Table of contents

New measurements of differential cross-sections for excitation of the a'' ¹Σ⁺_g (v' = 0, 1) state in molecular nitrogen reveal a cusp-like angular distribution. This feature is distinctly observed for the first time in the present electron energy-loss experiment as a result of finer scattering angle grid and impact energy coverage than previous measurements. This feature is similar to that observed in atomic targets such as He, Hg and Ba. The observed phenomenon suggests an interference effect related to configuration–interaction coupling between lower and excited states that are of the same symmetry. It is hoped that the present work will stimulate theoretical investigations into the physics that governs this cusp-like behaviour.

We review the experimental realization of a single bosonic Josephson junction for ultracold gases, which was made possible by the generation of a precisely controllable double-well potential for Bose–Einstein condensates. We will focus on the comparison of the experimentally obtained data with the predictions of a many-body two-mode model and a mean-field description and show that the observed static, thermal and dynamical properties can be described in terms of classical equations.

For a three-level Λ-type atom embedded in three-dimensional photonic crystals (PCs), the upper state population and spontaneous emission spectra are investigated with both transitions coupled to the same modified reservoir. By using an anisotropic dispersion model the analytic expressions of the upper state population and spontaneous emission spectra were obtained. The upper state population reaches a steady state value with a slight oscillation, and the spectra show a non-Lorentzian for both transition frequencies inside the band gap.

The VUV emissions of gaseous mixtures of krypton and xenon were investigated at room temperature. For this purpose, a pulsed, brief, selective multiphotonic excitation of the two lowest atomic states of the 5p⁵6s configuration of xenon was achieved. A spectroscopic and kinetic study was performed, for the first time, by comparing the radiation characteristics consecutive to initially populating either the metastable Xe[5p⁵6s(³P₂)] or the resonant Xe[5p⁵6s(³P₁)] states. The formation of the heteronuclear KrXe* excimers was clearly shown. The decay frequency of heteronuclear excimers correlated to the xenon metastable state obeys different scaling laws depending on the xenon pressure. The roles of heteronuclear and homonuclear excimers in the formation of VUV emissions of the gas mixture are discussed. Finally, two kinetic schemes showing the main reaction processes are proposed.

A problem to account for the direct electron–positron annihilation in positron–hydrogen scattering above the positronium formation threshold has been resolved within the time-independent formalism. The generalization of the optical theorem is derived for the case when an absorption potential is present in the Hamiltonian. With this theorem, the annihilation cross section is fully determined by scattering amplitudes. This allows us to separate out analytically the contribution of the positronium formation effect from the overall annihilation cross section. The rest is determined as the direct annihilation cross section. It is done uniformly below as well as above the positronium formation threshold. The multichannel three-body theory for scattering states in the presence of an imaginary absorption potential is developed in order to compute the direct e⁺e⁻ annihilation amplitude. Special attention has been paid to an accurate definition of the coordinate part of the absorption potential as the properly constructed zero-range potential, which corresponds to the delta function originating from the first-order perturbation theory. The calculated direct annihilation cross section below the positronium formation threshold is in good agreement with the results of other authors. The direct annihilation cross section computed with the formalism of the paper shows nonsingular behaviour at the positronium formation threshold. A number of e⁺e⁻ direct annihilation cross sections and positronium formation cross sections in the energy gap between Ps(1s) and H(n = 2) thresholds are reported. A sharp increase in the calculated direct annihilation cross section across the resonant energy is found for all first S- and P-wave Feshbach resonances.

A time-dependent close-coupling method is developed to treat the double ionization of helium by fast bare ion collisions. At high incident energies, charge transfer to the projectile is quite small, so that the two-electron wavefunction remains centred on the target, subject to a time-dependent projectile interaction. A multipole expansion of the projectile–atom interaction includes monopole, dipole, quadrupole and octopole terms. Time-dependent close-coupling calculations are carried out for α + He collisions at incident energies greater or equal to 1.0 MeV amu⁻¹. Over 100 coupled channels are needed to obtain total double ionization cross sections that are in good agreement with recent non-perturbative basis-set coupled channels calculations and absolute experimental measurements.

In the present work, we report on experimental and theoretical determination of the decay rates from the 7s_1/2 state in singly ionized lead. Lifetime measurements performed using time-resolved laser-induced fluorescence spectroscopy and lifetime computations carried out with the multiconfiguration Dirac–Fock method allow us to solve the discrepancies previously observed between theory and experiment for this level. New transition probabilities are proposed for the two depopulation channels, i.e. the 6p_1/2–7s_1/2 and 6p_3/2–7s_1/2 transitions of astrophysical interest in Pb II and Bi III, respectively.

We present elastic cross sections for scattering of electrons by a phosphine (PH₃) molecule using the R-matrix method for incident electron energies in the range 0.025–15 eV. We find a shape resonance in ²E scattering symmetry in the static-exchange (SE) and static-exchange plus polarization (SEP) approximations whose resonance positions are 5.57 eV and 2.34 eV, respectively. The Born correction is applied for the elastic transition to account for partial waves higher than l = 4 excluded in the R-matrix calculation. A comparison of our results in both approximations is carried out with other theoretical and the experimental work. Our calculations also reproduce the Ramsauer–Townsend effect that reflects the accuracy of electron–electron correlation included in our theoretical treatment.

A model of a four-level atom embedded in a double-band photonic crystal (PC) is presented. The atomic transitions from the upper two levels to the lower two levels are coupled by the same reservoir which is assumed in turn to be isotropic PC modes, anisotropic PC modes and free vacuum modes. The effects of the fine structure of the atomic ground state levels and the quantum interference on the spontaneous emission spectrum of an atom are investigated in detail. Most interestingly, it is shown for the first time that new spontaneous emission lines are produced from the fine splitting of atomic ground state levels in the isotropic PC case. Quantum interference induces additional narrow spontaneous lines near the transition from the empty upper level to the lower levels.

We present a numerical and theoretical study of intense-field single-electron ionization of helium at 390 nm and 780 nm. Accurate ionization rates (over an intensity range of (0.175–34) × 10¹⁴ W cm⁻², at 390 nm, and (0.275–14.4) × 10¹⁴ W cm⁻² at 780 nm) are obtained from full-dimensionality integrations of the time-dependent helium-laser Schrödinger equation. We show that the power law of lowest order perturbation theory, modified with a ponderomotive-shifted ionization potential, is capable of modelling the ionization rates over an intensity range that extends up to two orders of magnitude higher than that applicable to perturbation theory alone. Writing the modified perturbation theory in terms of scaled wavelength and intensity variables, we obtain to first approximation a single ionization law for both the 390 nm and 780 nm cases. To model the data in the high intensity limit as well as in the low, a new function is introduced for the rate. This function has, in part, a resemblance to that derived from tunnelling theory but, importantly, retains the correct frequency-dependence and scaling behaviour derived from the perturbative-like models at lower intensities. Comparison with the predictions of classical ADK tunnelling theory confirms that ADK performs poorly in the frequency and intensity domain treated here.

The zero-degree ejected-electron spectrum for protons incident on He at 25 keV is examined experimentally using the COLTRIMS technique. The momentum distribution of the emitted electrons for the transfer ionization (TI) reaction channel is measured in coincidence with the momentum vectors of the recoil ion and the scattered projectile. The momentum distribution of the electrons emitted around zero degree in the forward direction for the TI reaction channel shows two prominent structures: the electron-capture-to-the-continuum (ECC) peak and the saddle-point peak. From the measured fully differential electron emission cross sections with respect to the scattering plane we can deduce that the main ECC formation mechanism is electron promotion via quasimolecular orbitals.

Absolute line emission cross sections are presented for 1 keV amu⁻¹ charge-exchange collisions of multiply charged solar wind ions with H₂O. These cross sections can be used to model charge-exchange processes with cometary targets with similar binding energies such as H, O, CO₂ and CO. A parameter-free model is used to successfully predict the recently observed x-ray spectra of comet Linear C/1999 S4 and McNaught-Hartley C/1999 T1. We show that the resulting spectrum is extremely sensitive to the time variations of the solar wind composition.

We propose a new quantum protocol to teleport an arbitrary unknown N-qubit entangled state from a sender to a fixed receiver under the control of M (M < N) controllers. In comparison with other existing protocols, ours is more economical and more feasible. The quantum resource required is just M Greenberger–Horne–Zeilinger trios plus (N − M) Einstein–Podolsky–Rosen pairs. The techniques required are only N Bell measurements by the sender, a von Neumann measurement by a controller and N single-qubit transformations by the receiver. The rule for the receiver to reconstruct the desired state is derived explicitly in the most general case.

We consider a gas of neutral fermionic atoms at ultra-low temperatures, with the attractive interaction tuned to Feshbach resonance. We calculate the variation of the chemical potential and the energy per particle as a function of temperature by assuming the system to be an ideal gas obeying the Haldane–Wu fractional exclusion statistics. Our results for the untrapped gas compare favourably with the recently published Monte Carlo calculations of two groups. For a harmonically trapped gas, the results agree with the experiment, and also with other published works.

We have developed a parameter-free model wavefunction for the muonic molecular ions n₁n₂μ(n_i = p, d, t), which incorporates the correct cusp and coalescence conditions when two particles are close to each other, and the asymptotic property when one of the particles is far away. The predicted values for the energies and other properties of the molecular ions are close to the exact values and generally superior to the values from a 20 parameter, variational wavefunction.

Linear response theories based on the state specific multi-reference coupled electron-pair approximation-like methods (SS-MRCEPA) (Chattopadhyay and Mahapatra 2004 J. Phys. Chem. A 108 11664) (MRCEPA-LRT) have been amply utilized for computing excited-state potential energy surfaces (PES). Our MRCEPA-LRT methods provide energies in a size-intensive manner. As a pilot application, the effectiveness of the method is demonstrated by computing the PES of some low-lying excited states including the ground state of the BeH₂ molecule. This system is very effective and widely used to judge the potentiality of any state-specific theory since its ground state possesses degeneracy/quasi-degeneracy and also faces intruders at different regions of the PES.

A computer code based on the least-squares variational method (LSVM) is developed for the treatment of the collisions of positrons and electrons with diatomic molecules. The code was tested and employed for the investigation of the elastic scattering of positrons and electrons by hydrogen molecules within the framework of the adiabatic approximation. The system is described by a set of prolate spheroidal coordinates which is the most suitable representation of diatomic molecules. Reliable convergence of the scattering phaseshifts has been achieved by optimizing the nonlinear parameters and increasing the number of L² components of the trial wavefunctions. The cross section of the lowest partial wave is calculated at energies varying from 0.136 eV to 13.6 eV, under the assumption that positronium formation, exchange interactions, and all excitation channels are closed. A comparison is presented between our results and those determined by other authors.

Photoionization cross sections for the three alkali dimer cations (Li⁺₂, Na⁺₂ and LiNa⁺) were calculated at the equilibrium internuclear distance for parallel, perpendicular and isotropic orientations of the molecular axis with respect to the field. A model-potential method was used for the description of the cores. The influence of the model-potential parameters on the photoionization spectra was investigated. Two different methods, a time-independent and a time-dependent one, were implemented and used for computing the cross sections.

In this paper, we propose a ghost imaging scheme with third-order correlated thermal light. We show that it is possible to produce the spatial information of an object at two different places in a nonlocal fashion by means of a third-order correlated imaging process with a third-order correlated thermal source and third-order correlation measurement. Concretely, we propose a protocol to create two ghost images at two different places from one object. This protocol involves two optical configurations. We derive the Gaussian thin lens equations and plot the geometrical optics of the ghost imaging processes for the two configurations. It is indicated that third-order correlated ghost imaging with thermal light exhibits richer correlated imaging effects than second-order correlated ghost imaging with thermal light.

We propose a scheme to generate entangled coherent states (ECSs) of two microwave cavity fields coupling to a SQUID-based Cooper pair box (CPB) charge qubit by extending the single-cavity model of coupling between a single microcavity and a CPB charge qubit proposed by Liu et al (2005 Phys. Rev. A 71 063820). We show that by measuring the states of the CPB charge qubit, ECSs of the two cavity fields can be generated with a controllable interaction between the cavities and the CPB charge qubit.

Absolute differential cross sections (DCSs) for elastic electron scattering by a magnesium atom at incident electron energies of 10, 15, 20, 40, 60, 80 and 100 eV have been experimentally derived. The scattered electron intensities were measured over a wide range of scattering angles (θ) from 10° to 150°. The elastic-to-inelastic (3s3p ¹P₁ resonance state) intensity ratios at θ = 10° were measured separately. These ratios and DCSs for the resonance state (by Filipović et al 2006 Int. J. Mass Spectrom.251 66) were applied for the normalization. The absolute DCSs were extrapolated to 0° and 180° and then integrated to yield integral cross sections. The results were analysed and compared with available experimental data and theoretical calculations.

The problem of nonlocality in the dynamical three-body Casimir–Polder interaction between an initially excited and two ground-state atoms is considered. It is shown that the nonlocal spatial correlations of the field emitted by the excited atom during the initial part of its spontaneous decay may become manifest in the three-body interaction. The observability of this new phenomenon is discussed.

The absolute photoabsorption cross-section of cyclopropane has been measured between threshold and 30 eV using monochromated synchrotron radiation. The spectrum is dominated by prominent broad peaks arising from electronic transitions into valence-excited states. Much weaker structure ascribed to Rydberg states associated with excitation from the 3e' or 1a''₂ orbitals is also discernible. Some of these Rydberg states display vibrational progressions which can be correlated with similar structure observed in the corresponding photoelectron spectrum. Ab initio multireference configuration interaction calculations have been performed to obtain excitation energies for valence electron transitions into Rydberg or virtual valence orbitals. These theoretical predictions have enabled assignments to be proposed for most of the observed absorption bands.

The double-electron ejection process after photon absorption by an atom is considered near the inner-atomic shell threshold. The aim of our work is to clarify the dynamics of double-electron emission. For this purpose, it is of primary importance to investigate processes with well-defined final ionic states. The focus of our study in this case is resonant reactions with a low energy for one electron, whereas the energy of the other one is close to the energy of the Auger electron. In this case, important contributions to the cross section of three different processes have to be considered: (1) photoionization of the inner shell followed by Auger decay of the inner vacancy influenced by post-collision interaction (PCI); (2) the PCI capture of the slow photoelectron into a discrete state followed by valence multiplet decay, and (3) the double photoionization of the outer shell. A quantum-mechanical approach has been developed to take into account the contribution to the cross section of amplitudes of all three processes. This model is applied to the case of double photoionization of the Kr atom near the 3d-shell threshold. Our calculation predicts an interference effect in the channel of the ¹D final state, whereas in the ³P final state channel the role of interference is negligible.

We demonstrate electromagnetically induced transparency (EIT) in a sample of rubidium atoms, trapped in an optical dipole trap. Mixing a small amount of σ⁻-polarized light to the weak σ⁺-polarized probe pulses, we are able to measure the absorptive and dispersive properties of the atomic medium at the same time. Features as small as 4 kHz have been detected on an absorption line with 20 MHz line width.

We propose a scheme for generating entanglement of quantum states with continuous variables (coherent states and squeezed vacuum states) of electromagnetical fields. The scheme involves cross-Kerr nonlinearity. It was shown that the cross-Kerr nonlinearity required for generating the superposition and entanglement of squeezed vacuum states is smaller than that required for coherent states. It was also found that the fidelity monotonously decreases with both the increase of the amplitude of the input coherent field and the increase of the deviation of the nonlinear phase shift from π.

We present results of our simulations of the ionization of a hydrogen atom excited to a Rydberg wave packet in the presence of external parallel electric and magnetic fields. This is an example of an open, quantum system whose classical counterpart has been shown to display chaos in the time domain. Within the framework of classical mechanics, electrons escape through chaos induced pulse trains. We reproduce such previously observed signatures of classical chaos in the time-dependent current of ionizing electrons and study the interference effects between the outgoing pulse trains which is absent in the classical picture. Our attempts at manipulating the ionization pulse trains and the effect of core scattering coupled with the chaotic ionization are also discussed. We further investigate the onset of chaos as a function of the scaled energy of the system. We find that for relatively high magnetic fields, quantum-mechanical ionization current shows erratic fluctuations in contrast with the classical current which shows transition to regularity. We conclude that the oscillations result from the decrease in the number of the ionization channels for the higher magnetic field strengths. We further study the time-dependent autocorrelation function and its Fourier transform to look for the effects of the Landau quantization in the photoabsorption spectrum. Our results include calculations via the classical trajectory Monte Carlo method to compare our non-perturbative quantum-mechanical results with the underlying chaotic classical dynamics.

In their recent paper, Joshipura et al (2007 J. Phys. B: At. Mol. Opt. Phys.40 199–210) inadvertently neglected to compare the results of their calculation of integral cross sections for excitation of the sum of all the electronic states in NO to existing experimental data (Brunger et al 2000 J. Phys. B: At. Mol. Opt. Phys.33 809–19). In this comment such a comparison is now made, with the level of agreement between them found to be marginal.

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