Table of contents

The emission of highly charged ions from Xe clusters exposed to intense extreme ultraviolet laser pulses (λ ∼ 52 nm) from the free electron laser in Japan was investigated using ion momentum spectroscopy. With increasing average cluster size, we observed multiply charged ions Xe^{z +} up to z = 3. From kinetic energy distributions, we found that multiply charged ions were generated near the cluster surface. Our results suggest that charges are inhomogeneously redistributed in the cluster to lower the total energy stored in the clusters.

In this communication, we present an analytical theory of strong-field ionization of molecules, which takes into account the rearrangement of multiple interacting electrons during the ionization process. We show that such rearrangement offers an alternative pathway to the ionization of orbitals more deeply bound than the highest occupied molecular orbital. This pathway is not subject to the full exponential suppression characteristic of direct tunnel ionization from the deeper orbitals. The departing electron produces an 'attosecond correlation pulse' which controls the rearrangement during the tunnelling process. The shape and duration of this pulse are determined by the electronic structure of the relevant states, molecular orientation and laser parameters.

We theoretically investigate the behaviour of optical bistability (OB) and optical multistability (OM) in a generic double two-level atomic system driven by two orthogonally polarized fields (a π-polarized control field and a σ-polarized probe field). It is found that the behaviour of OB can be controlled by adjusting the intensity or the frequency detuning of the control field. Interestingly enough, our numerical results also show that it is easy to realize the transition from OB to OM or vice versa by adjusting the relative phase between the control and probe fields. This investigation can be used for the development of new types of devices for realizing an all-optic switching process.

We study the phenomenon of an electromagnetically induced phase grating in a double-dark state system of ⁸⁷Rb atoms, the two closely placed lower fold levels of which are coupled by a weak microwave field. Owing to the existence of the weak microwave field, the efficiency of the phase grating is strikingly improved, and an efficiency of approximately 33% can be achieved. Under the action of the weak standing wave field, the high efficiency of the phase grating can be maintained by modulating the strength and detuning of the weak microwave field, increasing the strength of the standing wave field.

Over the last few years arrays of evanescently coupled waveguides have been brought into focus as a particular representation of functionalized optical materials, in which the dispersion and diffraction of propagating light can be specifically tuned. Moreover, it turns out that the light evolution in these systems shares fundamental similarities to the quantum evolution of particle wavefunctions, so that waveguide arrays can act as a model system for emulating quantum mechanics. Recently, a novel technique was developed with which waveguides can be directly 'written' into various optical bulk materials using femtosecond laser pulses, which allows for the realization of a variety of innovative concepts which are not feasible using other fabrication methods. The aim of this tutorial is to give an introduction to this topic.

This paper contains the results of theoretical calculations of oscillator strengths and transition probabilities of 171 spectral lines arising from 4d¹⁰5sns (n = 6–9), 4d¹⁰5snp (n = 5, 6), 4d¹⁰5p², 4d¹⁰5snd (n = 5–7), 4d¹⁰5s4f and 4d¹⁰5s5g configurations, and 36 level radiative lifetimes of Sn III have been calculated. These values were obtained in intermediate coupling (IC) and using relativistic Hartree–Fock calculations including core-polarization effects in the potential and the corresponding corrections in the matrix elements. We use for the IC calculations the standard method of least-squares fitting from experimental energy levels by means of the Cowan computer code. The inclusion in these calculations of the 4d⁹5s²5p has allowed us to make a new assignment of the energy levels in Sn III. Transition probabilities, oscillator strengths and radiative lifetimes obtained are in good agreement with the experimental data.

Dipole and quadrupole oscillator strengths have been calculated for the series of transitions nl → n'l' (n ⩽ 5) in a hydrogen atom confined in the centre of a sphere with impenetrable walls. The non-monotonic dependence of the oscillator strengths on the radius of confinement has been discussed through their general forms obtained with a properly chosen radial wavefunction. It was qualitatively ascribed to variation of the radial electron density distributions of the states involved in the transitions. The effect of the confinement on the use of the dipole approximation has been studied, introducing a correction through the octupole term as well as performing the calculation with the full form of the interaction term.

This paper presents calculations of the oscillator strengths and A-values between the 3p⁶ns ²S^e (J = 1/2) (n = 4 to 10) states of Ti IV and the 3p⁶mp ²P^e (J = 1/2 and 3/2) (m = 4 to 10) states. The calculations were carried out using configuration interaction wave functions in which relativistic effects were included by using the Breit–Pauli approximation. To obtain an indication of the accuracy of the oscillator strengths, calculations were carried out using both the length and velocity formulations of the oscillator strengths. These calculations improve on the accuracy of the earlier work on the excitation of the 3p⁶4s ²S excited state of Ti IV to the 3p⁶mp ²P^o excited states (Kingston and Hibbert 2008 J. Phys. B: At. Mol. Opt. Phys.41 155001) by increasing the number of configurations in the calculations. Comparisons of the present calculations are made with other theoretical calculations.

This work reports density functional calculations of geometric, electronic and magnetic properties of freestanding iron–sulfur Fe₂S₂, Fe₃S₄ and Fe₄S₄ clusters which are the ones most frequently contained in proteins. We investigate neutral, anionic and cationic clusters using a method that employs linear combinations of atomic orbitals as basis sets, nonlocal norm-conserving pseudopotentials and a generalized gradient approximation to exchange and correlation. The results are discussed in connection with available experimental data. We mainly show that the ground-state geometries of these free clusters are consistent with their structures in core proteins and they are the same in the neutral, anionic and cationic states, but with small distortions. In all cases, an antiferromagnetic order between Fe atoms is always preferred to ferromagnetic and paramagnetic ones. The geometric distortions induced by magnetism decrease with cluster size and the maximum deviation between Fe–Fe distances is 11% in Fe₂S₂, but only 4% in Fe₃S₄ and 3% in Fe₄S₄ clusters.

In the framework of time-dependent density-functional theory, we study electron emission from Na clusters and the C₂H₄ molecule as induced by irradiation with an intense pulse. The collision of a charged projectile with C₂H₄ is also explored for comparison. We look in particular at the level depletion, i.e. the electron loss in each single-electron level separately. It is found that the distribution of electron loss depends sensitively on the photon frequency. Frequencies close to visible light remove electrons exclusively from the vicinity of the Fermi surface while light in the higher UV range (up to 20 eV for Na clusters and up to 136 eV for C₂H₄) depletes all levels, about equally strong, down to the deepest bound valence state.

Angular distributions of Auger electrons and subsequent fluorescence photons are studied theoretically and experimentally in the vicinity of the core excitations of NO. In the calculations, lifetime vibrational interference and electronic state interference were taken into account ab initio. The interference between excitation and deexcitation amplitudes for transitions via symmetry-different intermediate resonances 1s⁻¹2π²(²Δ, ²Σ^±), which is forbidden in the solid-angle-averaged or magic-angle-recorded decay spectra, plays a crucial role in the formation of the angularly resolved decay spectra. Experimentally, angular distribution parameters for the NO⁺(A¹Π → X¹Σ⁺) fluorescence induced by linearly polarized synchrotron radiation are determined in the vicinity of the N*O resonance in the Raman regime for core excitation. Theoretical results are in good agreement with the present experimental fluorescence spectra and with the available vibrationally and angularly resolved resonant Auger electron spectra.

We present theoretical single to quintuple ionization cross sections for Ne, Ar, Kr and Xe bombarded by H⁺ and He⁺. Post-collisional contributions due to Auger-like processes are taken into account using recent photoionization data. The present continuum distorted wave-eikonal initial state (CDW-EIS) and first Born approximation results are compared with the experimental data available in the energy range of 50–10 000 keV amu⁻¹ for H⁺ on Ne and Ar, and 50–1000 keV amu⁻¹ for the other cases. In general, the combination of the CDW-EIS with the post-collisional branching ratios describes well the multiple ionization data above 300 keV amu⁻¹, showing a clear tendency to coalesce with the first Born approximation at high energies. The surprising result of this work is the good performance of the first Born approximation which describes rather well the experimental data of double and triple ionization, even in the intermediate energy range (50–300 keV amu⁻¹), where direct ionization is the dominant contribution.

The electron capture in C⁶⁺–H collisions taking place in a weakly coupled plasma is studied by the two-centre atomic orbital close-coupling method. The interaction between charged particles is represented by the Debye–Hückel potential, appropriate for weakly coupled (Debye) plasmas. The adopted two-centre expansion basis contains all the states with n ⩽ 8 centred on C⁶⁺ and all the states with n ⩽ 2 centred on H⁺. The coupled channel calculations have been performed in the 0.5–100 keV/u energy range in which the results with the adopted basis are convergent. It is shown that the electron capture dynamics in Debye plasmas is strongly affected by the plasma screening of charged particle interactions mainly due to the reduction of electron binding energy and the number of reaction channels when the plasma screening increases. These effects are illustrated on the state-selective electron capture cross sections for a number of representative values of the interaction screening parameter and on the properties of a number of charge exchange spectral lines.

The ejected-electron spectra in the region of the lowest autoionizing states in Li and have been studied at an observation angle of 54.7° over the electron-impact energy range 0–6 eV above the excitation threshold at 56.4 eV. The strong near-threshold resonance excitation has been revealed for the and (1s → 3σ_g)¹Σ⁺_g molecular transitions. For the atomic , and molecular states the post-collision interaction (PCI) energy shift of corresponding lines has been measured and compared with electron-impact excitation functions for these states. The general appearance of the PCI data for the and states reveals a close agreement with the classical and semi-classical predictions, showing a minimal correlation between resonance excitation of the subshell and the PCI. Meanwhile, in the energy region of 0–1 eV the energy shift of the line reveals an abrupt decrease to nearly zero values, similar to that observed earlier for the (np⁵(n+1)s²)²P_3/2 lines in the ejected-electron spectra of Na (n = 2) and K (n = 3) atoms. The calculations show the possibility for extrema of the PCI shift to appear in those energy regions where the negative-ion resonances are present in the excitation function of the state.

We have investigated the angular distributions of O⁺(⁴S) ions produced from dissociative photoionization of O⁺₂ c⁴Σ⁻_u(ν = 0, 1) using the TPEPICO technique, i.e. by measuring the coincidence yield between threshold photoelectrons and photoions. The vibrational levels have distinctly different lifetimes, τ_ν, which diminish their inherent anisotropic photoion angular distribution characterized by a β parameter. We obtain τ₁ = 6.0 ± 0.3 × 10⁻¹⁴ s and a lower limit on τ₀ of ≈1 × 10⁻¹² s, in broad agreement with other experimental studies using different methods, and find that β = 0.40 ± 0.05, which is significantly at variance with the predicted value of ⩾1.6.

We compare results of various numerical calculations for electron impact ionization of the ground state Mg (3s²) leading to the final ionic ground state Mg⁺(3s) or the excited ionic states Mg⁺(3p, 4s, 3d). Although our numerical results exhibit some model dependence, especially for small values of the triple-differential cross section, we cannot resolve the significant differences between experiment and theory reported by Bolognesi et al (2008 J. Phys. B: At. Mol. Opt. Phys.41 065203). Extensive theoretical modelling allows us to identify possible sources of this strong disagreement and to suggest future investigations.

We have investigated the excitation process of hydrogen-like multicharged ions by the impact of different charged projectiles such as electrons, positrons, protons and antiprotons. The universal scaling behaviour for the differential and total cross sections is deduced within the framework of the non-relativistic perturbation theory, taking into account the one-photon exchange diagrams. Special emphasis is laid on the description of the reaction in the near-threshold energy domain, which requires the accurate account for interactions between all particles in the colliding system.

Gamma-ray positron annihilation spectra of the noble gases are simulated using computational chemistry tools for the bound electron wavefunctions and plane-wave approximation for the low-energy positron. The present annihilation line shapes, i.e. the full width at half maximum, Δε, of the γ-ray annihilation spectra for He and Ar (valence) agree well with available independent atomic calculations using a different algorithm. For other noble gases they achieve moderate agreement with the experimental measurements. It is found that the contributions of various atomic electron shells to the spectra depend significantly on their principal quantum number n and orbital angular momentum quantum number l. The present study further reveals that the outermost ns electrons of the noble gases exhibit spectral line shapes in close agreement with those measured, indicating (as expected) that the measurements are not due to a simple sum over the momentum densities for all atomic electrons. The robust nature of the present approach makes it possible for us to proceed to more complex molecular systems using the tools of modern computational chemistry.

The effect of quantum interference on the optical properties of a pumped-probe three-level V-type atomic system is investigated. The probe absorption, dispersion, group index and optical bistability beyond the two-photon resonance condition are discussed. It is found that the optical properties of a medium in the frequency of the probe field, in general, are phase independent. The phase dependence arises from a scattering of the coupling field into the probe field at a frequency which in general differs from the probe field frequency. It is demonstrated that beyond the two-photon resonance condition the phase sensitivity of the medium will disappear.

We propose a scheme for implementing a remote three-qubit controlled-Z gate with three atoms separately trapped in three distant cavities coupled by optical fibres. The influences of various decoherence processes, such as spontaneous emission of the atoms and photon leakage of the cavities and the optical fibres, on the fidelity are also investigated. It is found that the gate can be implemented with high fidelity even when these decoherence processes are considered. This scheme can be extended to the implementation of a remote N-qubit controlled-Z gate with N atoms separately trapped in N distant cavities coupled by optical fibres.

We study analytically the dynamic behaviours of quantum correlation measured by a quantum discord between two uncoupled qubits, which are immersed in a common Ohmic environment. We show that the quantum discord of the two non-interacting qubits can be greatly amplified or protected for certain initially prepared X-type states in the time evolution. In particular, it is found that stable amplification of the quantum discord exists for the case of two identical qubits, and the quantum discord can be protected for the case of two different qubits with a large detuning. It is also indicated that in general a sudden change of the quantum discord occurs in the time evolution at a critical time point t_c, and the discord amplification and protection may occur only in the time interval for certain X-type states. This sheds new light on the creation and protection of quantum correlation.

We present calculations of the double ionization of H₂ induced by an intense attosecond laser pulse at a photon energy of 40 eV using the time-dependent close-coupling method within the fixed nuclei approximation. We focus on two-photon absorption processes and examine how the response of the ejected electrons, in particular the single- and the double-energy differential probabilities, is affected by linear and circular polarizations at laser-field intensities ranging from 10 to 10. In general, we find that for both linearly and circularly polarized pulses, sequential peaks and non-sequential wells that appear in both the single- and double-energy differential probabilities are akin to the analogous two-electron photoemission processes in the helium atom driven by intense attosecond pulses. In addition, for the case of a linearly polarized pulse, a clear signature of the sequential double-electron above the threshold ionization process can be seen in these spectra.

This paper develops spherically symmetric models as well as a more accurate model for the calculation of the damage and the movement of free and quasi-free electrons which are produced from the irradiation of x-ray free electron lasers (XFELs). The behaviour of free and quasi-free electrons is studied for targets with various shapes such as spheres and ellipsoids by treating the space distribution of free and quasi-free electrons. Furthermore, the limits of the application of spherically symmetric models to various shapes are discussed. The spherically symmetric model developed here is also applied to a bio-molecule. The results obtained are useful for the analysis of the three-dimensional structures of large bio-molecules in the experiments of XFELs.