Table of contents

We present a complete derivation of two-particle states of the one-dimensional extended Bose–Hubbard model involving attractive or repulsive on-site and nearest-neighbour interactions. We find that this system possesses scattering resonances and two families of energy-dependent interaction-bound states which are not present in the Hubbard model with the on-site interaction alone.

A quantum mechanical model for laser-assisted Auger decay based on the numerical solution of a system of non-stationary Schrödinger equations is developed. The model is applied to a particular case of Ne KLL Auger transitions. The effect of an intense laser field on the angle-resolved spectra of Auger electrons generated by ultrashort (subfemtosecond) pulses is investigated. We demonstrate that for energetic Auger electrons the sideband structure, which appears due to interaction of electrons with the strong laser field, does not correspond to the generally accepted picture of regular equidistant peaks separated by the laser photon energy. Moreover, the structure strongly depends on the observation angle. The origin of this phenomenon is discussed, and it is demonstrated that the standard sideband picture appears after integration over all angles. The described structure can provide a useful tool for controlling the phase of the x-ray pulse relative to the laser pulse in laser-assisted Auger emission experiments.

A pertinent question in strong-field molecular physics is: what role does the permanent electric dipole moment of heteronuclear molecules play in their dissociation dynamics? Recently, Kiess et al (2008 Phys. Rev. A 77 053401) reported the first evidence for direct two-photon dissociation of an HD⁺ beam involving its permanent dipole moment, using 790 nm, 100 fs pulses. However, the measurement was convoluted by the fact that the H⁺ (H) and D⁺ (D) fragments could not be well resolved. Using high resolution coincidence 3D momentum imaging, which distinguishes all fragments, we find new evidence that challenges the previous findings. Specifically, we find that the small peak observed and assigned earlier to direct two-photon dissociation is instead due to one-photon dissociation of the v= 8 vibrational state by bond softening. Our vibrationally resolved spectra covering the intensity interval 3 × 10¹³ − 5 × 10¹⁴ W cm⁻² show that one-photon dissociation of HD⁺ dominates, with no clear evidence of permanent dipole transitions.

Photodetachment microscopy is carried out on a beam of ¹²⁷I⁻ ions with a nanosecond pulsed laser. The photoelectron interferograms are recorded by means of a digital camera that images the light spots produced by the amplified photoelectrons on a phosphor screen. This is the first implementation of such an optical imaging technique in photodetachment microscopy. Due to their sensitivity to the photoelectron energy, the recorded electron interferograms can be quantitatively analysed to produce a measure of the electron affinity of iodine ^eA(¹²⁷I) with an accuracy improved by more than a factor of 2 with respect to the best previous measurement. The result is 2467 287.4(29) m⁻¹ or 3.059 0463(38) eV.

This work reports measurements of nine Stark widths and nine Stark shifts of Xe II spectral lines. These line parameters have been measured in a pulsed arc plasma in electron density and temperature range (0.2–1.6) × 10²³ m⁻³ and (18 000–25 000) K, respectively. The results are compared to other experimental data as well as with theoretical values calculated from modified semiempirical formulae. Stark parameters of the lines belonging to the same multiplet are usually very close. The results obtained in this work show that in some cases very clear intramultiplet irregularities exist. These measurements, as well as some results from the previous works of this group, have been used to analyse the origin of these irregularities.

Radiative recombination of a free electron into an excited state of a bare, high-Z ion is studied, together with its subsequent decay, within the framework of the density matrix theory and Dirac's relativistic equation. Special attention is paid to the polarization and angular correlations between the recombination and the decay photons. In order to perform a systematic analysis of these correlations the general expression for the double-differential recombination cross section is obtained by making use of the resonance approximation. Based on this expression, detailed computations for the linear polarization of x-ray photons emitted in the (e, 2γ) two-step recombination of uranium ions U⁹²⁺ are carried out for a wide range of projectile energies.

The fragmentation dynamics of core-excited methanoic acid (formic acid) has been studied by means of partial anion- and cation-yield measurements around the C K ionization threshold. All detectable ionic fragments are reported, and significant differences among partial-ion yields are observed. Possible dissociation pathways are discussed. We confirm experimentally the mixed A* molecular/3s Rydberg character of the resonance at 292.1 eV. Atomic rearrangements after photoabsorption leading to the formation of novel cationic fragments and selective fragmentation to the CH₂O²⁺ ion are also reported.

The geometries, stability, electronic properties and magnetism of Sc_nO clusters up to n = 12 are systematically studied using density-functional theory. For the lowest energy structures of Sc_nO clusters, the equilibrium site of the O atom is located in the centre of the cluster and surrounded by the Sc atoms except for n = 2, 3 and 5. The calculated results show that clusters with n = 2, 6, 9 and 12 are more stable than their respective neighbours. In addition, the ionization potential (IP), the electron affinity (EA) and the HOMO–LUMO gaps are calculated and discussed. The total magnetic moments of Sc_nO clusters are larger than Y_nO except for n = 3 and 5, and also larger than the pure Sc_n at n = 4 and 7–12, indicating that the O atom can dramatically modify the magnetic properties of Sc_n, consistent with the observation of enhanced magnetic moments in the Sc_nO clusters. The lowest energy structure of the Sc₁₂O cluster is a perfect icosahedron with the largest magnetic moment, 10 μ_B, in the investigated clusters. In addition, we find that the total magnetic moment is not quenched in small Sc_nO (n = 2–12) clusters. The possible reasons for the high magnetic moments of the clusters are also discussed.

In this paper we present results of the structural, energetic and electronic properties of Al_n (n = 23, 26, 92) clusters calculated using density functional theory (DFT). Our main focus is the highly symmetric and energetically stable hollow Al₉₂ cluster. This cluster appears as a natural extension of the smaller clusters, Al₂₃ and Al₂₆, that manifest similar structural motifs of pentagonal and hexagonal pyramids. The Al₉₂ cluster maintains I_h symmetry and its central framework is similar to the structure of the well-known C₆₀ fullerene and the recently reported B₈₀ cluster. It has an additional Al atom below each hexagon centre and another one above each pentagon plane. To the best of our knowledge Al₉₂ is the largest Al hollow cluster studied so far.

Using projectile-like ions instead of projectile ions in a beam–foil source, we have reconfirmed the formation of circular Rydberg states. The projectile-like ⁶⁰₂₈Ni and ⁶³₂₉Cu ions have been produced by a ⁵⁶₂₆Fe ion beam and ⁶²₃₀Zn ions by a ⁵⁸₂₈Ni ion beam, at beam energies above the Coulomb barrier, bombarding a thin carbon foil. Time-resolved x-ray transitions of these projectile-like ions attain their maximum intensities at different delays. Such delays are attributed to successive cascading through yrast transitions from the circular Rydberg levels, which find important implications in understanding the origin of radio recombination lines from interstellar space.

Using a variant of the laser photoelectron attachment (LPA) method with an extended energy range (EXLPA), we have studied low-energy electron attachment to the molecules CCl₄ (Cl⁻ and Cl⁻₂ formation) and SF₆ (SF⁻₆ and SF⁻₅ formation) in a diffuse gas target (T_G = 300 K) from 0 eV up to 2 eV at energy widths down to 14 meV. In the EXLPA method, pulses of near-zero energy photoelectrons are produced in a guiding magnetic field, accelerated by a weak electric field, brought to the energy of interest prior to their traversal through the target region and subsequently accelerated and deflected onto a detecting plate. Anions due to electron attachment are extracted by a pulsed electric field, during which the photoelectron current is interrupted, and detected by a quadrupole mass spectrometer. The EXLPA anion yields are combined with absolute cross sections, obtained at very high resolution (≈1 meV) with the LPA method over the range 0–0.17 eV, to yield new recommended absolute partial and total attachment cross sections over the range 0–2 eV at the well-defined gas temperature T_G = 300 K. Our cross sections show characteristic deviations from previously reported results. At least in part, these differences can be attributed to the fact that in the earlier electron beam experiments the gas temperature was higher than 300 K. For SF₆, the branching fractions for SF⁻₅ formation at electron energies 0.002–0.43 eV and for different initial rovibrational distributions are compared with those recently predicted from kinetic modelling within the framework of statistical unimolecular rate theory. Satisfactory agreement is observed, but our data provide evidence that an additional path for producing SF⁻₆ and SF⁻₅ ions is available at electron energies above about 0.3 eV.

In this work, differential cross sections have been calculated for the excitation of atomic hydrogen (H) from its ground state (1s) into a selection of its excited states (2s, 2p₀, 2p_±1 and 3s). The present study was conducted at intermediate and high proton impact energies within the range 50 keV–1 MeV. A three-body model based on Faddeev-type equations is implemented, while near the energy shell the two-body Coulomb transition matrix was used to calculate the transition amplitude. Second and higher order approximations have been ignored in this case. The resulting amplitudes are subsequently utilized to calculate total and differential cross sections for the corresponding excitation processes. These calculated cross sections have then been compared with available experimental data and other theoretical results from the literature.

We consider spin exchange in ultracold collisions of hydrogen (H) and antihydrogen atoms. The cross sections for transitions between various hyperfine states are calculated. We show that the process of spin exchange in –H collisions is basically driven by the strong force between a proton and an antiproton.

The stability of the Lanczos algorithm for computing photodissociation cross sections is studied. The system is discretized on a grid and the discrete variable representation (DVR) is used to represent system operators. The Hamiltonian is augmented with an absorbing potential (AP) or smooth exterior scaling (SES), to enforce outgoing boundary conditions, making it complex symmetric. The main difference between the AP and the SES is that the former adds to the potential energy whereas the latter modifies the kinetic energy operator. Grozdanov et al (2004 J. Phys. B: At. Mol. Opt. Phys.31 173) observed the fact that the Lanczos recursions could slow down and even stagnate for certain choices of parameters in the AP or SES. Here we show that for the SES, it is important that the maximum kinetic energy of the DVR is adapted to the physical problem or else the Lanczos recursions might be unstable. A similar result was found for the AP; that is, the Lanczos algorithm in order to converge the strength of the absorbing potential should be of the order of the scattering energy of interest. It is shown that with a discretization adopted to the physical problem at hand and a proper choice of parameters, the Lanczos recursions converge and provide accurate results for both the absorbing potential and the smooth exterior scaling.

We study an atomic Josephson junction (AJJ) in the presence of two interacting Bose–Einstein condensates (BECs) confined in a double-well trap. We assume that bosons of different species interact with each other. The macroscopic wavefunctions of the two components obey a system of two 3D coupled Gross–Pitaevskii equations (GPEs). We write the Lagrangian of the system, and from this we derive a system of coupled ordinary differential equations (ODEs), for which the coupled pendula represent the mechanical analogues. These differential equations control the dynamical behaviour of the fractional imbalance and of the relative phase of each bosonic component. We perform the stability analysis around the points which preserve the symmetry and get an analytical formula for the oscillation frequency around the stable points. Such a formula could be used as an indirect measure of the inter-species s-wave scattering length. We also study the oscillations of each fractional imbalance around zero and nonzero—the macroscopic quantum self-trapping (MQST)—time averaged values. For different values of the inter-species interaction amplitude, we carry out this study both by directly solving the two GPEs and by solving the corresponding coupled pendula equations. We show that, under certain conditions, the predictions of these two approaches are in good agreement. Moreover, we calculate the crossover value of the inter-species interaction amplitude which signals the onset of MQST.

A dressed-state study of lasing without population inversion from a three-level atom interacting with a bi-chromatic laser field in the ladder configuration is presented. In our model we allow the atom to be dressed by both of the coupling and probe fields. The system under consideration is explored both analytically and numerically within the steady-state regime. A parameter study is performed in which we explore the influence of the change of control parameters (Rabi frequencies and incoherent pump rates, etc) on the system behaviour. The parameter study also applies to situations where both coherent fields have comparable intensities. Expressions for steady-state populations and coherences are derived, from which the constraints on the system control parameters allowing gain or absorption of various types, are calculated. The system is demonstrated to possess gain without inversion (GWI), as well as regular gain with inversion, for the appropriate choice of system control parameters. Inversion without lasing (IWL) is also found. The dressed-state approach offers a different insight into the system dynamics in which Rabi oscillations and spectral features appear natural. We believe that the dressed-state approach provides a more adequate description of the system dynamics than that provided by the bare state basis.

We propose a set of novel protocols to remotely prepare a general two-qubit state by joint actions of two separate people who independently share the classical knowledge of the state, without and with quantum control of a supervisor. Various quantum resources are used as the quantum channel. Although the execution complexity differs for different protocols, the total success probability is protocol independent.

We investigate the effects of spontaneously generated interference on the spectrum of quantum fluctuations in phase quadratures of resonant fluorescence emitted from the transition of j = 1/2 to j = 1/2 of a monochromatically driven four-level atom with antiparallel dipole moments. It is found that the spectrum exhibits obvious characteristics of quantum interference in both cases of weak and strong laser fields. For the Zeeman-degenerate system, strong radiation squeezing in weak atomic excitation can occur at the central frequency of the spectrum, which is quite different from the case without consideration of the interference. Moreover, when we apply an external magnetic field to the atom, we find that the quantum interference can lead to a narrowing spectrum with much stronger squeezing by adjusting the magnetic field strength. These results suggest that the phase-dependent fluorescence spectrum can serve as a probe of the quantum interference effects in a realistic system such as a trapped ¹⁹⁸Hg⁺ ion or charged quantum dots.

We investigate the change of entanglement for distributing an arbitrarily entangled two-qubit pure state via three typical kinds of noisy quantum channels—amplitude damping, phase damping and depolarizing quantum channels. We find that for one-qubit transmission, the output distant entanglement (measured by concurrence) reduces proportionately with respect to its initial amount, and the entanglement-decay ratio is determined only by the noisy characteristics of quantum channels. For two-qubit transmission, the rule of the entanglement change is complex. For phase damping quantum channels, the four Bell states and are all the optimal entanglement-distribution (OED) input states. But for amplitude damping quantum channels, only |ψ^±⟩ are the OED input. For depolarizing quantum channels, and in the case of symmetrical quantum channels, the four Bell states are still the OED input. We also discuss the problem of entanglement sudden death (ESD) and find that for all the three types of quantum channels, ESD can be found.

Non-sequential double ionization (NSDI) of helium in an intense few-cycle laser pulse is investigated by applying the three-dimensional semi-classical rescattering method. It is found that the momentum distribution of He²⁺ shows a single–double–single peak structure as the pulse intensity increases. According to the different mechanisms dominating the NSDI process, the laser intensity can be classified into three regimes where the momentum distribution of He²⁺ exhibits different characteristics. In the relatively high intensity regime, an NSDI mechanism named the 'laser-assisted collision ionization' is found to be dominating the NSDI process and causing the single-peak structure. This result can shed light on the study of non-sequential ionization of a highly charged ion in a relatively intense laser pulse.

Soft x-rays are produced when high intensity pulsed lasers irradiate solid targets. The emitted x-rays can be suitable for different applications such as lithography and biological imaging. The free–bound and free–free (bremsstrahlung) x-ray emission at wavelengths 2.3–4.4 nm and 13.6 nm from targets with porous surfaces irradiated by longer (>50 ps) laser pulses has been investigated numerically. The computer code EHYBRID was used to simulate the hydrodynamics of a copper plasma produced by a 532 nm wavelength laser pulse irradiating a copper nano-layer on a copper target at intensities up to 10¹⁵ W cm⁻². The target is modelled by assuming a uniform solid metal substrate coated by a nano-layer with density less than the solid density. The simulation results show that the x-ray yield can be enhanced significantly by using such low-density targets and is sensitive to the nano-layer density. For any specific condition there is a density ratio at which an optimum x-ray yield occurs. Using a pre-pulse to increase x-ray yield was also investigated and the results are compared to the x-ray yield for low-density targets.

We theoretically study high-order harmonic generation when a helium ion is exposed to a two-colour laser field, which is synthesized by a 5 fs/800 nm laser pulse and a 64 fs/2400 nm laser pulse. Our numerical results show that the harmonic spectrum exhibits an ultrabroad extreme ultraviolet supercontinuum when the initial state is prepared as a coherent superposition of the ground state and the first excited state. By superposing a series of properly selected harmonics, an isolated attosecond pulse with a duration of 47 as is obtained. Compared with the case of the ground state in a one-colour field, the intensity of this isolated attosecond pulse is six orders of magnitude higher. We also demonstrate these results in terms of the time–frequency analysis and the semiclassical three-step model.

We investigate the spectra of two-colour few-cycle pulses in a semiconductor quantum well (QW) with consideration of quantum size effects. Our results show that, in the few-cycle regime, the spectral peak at 4ω₀ can be achieved even under the condition of a two-colour ω₀ − 3ω₀ pulse combination. Moreover, when the quantum size is modified, the enlargement or suppression of the spectral peak at 4ω₀ or 5ω₀ can be selectively controlled by the relative phase of the two-colour few-cycle pulses.

We consider the application of the line strength formula recently derived by Watson (2006 J. Phys. B: At. Mol. Opt. Phys.39 L291) to transitions between states of high principal quantum number in hydrogenic atoms and ions (Rydberg–Rydberg transitions). Apparent difficulties in the implementation of this formula are overcome by the use of recurrence relations derived by the ladder operator technique of Infeld and Hull (1951 Rev. Mod. Phys.23 21), and set out in an earlier paper by the present author (2006 J. Phys. B: At. Mol. Opt. Phys.39 2641). The use of the McLean–Watson formula for such cases is illustrated by the determination of the radiative lifetimes for levels with n ≈ 1000 and comparison of present results with approximate formulae. Interest in this work on the radial matrix elements for large n and n' is related both to measurements of radio recombination lines from tenuous space plasmas, e.g. Stepkin et al (2007 Mon. Not. R. Astron. Soc.374 852) and to the calculation of Stark broadening for such spectra, e.g. Gigosos et al (2007 Astron. Astrophys.466 1189), Stambulchik et al (2007 Phys. Rev. E 75 016401) and Stambulchik and Maron (2008 J. Phys. B: At. Mol. Opt. Phys.41 095703). In addition, we discuss the question of inaccuracy caused by the omission of fine structure in such calculations, and the numerical stability of the recurrence relations used to implement the line strength formulae.

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