Table of contents

A non-stationary method of calculation of energy distributions of emitted electrons in near-threshold resonant (sequential) double photoionization of atoms by attosecond pulses is developed. The method is based on the numerical solution of a system of time-dependent equations describing photoionization and the following Auger decay. As an example, a double-differential cross section of the sequential double photoionization of Ne is considered. It is shown that the shape of the electron energy distribution strongly depends on the pulse duration.

The photo-double ionization cross section for the helium atom is calculated in the near-threshold region by direct solution of the time-dependent Schrödinger equation. Full close-coupling results for the 1s²¹S ground state are found to be in excellent agreement with experimental measurements. The calculations confirm the validity of the Wannier power law from 0.1 eV to about 1.7 eV excess energy and find no oscillations in the threshold cross section beyond numerical uncertainty. Further time-dependent calculations are made in a simpler s-wave counterlinear model for both the 1s²¹S ground and 1s2s ¹S excited states. Although numerical uncertainties are significantly reduced in the helium model calculations, again no oscillations in the threshold cross sections are found beyond the remaining numerical uncertainty.

We investigate the impact of an external magnetic field on ultra-long-range and ultracold Rydberg molecules. The Born–Oppenheimer potential surfaces are analysed and discussed for different values of the magnetic field strength. The magnetic field provides an angular confinement turning a rotational degree of freedom into a vibrational one. We explore the vibrational dynamics and observe a pronounced transition in the level spacing from a linear splitting via an irregular regime to a 2D harmonic oscillator-like behaviour. Scaling arguments for the dependence of the potential energy surfaces on the field strengths are provided. The occurrence of a monotonic lowering of the magnitude of the electric dipole moment with increasing magnetic field strength is shown.

Femtosecond laser pulses have proven to provide valuable insight into the dynamics of microscopic systems by using pump–probe techniques. Applied to atomic clusters even a single pulse of varying pulse duration can reveal how and when energy from the laser pulse is transferred effectively to the cluster. We review the main experimental observables for energy transfer to a cluster and the major theoretical approaches which have been devised. Most importantly, we compare the cluster response to standard 780 nm light pulses with the response to 100 nm pulses, already obtained at a VUV free electron laser (FEL) source, and with 3 nm light which will be available from x-ray FEL sources.

A direct entanglement concentration protocol for generalized GHZ states is proposed using entanglement swapping in cavity QED. Via this scheme, multi-atom maximally entangled states can be prepared free from the effects of the cavity decay and the thermal field, due to the fact that the interaction here is a large-detuned one between atoms and the cavity.

Energies of the 4s²np_j (n = 4–8), 4s²ns_1/2 (n = 5–8), 4s²nd_j (n = 4–7), 4s²nf_j (n = 4–5) and 4s²5g_j states in neutral gallium are obtained using a relativistic many-body perturbation theory (RMBPT) method. First-, second-, third- and all-order Coulomb energies and first-order Breit and second-order Coulomb–Breit corrections are calculated. Reduced matrix elements, oscillator strengths, transition rates and lifetimes are determined for the 130 possible electric-dipole transitions. Hyperfine constants A are determined for 28 4s²nl_j states in ⁶⁹Ga I and ⁷¹Ga I isotopes. All above-mentioned properties are obtained in the relativistic single–double (SD) approximation, where single and double excitations of Dirac–Fock wavefunctions are included to all orders of perturbation theory. Using SD wavefunctions, accurate values are obtained for energies of the lowest states and for the possible electric-dipole matrix elements between these states. With the aid of the SD wavefunctions, we also determine transition rates, oscillator strengths and lifetimes. These calculations provide a theoretical benchmark for comparison with experiment and theory.

The ionization cross-section of the 3d spin–orbit components of the Cs atom has been measured from about 12 to 70 eV above their respective thresholds. The measured relative ionization cross-section of the 3d_5/2 channel exhibits a pronounced minimum above threshold followed by a second maximum near the 3d_3/2 ionization onset and thus qualitatively confirms the theoretical predictions of a spin–orbit activated interchannel coupling (Amusia et al 2002 Phys. Rev. Lett88 093002).

The 4d photoabsorption spectra of singly to triply ionized indium have been recorded using the dual laser plasma technique in the 24.5–55 eV photon energy range. The data substantiate and add to previously classified lines in In II and In IV while many new lines are tentatively identified in In III. Transitions were assigned with the aid of Hartree–Fock calculations.

Various random point processes occurring in the detection of photons are considered. Statistics of the number of points registered within a finite time interval and of the times between these points are analysed. Based on the second order moments of these statistics, new criteria for characterizing classical and nonclassical properties of photon statistics are proposed.

Cross sections and branching ratios are presented for the dissociative recombination of the CF⁺₂ and CF⁺₃ ions with electrons. It is found that the channel producing CF + F is dominant for the reaction with CF⁺₂ and the production of CF₂ + F is dominant for the reaction with CF⁺₃. The cross sections for these two ions are very similar.

The dissociative photoionization processes of H₂ and D₂ in an intense, short-pulsed soft x-ray field (λ = 29.6 nm, 3 × 10¹² W cm⁻², 15 fs) generated as the 27th high-order harmonic of a Ti:sapphire laser is investigated for the first time by time-of-flight mass spectrometry. The ejected H⁺ fragment ions from H₂ are distributed in the kinetic energy range of 3–20 eV peaked at 10.0 eV, and the yield exhibits a nonlinear dependence of the order of 1.7(1) on the light field intensity of the 27th harmonic. Similarly, the D⁺ fragment ions from D₂ are distributed in the 3–20 eV range peaked at 10.3 eV, and the yield exhibits a nonlinear dependence of the order of 1.6 ± 0.2. The kinetic energy distributions of H⁺ and D⁺ and the nonlinearity in their yields are interpreted well by the two coexisting ionization processes: (i) one-photon dissociative ionization and (ii) dissociative two-photon above-threshold-ionization (ATI) mainly into the repulsive 2pπ_u state.

A millimetre wave source operating in the 240–380 GHz frequency range and its application to the study of high n Rydberg states are described. The level intervals between fine structure components of ns, np and nd (n = 60–80) Rydberg states of krypton have been determined at sub-MHz resolution and accurate effective quantum defects and binding energies have been derived.

We propose the use of periodic arrays of permanent magnetic films for producing magnetic lattices of microtraps for confining, manipulating and controlling small clouds of ultracold atoms and quantum degenerate gases. Using analytical expressions and numerical calculations we show that periodic arrays of magnetic films can produce one-dimensional (1D) and two-dimensional (2D) magnetic lattices with non-zero potential minima, allowing ultracold atoms to be trapped without losses due to spin flips. In particular, we show that two crossed layers of periodic arrays of parallel rectangular magnets plus bias fields, or a single layer of periodic arrays of square-shaped magnets with three different thicknesses plus bias fields, can produce 2D magnetic lattices of microtraps having non-zero potential minima and controllable trap depth. For arrays with micron-scale periodicity, the magnetic microtraps can have very large trap depths (∼0.5 mK for the realistic parameters chosen for the 2D lattice) and very tight confinement.

We used the momentum-space coupled-channels optical (CCO) method to investigate the open-shell carbon atom. The ionization of 2p outer shell, 2p3ℓ excitation and total cross sections have been presented at the energies below 200 eV. The resonance structures in the low energy in the 2p3s and 2p3p excitation cross sections have been reported in this paper. The present CCO calculations have been compared with other theoretical and experimental measurements.

We present new experimental data on the term energies and quantum defects of the even-parity triplet states of zinc using a two-step laser excitation scheme in conjunction with a thermionic diode ion detector. The first laser was fixed at 32 501.399 cm⁻¹ to populate the 4s4p ³P₁ intermediate level. The 4s4p ³P_0,2 fine structure components get populated through collisional energy transfer. The second laser was scanned covering the energy region 43 200 to 42 000 cm⁻¹ revealing the highly excited states up to the first ionization limit. Our observations include 4snd ³D₂ (14 ⩽ n ⩽ 55) and 4sns ³S₁ (15 ⩽ n ⩽ 35) Rydberg series excited from the 4s4p ³P₁ level. In addition, 4snd ³D₃ (13 ⩽ n ⩽ 49) and 4snd ³D₁ (10 ⩽ n ⩽ 20) series including few members of the 4sns ³S₁ series have also been observed from the 4s4p ³P₂ and 4s4p ³P₀ levels, respectively. The binding energy of 4s4p ³P₁ has been determined as 43 267.93 ± 0.02 cm⁻¹, which consequently yields the first ionization potential of zinc as 75 769.35 ± 0.05 cm⁻¹, which is in good agreement with that reported previously.

In this work we present theoretical ionization yields of helium for photon energies where direct two-photon double electron ejection is allowed. We explore the photon energies and the laser peak intensities of the xuv pulse where the direct double ionization of helium is expected to dominate over the sequential channel. This is achieved with the separate calculation of the partial double ionization yield (sequential and direct). We generally conclude that a careful choice of the photon energy in combination with the peak xuv intensity can result in the clear dominance of one signal (direct) over the other (sequential). Avoiding photons where resonant He⁺ enhancement may appear, relative double ionization signals are less sensitive to pulse duration and the ideal candidate for the observation of the direct double ionization signal appears to be photons around 45 eV (29th harmonic of a Ti:Sapphire 800 nm laser), assuming the radiation is restricted to high order harmonic generation sources. For continuously tunable sources, such as the free electron lasers, any frequency around the minima between intermediate resonances will be as convenient. In addition, we have demonstrate that beyond a certain intensity, depending on wavelength and pulse duration, the signal from the direct will be overwhelmed by the sequential.

The ionization of atoms and negative ions by an electric pulse of Gaussian time shape is studied. At 'small' (in the sense of application of perturbation theory) amplitude of the Gaussian pulse, a zigzag behaviour of the ionization probability as a function of the pulse duration is revealed. The zigzag behaviour is due to the competition between perturbation and tunnelling mechanism. Such behaviour is absent for the excitation probability. The maximum at short pulse duration is reproduced by the first-order perturbation theory and the position of the maximum is proportional to the inverse of the transition frequency.

Nonclassicality conditions for an oscillator-like system interacting with a hot thermal bath are considered. Nonclassical properties of quantum states can be conserved up to a certain temperature threshold only. In this case, affection of the thermal noise can be compensated via transformation of an observable, which tests the nonclassicality (witness function). Possibilities for experimental implementations based on unbalanced homodyning are discussed. At the same time, we demonstrate that the scheme based on balanced homodyning cannot be improved for noisy states with the proposed technique and should be applied directly.

This paper provides a theoretical complement to the experimental measurement of the population of excited dμ(2s) and dμ(1s) atoms in a deuterium. The population of these atoms plays an important role in a muon catalyzed fusion cycle. Symmetric and non-symmetric muonic molecular ions have been predicted to form in excited states in collisions between excited muonic atoms and hydrogen molecules. One example is the ddμ*, which is a muonic deuterium–deuterium symmetric ion in excited state and is initially produced in the interaction of dμ(2s) atoms with deuterium nuclei. Our calculations interpret the experimental findings in terms of the so-called side-path model. This model essentially deals with the interaction mentioned above in which the ddμ* ion undergoes Coulomb de-excitation where the excitation energy is shared between a dμ(1s) atom and one deuterium. The structure of ddμ* is studied here using the numerical, variational method and the given wavefunctions. Few resonance energies for ddμ* molecular states are calculated below the 2s threshold. For more precise assessment of the reliability of the given wavefunctions, the nucleus sizes and Coulomb decay rates for the zeroth, first and second vibrational meta-stable states of the mentioned ion are also calculated. The obtained results are close to those previously reported. The advantage of the given method over previous methods is that the used wavefunction has only two terms, which simplifies the calculations with the same results as those from the complicated coupled rearrangement channel method with a Gaussian basis set. These energies are the base data required for size, formation and decay rate calculations of the ddμ* ion.

Ion-induced fragmentation of H₂O molecules was investigated experimentally by ³He²⁺ impact as a function of the energy in the range from 1 to 5 keV, as well as a function of the charge state of 20 keV ²⁰Ne^q+ projectiles (q = 3 and 7). The fragments were detected in the angular range from 25° to 135° with respect to the incident beam direction. Particular emphasis is given to protons originating from collisions at large impact parameters involving a Coulomb explosion mechanism. Absolute cross sections dσ/dΩ, differential in the emission angle, are found to be anisotropic. Protons are preferentially emitted at angles near 90°, with cross sections being ∼50% larger than those at forward and backward angles. Possible mechanisms causing anisotropic emission of protons from fragmenting H₂O are discussed.

The Harris–Nesbet variational method is used to determine the series of doubly-excited resonances ³P^o of He at energies below the N = 2 threshold of He⁺. We succeeded in locating 16 ³P^o resonances of the three series a, b and c below this threshold with this collision method. We also determined, for the first time, the widths of all of these resonances of the three series, including those of the high-lying ones.

Because of the high cosmic abundance of iron, data on Fe are of particular interest in astrophysics, whereas data on the neighbouring element Co with its lower abundance usually are not. However, we have recorded data on Co XI in order to improve an extrapolation towards the line strength of the prominent 'red iron' corona line of Fe X which has turned out to be difficult to measure in the laboratory directly. In addition, new data on the two longer-lived excited levels in the ground configuration of Co XIII corroborate by isoelectronic comparison earlier data on Fe XII.

We have calculated fivefold differential cross sections (FDCS) for the double ionization of He by electron and positron impact at the incident energy of 601 eV. The calculation is based on the Glauber approximation (GA) method which includes projectile–target correlation in the initial state and has contributions from higher order. A comparison is made of the present FDCS with the corresponding results of other theoretical calculations and experiment. It is found that there is a significant difference between positron- and electron-impact cross sections at this low incident energy. The shape of present FDCS is found to be in qualitative agreement with experiment.

Nonresonant two-photon annihilation of an electron–positron pair in the field of a moderately strong circularly polarized wave is investigated theoretically. The partial cross-section can be summed over all processes of emission and absorption of photons. It is shown that all essentially quantum contributions caused by the Bunkin–Fedorov quantum parameter are compensated for both Feynman diagrams and are not compensated for the quantum process of interference of these diagrams. The external field has an effect on the interference term through the Bunkin–Fedorov quantum parameter. As a result the nonresonant differential cross-section can differ from that in the absence of the external field.

We have performed calculations for electron collisions with tetrahydrofuran (THF) using the UK molecular R-matrix codes. This is the largest molecule ever treated with the R-matrix method, and the only biologically relevant molecule of this size studied theoretically in the inelastic regime. We report ab initio integral cross section for incident energies up to 10 eV. No shape resonances have been found for this system, but a few core-excited resonances are present.

The effects of electron correlation and second-order terms on theoretical total cross sections of transfer ionization in collisions of the helium atom with fast H⁺, He²⁺ and Li³⁺ ions are studied and reported. The total cross sections are calculated using highly correlated wavefunctions with expansion of the transition amplitude in the Born series through the second order. The results of these calculations are in sensible agreement with experimental data.

Recently, Gottesman et al (2001 Phys. Rev. A 64, 012310) showed how to encode a qubit into a continuous variable quantum system. This encoding was realized by using non-normalizable quantum codewords, which therefore can only be approximated in any real physical set-up. Here we show how a neutral atom, falling through an optical cavity and interacting with a single mode of the intracavity electromagnetic field, can be used to safely encode a qubit into its external degrees of freedom. In fact, the localization induced by a homodyne detection of the cavity field is able to project the near-field atomic motional state into an approximate quantum codeword. The performance of this encoding process is then analysed by evaluating the intrinsic errors induced in the recovery process by the approximated form of the generated codeword.