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The propagation of a modulated positron beam in a planar crystal channel is investigated. It is demonstrated that the beam preserves its modulation at sufficiently large penetration depths, which opens the prospect of using a crystalline undulator as a coherent source of hard x-rays. This finding is a crucial milestone in developing a new type of laser radiating in the hard x-ray and gamma-ray range.

A basic theoretical introduction is given for the phenomenon of electronic transport through molecular junctions. The electrode–molecule–electrode system is represented using a model Hamiltonian framework based on separation between the molecular and the electrode single-particle subspaces, using projection operators. The Landauer formulation of the steady-state current through the junction is introduced and the transmission function is derived from basic single-particle quantum scattering theory. Detailed implementations to a generic tight-binding model demonstrate the typical characteristics of the transmission function, and resonant transport through discrete quantum molecular states is analysed in detail. An alternative formulation based on the time-dependent Liouville–von Neumann equation leads to a quantum kinetic representation of the current in terms of rate constants for electron hopping between the molecule and the electrodes. The generalization of this approach to inelastic transport is discussed.

Relativistic configuration interaction results are presented for the Kαα x-rays arising from the decay of doubly ionized K shell of atoms with 12 ⩽ Z ⩽ 80. The x-ray energies and dipole rates are calculated in the active space approximation using multi-configuration Dirac–Fock wavefunctions with the inclusion of the Breit interaction and quantum electrodynamics corrections. The calculations include all single and double excitations from core-valence interactions. The resulting length and velocity gauge transition rates are well converged and are in good agreement with the allowed transition, while the intercombination line rate is seen to be gauge dependent. Conclusions concerning the relative importance of correlation and the Breit interaction on the intensity ratios are obtained in some detail. The sensitivity of the relative intensities of the fine structure lines to the coupling variations in both length and velocity gauges is analysed. The calculated results are compared with previous experimental and theoretical data.

We present the implementation of tailored trapping potentials for ultracold gases on an atom chip. We realize highly elongated traps with box-like confinement along the long, axial direction combined with conventional harmonic confinement along the two radial directions. The design, fabrication and characterization of the atom chip and the box traps are described. We load ultracold (≲ 1 µK) clouds of ⁸⁷Rb in a box trap, and demonstrate Bose-gas focusing as a means to characterize these atomic clouds in arbitrarily shaped potentials. Our results show that box-like axial potentials on atom chips are very promising for studies of one-dimensional quantum gases.

We explore how the extraordinary properties of Rydberg atoms can be employed to impact the motion of ultracold ground state atoms. Specifically, we use an off-resonant two-photon laser dressing to map features of the Rydberg states on ground state atoms. It is demonstrated that the interplay between the spatially varying quantization axis of the considered Ioffe–Pritchard field and the fixed polarizations of the laser transitions provides the possibility of substantially manipulating the ground state trapping potential.

Radiative lifetimes have been measured for 123 levels of neutral erbium using time-resolved laser-induced fluorescence on a slow beam of erbium atoms. Of the 123 levels, 56 are even parity and range in energy from 26 993 to 40 440 cm⁻¹ and 67 are odd parity ranging from 16 070 to 38 401 cm⁻¹. This set of Er i lifetimes is much more extensive than others published to date, with 90 of the 123 level lifetimes measured for the first time. These lifetimes will provide the absolute calibration for a large set of measured Er i transition probabilities. Spectroscopic studies of rare earth elements including erbium are motivated by research needs in both the astrophysics and lighting communities.

We investigate the accuracy of different common methods for calculating the momentum distribution of the nuclei following dissociation of H⁺₂ by a short, intense laser pulse. This analysis can be performed either exactly by projecting onto scattering states of the system or by using a Fourier transformation of the final wavefunction. When and how this Fourier transform is made determines the validity of the approximation. While the values of the total observables were comparable, the Fourier transform methods require further free propagation time to converge to the exact result.

The excited electronic states of ArXe molecules in the region 77 000–80 200 cm⁻¹ were studied using the (2+1) and (3+1) resonance-enhanced multiphoton ionization methods. The use of different methods of multi-photon excitation and Ar⁺ ion registration allowed us to obtain some new data. Molecular constants were obtained for previously unknown excited states of molecules with the following dissociation limits: ArXe* → Ar¹S₀+Xe*6р[5/2]₃ with Ω = 2, 3 symmetry; Ar¹S₀+Xe*6p[3/2]₂ with Ω = 1, 2 symmetry; Xe⁰S₁ → Xe*6s'[1/2]⁰₁ with Ω = 0⁺ symmetry.

The polarization labelling spectroscopy technique is used to characterize three previously unknown ¹Π_u electronic states in the ⁷Li₂ molecule. The states are identified, together with the 4 ¹Π_u and 6 ¹Π_u states reported before, as low-lying members, n = 3 to 7, of a molecular Rydberg series corresponding to the excitation of the npπ_u electron. The dependence of their properties on the principal quantum number n is discussed.

With the aim of finding an appropriate description of the state of an ejected electron from a linear three-centre molecular target, a wavefunction is constructed in a closed analytical form by solving the Schrödinger equation of an unbound electron (with wave vector k) in a Coulomb field of three fixed charged nuclei. The model, which is an extension of a two-centre model developed in the past, fulfils the correct boundary conditions asymptotically up to the order O((kr)⁻²). It is employed, in the frame of a perturbative first Born three-centre procedure, to the determination of the multiply differential cross sections (MDCS) of the (e,2e) simple ionization of the valence 1π_g level of CO₂, for which experimental results were given recently. The ionization of the inner 1π_u and 3σ_u levels of CO₂ are also investigated by this approach. The study of the variation of the MDCS with the direction of the scattered electron and the ejected electron in the case of oriented three-centre targets shows interference patterns similar to those created by the diffraction of light by three apertures.

We report an extensive study on partial-ion-yield spectroscopy around the Cl 1s and 2p ionization thresholds for Cl₂. All positive ion channels, several with the same mass/charge ratio, which could be distinguished by taking the advantage of the ³⁷Cl isotope, have been measured at a photon resolution of nearly 6500. At the Cl 1s ionization threshold, no significant differences are reported between the absorption and the partial-ion yields. In contrast, near the 2p ionization thresholds, we detect large variations in the fragmentation patterns following excitations to the Rydberg series when comparing the atomic fragment ions to the molecular fragment ions. We attribute the different behaviours to the more-or-less diffuse nature of Rydberg states with different angular momenta.

State-selective charge exchange cross sections and momentum spectra are calculated for collisions of Xe¹⁸⁺ and Xe⁵⁴⁺ ions with Na(3s) and Na*(3p) over the energy range of 0.1–10.0 keV/amu. The classical trajectory Monte Carlo method is used which includes all two-body interactions. The n-level cross sections are found to be rather insensitive to collision energy below 1 keV/amu. In contrast, the transverse momentum cross sections for specific n-levels change rapidly with energy. However, this latter variation in energy is found to be in general agreement with simple scaling rules. Experimental state-selective data are available for Xe¹⁸⁺ over a limited energy range; they are found to be in reasonable accord with the calculations.

The variation in the electric dipole moments of H₂O, CH₃F, CH₃Cl and CH₃Br as their geometries relax due to interaction with a positron is evaluated. The results are in good agreement with a recently observed empirical dependence of the positron binding energy on molecular properties (Danielson et al2009 J. Phys. B: At. Mol. Opt. Phys.42 235203). For binding energies larger than 100 meV relaxation could alter significantly the analysis of the binding, but it is in the prospect of generating effective potentials for positron scattering by molecules that the effect can be more important.

Photoionization cross sections of the Be-like O^{4 +} ion in the photon energy region from the first threshold up to the O^{5 +} 3d threshold have been calculated using a non-iterative variational R-matrix method combined with multichannel quantum-defect theory for the ground 2s²¹S and excited 2s2p ^{1, 3}P states. The partial cross sections are presented and the autoionizing resonance structures arising from the ground and excited states are identified and characterized. Our calculational results, which show excellent agreement between length and velocity gauges, are compared with the available experiment and previous calculations, and good agreement is found.

We propose an efficient scheme to avoid collision-induced dynamical instabilities in the generation of heteronuclear molecular Bose–Einstein condensates based on an R-type photoassociative adiabatic passage. This scheme allows us to completely compensate the intraspecies collisions by tunable interspecies collisions, and thereby achieve a robust atom–molecule conversion which is equivalent to that in a collision-free case. By taking the formation of stable ⁴¹K⁸⁷Rb molecules as an example, we study the feasibility of the scheme.

The aim of this work is to improve the treatment of density effects in non-local thermodynamic equilibrium plasmas. The density effect on atomic structure (wavefunctions and energy levels) is modelled by an ion-sphere potential. The modification of the atomic potential, continuum lowering and appearance of resonances are presented. In particular, we show that the continuum resonances are linked to the electrons in the subshells passed into the continuum. Their presence determines the existence of partially bound configurations, which must be taken into account in the collisional-radiative model. We introduce in the set of rate equations a supplementary ionization process due to the plasma environment. This process (and its inverse) enters into the balance of all the other processes. It is equivalent to tunnelling ionization where an outer electron located above the ionization threshold (and trapped by the potential barrier) crosses the barrier. As an application, we studied the influence of temperature and density on the average ionization and the ionic populations of a carbon plasma. We compared these calculations with the traditional method based on the chemical picture with continuum lowering.

The differential cross sections for the dissociative single and double excitations resulting in H(2p) formation with the excitation energy of 19–46 eV in electron–CH₄ collisions have been measured as a function of electron scattering angle in the range 4°–48° at 80 eV incident electron energy by means of angle-resolved electron energy-loss spectroscopy in coincidence with detecting Lyman-α photons. This is the first measurement of the differential cross sections for the dissociative double excitations as a function of electron scattering angle in electron–molecule collisions. Their fractions have been compared with those at the optical limit calculated from the density of the dipole oscillator strength for the emission of Lyman-α photons previously measured by our group. The dissociative double excitations in 80 eV electron collisions seem to be brought about in a very different way from those at the optical limit where they arise from the electron correlation in a methane molecule. The differential cross sections have also been discussed in terms of momentum transfer, leading to a universal curve.

Two-photon ionization of rubidium atoms in a magneto-optical trap and a Bose–Einstein condensate (BEC) is experimentally investigated. Using 100 ns laser pulses, we detect single ions photoionized from the condensate with a 35(10)% efficiency. The measurements are performed using a quartz cell with external electrodes, allowing large optical access for BECs and optical lattices.

We revisit the composite fermion (CF) construction of the lowest angular momentum yrast states of rotating Bose gases with weak short-range interaction. For angular momenta at and below the single vortex, L ⩽ N, the overlaps between these trial wavefunctions and the corresponding exact solutions increase with increasing system size and appear to approach unity in the thermodynamic limit. In the special case L = N, this remarkable behaviour was previously observed numerically. Here we present methods to address this point analytically and find strongly suggestive evidence in favour of similar behaviour for all L ⩽ N. While not constituting a fully conclusive proof of the converging overlaps, our results do demonstrate a striking similarity between the analytic structure of the exact ground state wavefunctions at L ⩽ N and that of their CF counterparts. Results are given for two different projection methods commonly used in the CF approach.

In this work, the spectral properties of a singly charged vortex in a Bose–Einstein condensate confined in a highly anisotropic (disc-shaped) harmonic trap are investigated. Special emphasis is placed on the analysis of the so-called anomalous (negative energy) mode of the Bogoliubov spectrum. We use analytical and numerical techniques to illustrate the connection of the anomalous mode to the precession dynamics of the vortex in the trap. Effects due to inhomogeneous interatomic interactions and dissipative perturbations motivated by finite-temperature considerations are explored. We find that both of these effects may give rise to oscillatory instabilities of the vortex, which are suitably diagnosed through the perturbation-induced evolution of the anomalous mode, and monitored by direct numerical simulations.

Internal modes and internal oscillation of ten-pearled necklace solitons in Bessel photonic lattices with defocusing Kerr nonlinearity are investigated after the derivation of solutions of such solitons and the analysis of their stability. It is shown that the internal modes have both real and imaginary parts in such cases, which is different from the one-dimensional case. While white noise gives rise to the jitter of the intensity of stable necklace solitons, the stable solitons perform long-distance and quasi-periodic oscillation of intensity and shape under the perturbation of internal modes.

A method of dynamic control of absorption and dispersion of a two-level quantum system (atoms, ions or quantum dots) doping a photonic band gap (PBG) material via variation of the intensity and frequency of an external laser field is proposed. The frequency of an optical transition should be inside a PBG and located near or at a photonic band edge. In this case the laser field 'dresses' the quantum system (Mollow splitting) while the decay rates of the dressed states become very different due to a photonic band edge and depend on the form of spectral density of electromagnetic modes as well as intensity and frequency of the laser field. This enables us to control absorption and dispersion of a signal laser field, which is near resonant to the quantum transition of a dopant.

Aluminium K_α emission (1.5 keV) produced by an 8 J, 500 ps, Nd:glass laser incident at 45° onto a layered target of 0.8 µm thick aluminium (front side) and 1 µm thick iron (backside) has been used to probe the opacity of iron plasma. Source broadened spectroscopy and continuum emission analysis show that whole beam self-focusing within the aluminium plasma results in a two-temperature spatial distribution. Thermal conduction from the laser-irradiated aluminium into the iron layer, enhanced by the whole beam self-focusing, results in a temperature of ∼10–150 eV in the iron layer. The iron opacity at a photon energy of 1.5 keV is shown to be strongly modified from cold values in agreement with IMP code opacities. Results presented here represent a feasibility study to seed future work using table-top laser systems for plasma opacity experiments.

We evaluate exactly the non-Markovian effect on the decoherence dynamics of a qubit interacting with a dissipative vacuum reservoir and find that the coherence of the qubit can be partially trapped in the steady state when the memory effect of the reservoir is considered. Our analysis shows that it is the formation of a bound state between the qubit and its reservoir that results in this residual coherence in the steady state under the non-Markovian dynamics. A physical condition for the decoherence suppression is given explicitly. Our results suggest a potential way to decoherence control by modifying the system–reservoir interaction and the spectrum of the reservoir to the non-Markovian regime in the scenario of reservoir engineering.

In this paper we present a scheme for generation of two-photon EPR and W states in the cavity QED context. The scheme requires only one three-level Rydberg atom and two or three cavities. The atom is sent to interact with cavities previously prepared in vacuum states, via a two-photon process. An appropriate choice of the interaction times allows one to obtain the mentioned states with maximized fidelities. These specific times and the values of success probability and fidelity are discussed.

In this paper we introduce a Hamiltonian model which represents the interaction between SU(1, 1) and SU(2) quantum systems. The Heisenberg equations of motion are invoked to derive the exact time-dependent dynamical operators. The phenomena of the collapse and revival are observed for the intermediate states, however, for a large value of the excitation number m. It is also shown that the phenomenon of squeezing is observed in the intermediate state. Moreover, it is sensitive to the variation in the coupling parameter λ, the detuning parameter Δ, as well as in the Bargmann index k. The examination of the correlation function shows that the system displays anti-bunching for all periods of time except for a large value of the excitation number when k = 3/4. Our discussion for the variance squeezing also shows that the phenomenon of squeezing is pronounced in the quadrature variances for the odd parity case when the detuning parameter is large.

We theoretically propose a scheme to study coherent population oscillation (CPO) spectra in a system of a quantum dot coupled to a nanomechanical resonator. Due to nanomechanical vibrations CPO in a single quantum dot demonstrates novel features. In analogy with CPO in quantum optics, we refer to this effect as mechanically induced coherent population oscillation (MICPO). In this scheme, the vibration of the nanomechanical resonator makes a contribution to additional auxiliary energy levels so that the transparency phenomenon could be realized in such a system. The optical spectrum shows that the transparency based on MICPO can be controlled simply by the intensity of the optical field, the coupling strength and other relevant parameters. Furthermore, our technique provides detection in a real experiment to measure the decay rate of the nanomechanical resonator.

Using the Langevin formalism we study the effects an external electromagnetic field induces in a system made of a pumped two-level system (TLS) and a metallic nano-particle (NP) that interact together via their near-fields. The surface plasmons of the NP greatly enhance the scattered light. With the absence of the external EM field the spectral width of the scattered light is broader than that of the system, covering almost the entire optical range. However, with the inclusion the external EM field, a reduction in the spectral width of the scattered light of order of 10–50 times below that of the system is observed for certain parameter regimes.This system exhibits also bistability in the population difference of the TLS with the external field acting as an order parameter, but only for certain values of the noise quanta.

In this paper, we show that it is possible to achieve continuous-variable entanglement of three optical fields in a four-level Y-type atomic system. The three fields are amplified through the Rabi resonances of the different driven transitions. Two fields from the V configuration are in one quantum beat and in the mixing parametric interactions with the third field from the lower transition. This yields entanglement between the pair of beat modes and the third field. The best achievable state in the output approaches the Einstein–Podolsky–Rosen entangled state.

The ionization of Ar and Xe in a dichroic pulsed laser field, comprising the fundamental and second harmonic of a Ti:sapphire laser, was studied. The ion yield was found to depend on the angle between the polarization vectors of the two fields. The dependence was explained by proposing a model that considered the ionization process to be resulting from the action of two independent mechanisms, namely quasi-static tunnelling and multiphoton ionization.

Polar molecules such as CO are interesting target systems for high-order harmonic generation (HHG) as they can be oriented with current laser techniques, thus allowing the study of systems without inversion symmetry. However, the asymmetry of the molecule also means that the molecular orbitals are Stark shifted in energy due to their interaction with the driving laser. We extend the strong-field approximation of HHG by incorporating the Stark shift into the Lewenstein model and discuss its impact on two different gating mechanisms in CO. In system-induced gating an oriented target molecule serves as a gate by selecting every other half-cycle due to an increased (decreased) ionization rate. In field-induced gating the waveform of the driving laser is tailored such that the harmonic emission from an aligned molecule is damped (enhanced) every other half-cycle. We show that the Stark shift weakens the strength of system-induced gating and also determines the relative contribution from opposite orientations in field-induced gating. Finally, we propose a novel scheme for extending the high-order harmonic cutoff by letting the two gating mechanisms counteract each other, thus allowing for a higher laser intensity without increased ionization of the target gas.