threshold, followed by a broadband with a boomerang structure due to the known π* shape resonance. The cross section for the excitation of the O–H stretch vibration behaves differently, it also peaks at threshold, but then drops only slowly, with narrow cusp structures, and only a very weak influence of the π* shape resonance. The cusp structures are even more pronounced in the cross section for the excitation of the O–H stretch overtone. The elastic cross section rises steeply at low energies. The π* shape resonance decays also by the ejection of very slow electrons, exciting a vibrational quasicontinuum at large energy losses.

A power source is presented capable of providing ac currents with amplitudes of up to 50 A and frequencies near 10 kHz. The current amplification is actively stabilized and exhibits fractional current noise better than one part in 10⁵. The source was developed for use with an atom interferometer based on magnetically trapped condensate atoms. The relationship between current noise and interferometer noise is developed, and it is shown that the demonstrated precision should permit interferometry with coherence times of 1 s or more.

The probe absorption–dispersion spectra of a driven three-level atom in a double-band photonic crystal have been investigated. We use the model which assumes the upper levels of the atomic transitions coupled via a classical driving field. One of the transitions interacts with the free vacuum modes, and the other transition couples to the modes of the modified reservoir (photonic band gap). The effect of the classical driving field on the absorption–dispersion spectra of an atom is investigated in detail. Most interestingly, it is shown that the atom becomes transparent to a probe laser field coupled to the free space transition, and slow group velocities are obtained near the transparency window.

The three series of doubly excited ¹P^o states at energies below the N = 2 threshold of He⁺ were determined, employing the Harris–Nesbet variational method. We have located altogether 18 of these resonances below this threshold as well as determined the widths for all of them, including the widths of the high-lying ones. Some of these resonances were located by us for the first time, and had not yet been available in the literature. Our results are compared to those obtained by various other research groups using different numerical methods of approach.

It is shown that variational solutions corresponding to the bound states of the Dirac–Coulomb (DC) eigenvalue problem may be obtained using a complex-coordinate rotation method. The discrete eigenvalues of the DC Hamiltonian are treated as resonances coupled to the Brown–Ravenhall continuum (the superposition of the negative- and positive-energy one-electron continua). The method leads to a clean separation of the bound-state energies from the continuum. It is shown that the effects related to the resonance character of the bound states being an artefact of the DC approximation, in particular the instability of the ground state, are proportional to the third power of the fine structure constant α. Thus, the results correctly describe the physical reality up to the terms proportional to α². The approach has been implemented for the case of the ground state of two-electron atoms described by the Dirac–Coulomb Hamiltonian in a space of the Hylleraas-type trial functions. The variational energies, calculated for Z = 2, 80, 90, correspond to the best available in the literature.

We propose a novel method to describe realistically ionization processes with absorbing boundary conditions in basis expansion within the formalism of so-called non-adiabatic quantum molecular dynamics. This theory couples self-consistently a classical description of the nuclei with a quantum-mechanical treatment of the electrons in atomic many-body systems. In this paper, we extend the formalism by introducing absorbing boundary conditions via an imaginary potential. It is shown how this potential can be constructed in time-dependent density functional theory in basis expansion. The approach is first tested on the hydrogen atom and the pre-aligned hydrogen molecular ion H₂⁺ in intense laser fields where reference calculations are available. It is then applied to study the ionization of non-aligned H₂⁺ and H₂. Striking differences in the orientation dependence between both molecules are found. Surprisingly, enhanced ionization is predicted for perpendicularly aligned molecules.

Momentum-space coupled-channels-optical (CCO) method has been used to study the excitation process of the 2p^{4 3}P → 2p³3s ³S^o transition of oxygen by electron impact at energies of 15, 17.5, 20, 22.5, 25, 27.5, 30, 50 and 100 eV. Important continuum states of a target are included via a complex equivalent-local optical potential. Direct ionization cross sections have been calculated to check the validity of the method. Differential cross section and integral cross section have been calculated and compared with experimental measurements and other theoretical results.

Explicit analytic expressions, involving elementary functions, are given for occupation probabilities of atomic states after one or a few cycles of a strong short laser pulse. These formulae are strictly valid in the asymptotic limit of large momentum transfer from the field to the atom during a half cycle of the pulse. The cases of a hydrogenic atom, of a negative ion and of multielectron atoms in the independent-electron approximation are considered. The asymptotic expressions, derived in the limit of infinite momentum transfer from the field, have also the correct limit for vanishingly small momentum transfers, so that fortuitously they are valid for arbitrary momentum transfer.

With one weak probe field and two strong pumping fields, superluminal optical solitons are formed in a lifetime-broadened four-level tripod atomic medium. With proper parameters, both dark and bright solitons can occur in the highly resonant medium. The corresponding group velocity of the solitons can be superluminal. Meanwhile, the conditions for superluminal solitons occurrence are given.

The Stark widths (W) of 12 neutral and 16 singly ionized tin (Sn I and Sn II, respectively) spectral lines have been measured in a laboratory helium plasma at 13 000 K electron temperature and 5.1 × 10²² m⁻³ electron density. Many of them are the first data in the literature. At mentioned plasma conditions, the Stark broadening has been found to be the dominant mechanism in line shape formation. The modified version of the linear, low-pressure, pulsed arc was used as a plasma source operated in helium with tin atoms, as impurities, evaporated from tin cylindrical plates located in the homogeneous part of the discharge, providing conditions free of self-absorption. Our Sn II W values are compared to the recent theoretical data calculated on the basis of the modified semi-empirical approach and, also, to the existing experimental W values. Our normalized Stark widths are much smaller (up to a factor of 4, on average) than those measured in a laser-produced plasma and in a plasma created by the shock-wave tubes. An agreement, within the accuracy of the experiment and uncertainties of the used theoretical approach, with the recent calculated W data was found. Our normalized Sn I Stark widths are much smaller than those observed in the mentioned plasma sources.

We have measured the vibrationally resolved partial cross sections and asymmetry parameters for C K-shell photoionization of the CO₂ molecule in the Σ_u shape resonance region above the C K-shell ionization threshold. The positions of both the maxima of and the minima of move towards the C K-shell threshold with increasing symmetric stretching vibrational excitation v'₁ in the C 1s single-hole state. Calculations employing the relaxed-core Hartree–Fock approach reproduce the observed vibrational effects.

The eigenvalue problem for an electron that is moving in a superposition of the attractive Coulomb potential and the Yukawa potential is solved by using the shifted 1/N method. The calculations of the energy levels have been carried out for both cases of three and the two dimensions. The energy levels for 1s, 2p⁻, 3s⁻, 3p⁻, 3d⁻ and 4f⁻ for the two dimensions case are calculated as a function of the potential strengths A and B and the screening parameter λ. It is shown that for a given principal quantum number n, the energy eigenvalues increase (decrease) with increasing ℓ for the 3D case and with increasing |m| for the 2D and that for 2s and 2p⁻ levels (Lamb shift) is also discussed.

Experimental measurements on angular distribution of ions and ion-energy spectra from planar slab targets of Al, Ti and binary targets of Al, Ti with different stoichiometries are reported. The plasma was produced by a 125 mJ, 5 ns, 1.06 µm Nd:YAG laser, incident on a planar target at a fixed angle of −45°. The laser was focused on an ablation area of about 0.3 mm² at a laser intensity of about 10¹⁰ W cm⁻². The characteristics of the ions were obtained using a retarding-potential analyser and a quartz crystal. Results on time-of-flight spectra, ion fractions, average ionization, angular distributions of particles and their kinetic energy are presented. Full width at half maximum of the angular distributions was obtained and the results are discussed. Measurements on total integrated energy per particle are also reported. They show evidence of energy transfer from light to heavy particles in agreement with the theoretical simulation results of earlier workers. Ion acceleration due to an in-built electrostatic potential is discussed in detail and its effect was found to be marginal.

The photoabsorption spectra of La V, La VI and La VII have been recorded in the 85–96 eV region using the dual laser plasma technique. 4d → 5p transitions from their ground configurations were observed and identified with the aid of Hartree–Fock with configuration interaction calculations. The excited states were found to decay by autoionization involving 5s or 5p electrons and rates for the different processes and resulting linewidths were calculated and compared with experiment.

We consider a novel system of two-component atomic Bose–Einstein condensate in a double-well potential. Based on the well-known two-mode approximation, we demonstrate that there are obvious avoided level-crossings when both interspecies and intraspecies interactions of two species are increased. The quantum dynamics of the system exhibits revised oscillating behaviours compared with a single component condensate. We also examine the entanglement of two species. Our numerical calculations show that the onset of entanglement can be signalled by a violation of the Cauchy–Schwarz inequality of a second-order cross-correlation function. Consequently, we use Von Neumann entropy to quantify the degree of entanglement.

We have resolved closely spaced hyperfine levels in the 3P_3/2 state of ²³Na with a technique of coherent-control spectroscopy that uses co-propagating beams. The probe beam is locked to one hyperfine transition, while the control beam is scanned across a neighbouring transition. An acousto-optic modulator placed in the path of the control beam provides calibrated frequency offsets for measuring the hyperfine intervals. We thus obtain precise values for the hyperfine constants in the 3P_3/2 state: A = 18.530(3) MHz and B = 2.721(8) MHz, which improve previous values significantly.

On the basis of a multiconfiguration Dirac–Fock method, a systematic study has been carried out for the decay process of the 1s2s²²S_1/2 state of Li-like ions. It is found that the decay properties of these states are quite different along the isoelectronic sequence: for low-Z ions up to Z = 30, the Auger decay channel is dominant; for medium-Z ions up to Z = 80, a two-electron one-photon radiative decay caused by a strong electron correlation becomes a competitive channel; and for very heavy ions about Z = 90, the magnetic dipole radiative decay becomes important too. In addition, the QED contributions to the excitation energy and the contributions from the Breit interaction to the Auger decay rates are stressed for heavy ions in particular.

We present a thorough analysis of the electronic detection of charged particles, confined in a Penning trap, via image charges induced in the trap electrodes. Trapping of charged particles in an electrode structure leads to frequency shifts, which are due to image charge and space charge effects. These effects are of importance for Penning trap experiments which involve high charge densities or require high precision in the motional frequencies. Our analysis of image charges shows that only (higher order) odd powers of the particle displacement lead to induced charge differences, giving rise to a signal. This implies that, besides the centre-of-mass frequency of a trapped particle cloud, also higher order individual particle frequencies induce a signal, which can be picked up by an electronic detection circuit attached to the trap electrodes. We also derive analytic expressions for the image charge and space charge induced frequency shifts and perform simulations of space charge effects. In relation to this, we discuss the consequences of the shifted particle frequencies for resistive cooling of the particle motion.

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