Journal of Physics B: Atomic, Molecular and Optical Physics announces the publication of its 2013 Highlights collection.
The Highlights, selected by the journal's Editorial and Advisory Boards, give a taste of the outstanding and excellent research published in the journal in the course of the last calendar year.
At the journal level, the highlight of the year, affecting all our authors and referees, has been the move, mid-year, to ScholarOne, our new Editorial system. Editorially speaking, once again, in 2013, we have had the privilege to work with world class guest editors and published our first invited articles to illustrate our Frontiers of FEL science special issue.
We would like to express our appreciation to all members of the AMO community who have contributed to the great success of the journal.
We hope you will enjoy this new edition of the Highlights.
Isabelle Auffret-Babak
Publisher
All selected Highlights are free to read until the 31 December 2014.
(Please click here to view previous editions of our highlight: 2008,
2009,
2010,
2011 and 2012.)
Multidimensional high harmonic spectroscopy: a semi-classical perspective on measuring multielectron rearrangement upon ionization
Valeria Serbinenko and Olga Smirnova 2013 J. Phys. B: At. Mol. Opt. Phys. 46 171001
High harmonic spectroscopy has the potential to combine attosecond temporal with sub-Angstrom spatial resolution of the early nuclear and multielectron dynamics in molecules. It involves strong-field ionization of the molecule by an infrared (IR) laser field followed by time-delayed recombination of the removed electron with the molecular ion. The time-delay is controlled on the attosecond time scale by the oscillation of the IR field and is mapped into the harmonic number, providing a movie of molecular dynamics between ionization and recombination. One of the challenges in the analysis of a high harmonic signal stems from the fact that the complex dynamics of both ionization and recombination with their multiple observables are entangled in the harmonic signal. Disentangling this information requires a multidimensional approach, capable of mapping ionization and recombination dynamics into different independent parameters. We suggest multidimensional high harmonic spectroscopy as a tool for characterizing ionization and recombination processes separately allowing for simultaneous detection of both the ionization delays and sub-cycle ionization rates. Our method extends the capability of the two-dimensional set-up suggested recently by Shafir et al on reconstructing ionization delays, while keeping the reconstruction procedure as simple as in the original proposal. The scheme is based on the optimization of the high harmonic signal in orthogonally polarized strong fundamental and relatively weak multicolour control fields.
Mechanical memory for photons with orbital angular momentum
H Shi and M Bhattacharya 2013 J. Phys. B: At. Mol. Opt. Phys. 46 151001
We propose to use an acoustic surface wave as a memory for a photon carrying orbital angular momentum. We clarify the physical mechanism that enables the transfer of information, derive the angular momentum selection rule that must be obeyed in the process and show how to optimize the optoacoustic coupling. We theoretically demonstrate that high fidelities can be achieved, using realistic parameters, for the transfer of a coherent optical Laguerre–Gaussian state, associated with large angular momentum, to a mechanical shear mode. Our results add a significant possibility to the ongoing efforts towards the implementation of quantum information processing using photonic orbital angular momentum.
Molecular alignment using circularly polarized laser pulses
C T L Smeenk and P B Corkum 2013 J. Phys. B: At. Mol. Opt. Phys. 46 201001
We show that circularly polarized femtosecond laser pulses produce field-free alignment in linear and planar molecules. We study the rotational wavepacket evolution of O2 and benzene created by circularly polarized light. For benzene, we align the molecular plane to the plane of polarization. For O2, we demonstrate that circular polarization yields a net alignment along the laser propagation axis at certain phases of the evolution. Circular polarization gives us the ability to control alignment of linear molecules outside the plane of polarization, providing new capabilities for molecular imaging.
Quantum reflection from an oscillating surface
Benedikt Herwerth et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 141002
We describe an experimentally realistic situation of the quantum reflection of helium atoms from an oscillating surface. The temporal modulation of the potential induces clear sidebands in the reflection probability as a function of momentum. These sidebands could be exploited to slow down atoms and molecules in the experiment.
Electron elastic back-scattering from aligned CO
molecular ions in the 15–30 eV energy range
C Cornaggia 2013 J. Phys. B: At. Mol. Opt. Phys. 46 191001
Angular distributions of electrons elastically back-scattered from aligned CO
ions are extracted in the 15–30 eV energy range from electron spectra recorded in field-free-aligned CO2 molecules using a femtosecond pump–probe scheme. The angular distributions are found to exhibit a steeper increase as the scattering angle goes from 150° to 180° for molecular ions aligned with the incident electron momentum.
Atomic ionization of hydrogen-like ions by twisted photons: angular distribution of emitted electrons
O Matula et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 205002
We investigate the angular distribution of electrons that are emitted in the ionization of hydrogen-like ions by twisted photons. Analysis is performed based on the first-order perturbation theory and the non-relativistic Schrödinger equation. Special attention is paid to the dependence of the electron emission pattern on the impact parameter b of the ion with respect to the centre of the twisted wave front. In order to explore such a dependence, detailed calculations were carried out for the photoionization of the 1s ground and 2 py excited states of neutral hydrogen atoms. Based on these calculations, we argue that for relatively small impact parameters, the electron angular distributions may be strongly affected by altering the position of the atom within the wave front. In contrast, if the atom is placed far from the front centre, the emission pattern of the electrons is independent of the impact parameter b and resembles that observed in the photoionization by plane wave photons.
Open access
Positron annihilation in small molecules
M Charlton et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 195001
Positron annihilation in room temperature samples of the molecular gases N2, O2, CO, N2O and CH4 has been studied in the density range below 10 amagat using the positron lifetime technique. Careful analyses of the density dependence of the free positron annihilation rates have been performed that have allowed the annihilation parameter, 〈Zeff〉, to be extracted. We compare our results to those in the literature, and give recommended 〈Zeff〉 values from experiment. We have also synthesized the existing data for H2 and derived a recommended value for the annihilation parameter for this molecule.
Open access
Time-dependent calculations of electron energy distribution functions for neon gas in the presence of intense XFEL radiation
J Abdallah Jr et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 235004
Boltzmann electron kinetic simulations are performed to study the time development of the electron energy distribution function (EEDF) in plasma resulting from neon gas subject to a 40 fs x-ray free electron laser radiation source. The simulations are performed without any assumptions of electron temperature. The distributions are calculated as a function of time through 40 fs using Boltzmann kinetics, including the appropriate processes that alter state populations and electron energy. The calculations are also extended into the picosecond regime, after the laser pulse, to track the thermalization of free electrons. Results are presented that predict the evolution of the charge-state distribution, effective temperature, the photon spectrum, as well as the EEDF.
An ab initio study of singlet and triplet Rydberg states of N2
Duncan A Little and Jonathan Tennyson 2013 J. Phys. B: At. Mol. Opt. Phys. 46 145102
Potential energy curves for electronically excited states of molecular nitrogen are calculated using three different ab initio procedures. The most comprehensive of these involves the use of scattering calculations, performed at negative energy using the UK molecular R-matrix method. Such calculations are used to characterize all the Rydberg states of N2 with n ⩽ 6 and ℓ ⩽ 4 as well as many higher states including some Rydberg states associated with the first excited A 2Πu state of N. Many of these states are previously unknown. The calculations are performed at a dense grid of internuclear separations allowing the many avoided crossings present in the system to be mapped out in detail. Extensive comparisons are made with the previously available data for excited states of N2.
Multiphoton core ionization dynamics of polyatomic molecules
Daniele Toffoli and Piero Decleva 2013 J. Phys. B: At. Mol. Opt. Phys. 46 145101
The two-photon core ionization dynamics of gas-phase methane, carbon monoxide and nitrogen have been studied with a recent implementation of the lowest order perturbation theory in the framework of density functional theory and a multicentric basis set expansion of bound and scattering states. Ionization cross sections and angular asymmetry parameters have been calculated for the case of a single radiation beam and for both linear and circular light polarizations in the fixed nuclei approximation. Expected resonances due to core valence excitations enhance the cross section by several orders of magnitude.
Unusual under-threshold ionization of neon clusters studied by ion spectroscopy
K Nagaya et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 164023
We carried out time-of-flight mass spectrometry for neon clusters that were exposed to intense free electron laser pulses with the wavelength of 62 nm, which induce optical transition from the ground state (2s2 2p6) to an excited state (2s2 2p5 nl ) in the Ne atoms. In contrast to Ne+ ions produced by two-photon absorption from isolated Ne atoms, the Ne+ ion yield from Ne clusters shows a linear dependence on the laser intensity (I). We discuss the ionization mechanisms which give the linear behaviour with respect to I and expected features in the electron emission spectrum.
Inner-shell multiple ionization of polyatomic molecules with an intense x-ray free-electron laser studied by coincident ion momentum imaging
B Erk et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 164031
The ionization and fragmentation of two selenium containing hydrocarbon molecules, methylselenol (CH3SeH) and ethylselenol (C2H5SeH), by intense (>1017 W cm−2) 5 fs x-ray pulses with photon energies of 1.7 and 2 keV has been studied by means of coincident ion momentum spectroscopy. Measuring charge states and ion kinetic energies, we find signatures of charge redistribution within the molecular environment. Furthermore, by analyzing fragment ion angular correlations, we can determine the laboratory-frame orientation of individual molecules and thus investigate the fragmentation dynamics in the molecular frame. This allows distinguishing protons originating from different molecular sites along with identifying the reaction channels that lead to their emission.
Electron-impact excitation and ionization of W3+ for the determination of tungsten influx in a fusion plasma
C P Ballance et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 055202
Tungsten will be employed as a plasma facing material in the ITER fusion reactor under construction in Cadarache, France; therefore, there is a significant need for accurate electron-impact excitation and ionization data for the ions of tungsten. We report on the results of extensive calculations of ionization and excitation for W3+ that are intended to provide the atomic data needed for the determination of impurity influx diagnostics of tungsten in several existing tokamak reactors. The electron-impact excitation rate coefficients for this study were determined using the relativistic R-matrix method. The contribution to direct electron-impact ionization was determined using the distorted-wave approximation, the accuracy of which was verified by an R-matrix with pseudo states calculation. Contributions to total ionization from excitation autoionization were also generated from the relativistic R-matrix method. These results were then employed to calculate values of ionization per emitted photon, or SXB ratios, for four carefully selected spectral lines; these data will allow the determination of impurity influx from tungsten facing surfaces. For the range of densities of importance in the edge region of a tokamak reactor, these SXB ratios are found to be nearly independent of electron density but vary significantly with electron temperature.
Electron impact excitation of the lowest doublet and quartet core-excited autoionizing states in Rb atoms
A Borovik et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 015203
Electron impact excitation of the (4p55s2)2P3/2,1/2 and (4p54d5s)4P1/2,3/2,5/2 autoionizing states in rubidium atoms was studied experimentally by measuring the ejected-electron excitation functions and theoretically by employing a fully relativistic Dirac B-spline R-matrix (close-coupling) model. The experimental data were collected in an impact energy range from the respective excitation thresholds up to 50 eV with an incident electron energy resolution of 0.2 eV and an observation angle of 54.7°. Absolute values of the excitation cross sections were obtained by normalizing to the theoretical predictions. The observed near-threshold resonance structures were also analysed by comparison with theory. For the 2P3/2,1/2 doublet states, a detailed analysis of the R-matrix results reveals that the most intense resonances are related to odd-parity negative-ion states with dominant configurations 4p55s5p2 and 4p54d5s6s. The measured excitation functions for the 2P1/2 and 4PJ states indicate a noticeable cascade population due to the radiative decay from high-lying autoionizing states. A comparative analysis with similar data for other alkali atoms is also presented.
Positronium formation in the noble gases
R P McEachran and A D Stauffer 2013 J. Phys. B: At. Mol. Opt. Phys. 46 075203
We present results based upon our ab initio relativistic optical potential method for the simulation of positronium formation in the noble gases neon, argon, krypton and xenon. The method used in this simulation is based upon an approach, first suggested by Reid and Wadehra, whereby the ionization threshold of the atom is effectively reduced by 6.8 eV, the binding energy of positronium in its ground state. This procedure is then modified in order to account for the fact that positronium formation cross sections tend to zero much more rapidly than the corresponding direct ionization cross sections as the energy of the incident positron increases. The agreement of our positronium formation cross sections with experiment is quite good for argon, krypton and xenon over most of the energy region while for neon our cross sections are generally too small at all energies. Nonetheless, our method predicts positronium formation cross sections in better agreement with experiment than the much more sophisticated theoretical approaches used in the past. We also show that adding positronium formation in this way improves agreement with the measured elastic differential cross section at small scattering angles.
Excitation of the six lowest electronic transitions in water by 9–20 eV electrons
K Ralphs et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 125201
We report differential and integral cross sections for excitation of the 3B1, 1B1, 3A2, 1A2, 3A1 and 1A1 states of H2O by 9–20 eV electrons. The measurements are taken by conventional differential electron energy loss spectroscopy techniques, while the calculations employ the Schwinger multichannel method within a ten-channel approximation. The new data are compared with previous experimental and theoretical results. The present measurements and calculations agree reasonably well both with each other and with prior theoretical efforts but show discrepancies with prior measurements. Reasons for those discrepancies are considered and discussed.
Motional, isotope and quadratic Stark effects in Rydberg–Stark deceleration and electric trapping of H and D
S D Hogan et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 045303
Hydrogen and deuterium Rydberg atoms, initially moving at velocities of 600 and 560 m s−1, respectively, in pulsed supersonic beams, have been decelerated and electrostatically trapped following adiabatic 90° deflection from their initial axis of propagation to minimize collisions with the trailing edge of the gas pulses. The time evolution of the potential energy surfaces, over which the atoms undergoing deceleration travel during the trap-loading process, is analogous to that of a moving electrodynamic trap. It has been studied in the laboratory-fixed frame of reference and in the continuously moving frame of reference defined by the instantaneous position of the electric-field minimum around which the atoms are located. The importance of the quadratic Stark effect in the deceleration of samples in Rydberg states with principal quantum numbers above 35 has also been investigated by comparison of experimental results with predictions resulting from the numerical calculation of particle trajectories. The data presented for deuterium atoms represent the first application of Rydberg–Stark deceleration and trapping for this atom. Comparison of the rate of loss of n = 30 H and D atoms from the trap enables one to conclude that it is not affected by the particle dynamics during deceleration and trap loading.
Dynamical probing of a topological phase of bosons in one dimension
Emanuele G Dalla Torre 2013 J. Phys. B: At. Mol. Opt. Phys. 46 085303
We study the linear response to time-dependent probes of a symmetry-protected topological phase of bosons in one dimension, the Haldane insulator (HI). This phase is separated from the ordinary Mott insulator (MI) and density-wave (DW) phases by continuous transitions, whose field theoretical description is reviewed here. Using this technique, we compute the absorption spectrum to two types of periodic perturbations and relate the findings to the nature of the critical excitations at the transition between the different phases. The HI–MI phase transition is topological and the critical excitations possess trivial quantum numbers: they correspond to particles and holes at zero momentum. Our findings are corroborated by a non-local mean-field approach, which allows us to directly relate the predicted spectrum to the known microscopic theory.
Realizing non-Abelian gauge potentials in optical square lattices: an application to atomic Chern insulators
N Goldman et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 134010
We describe a scheme to engineer non-Abelian gauge potentials on a square optical lattice using laser-induced transitions. We emphasize the case of two-electron atoms, where the electronic ground state g is laser-coupled to a metastable state e within a state-dependent optical lattice. In this scheme, the alternating pattern of lattice sites hosting g and e states depicts a chequerboard structure, allowing for laser-assisted tunnelling along both spatial directions. In this configuration, the nuclear spin of the atoms can be viewed as a 'flavour' quantum number undergoing non-Abelian tunnelling along nearest-neighbour links. We show that this technique can be useful to simulate the equivalent of the Haldane quantum Hall model using cold atoms trapped in square optical lattices, offering an interesting route to realize Chern insulators. The emblematic Haldane model is particularly suited to investigate the physics of topological insulators, but requires, in its original form, complex hopping terms beyond nearest-neighbouring sites. In general, this drawback inhibits a direct realization with cold atoms, using standard laser-induced tunnelling techniques. We demonstrate that a simple mapping allows us to express this model in terms of matrix hopping operators that are defined on a standard square lattice. This mapping is investigated for two models that lead to anomalous quantum Hall phases. We discuss the practical implementation of such models, exploiting laser-induced tunnelling methods applied to the chequerboard optical lattice.
Terahertz relativistic spatial solitons in doped graphene metamaterials
H Dong et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 155401
We propose an electrically tunable graphene-based metamaterial that shows a large nonlinear optical response at THz frequencies. The responsible nonlinearity comes from the intraband current, which we are able to calculate analytically. We demonstrate that the proposed metamaterial supports stable 2D spatial solitary waves. Our theoretical approach is not restricted to graphene, but can be applied to all materials exhibiting a conical dispersion supporting massless Dirac fermions.
Interface localization of light in disordered photonic lattices
Dragana Jović 2013 J. Phys. B: At. Mol. Opt. Phys. 46 145401
The Anderson localization of light at the interface separating square and hexagonal photonic lattices is demonstrated numerically. The influence of varying lattice intensities and disorder level on the transverse localization of light is discussed. Both suppression and enhancement of light localization in the presence of the interface, depending on the difference in the lattice intensity in both regions, and the position of the excited lattice site are demonstrated. Such localization is compared to the cases with no interfaces. Also, it is analysed how the presence of a phase-slip defect modifies the phenomenon of Anderson localization of light at the interface.
Open access
Coulomb explosion of diatomic molecules in intense XUV fields mapped by partial covariance
O Kornilov et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 164028
Single-shot time-of-flight spectra for Coulomb explosion of N2 and I2 molecules have been recorded at the Free Electron LASer in Hamburg (FLASH) and have been analysed using a partial covariance mapping technique. The partial covariance analysis unravels a detailed picture of all significant Coulomb explosion pathways, extending up to the N4+–N5+ channel for nitrogen and up to the I8+–I9+ channel for iodine. The observation of the latter channel is unexpected if only sequential ionization processes from the ground state ions are considered. The maximum kinetic energy release extracted from the covariance maps for each dissociation channel shows that Coulomb explosion of nitrogen molecules proceeds much faster than that of the iodine. The N2 ionization dynamics is modelled using classical trajectory simulations in good agreement with the outcome of the experiments. The results suggest that covariance mapping of the Coulomb explosion can be used to measure the intensity and pulse duration of free-electron lasers.
Open access
Orientation determination in single-particle x-ray coherent diffraction imaging experiments
O M Yefanov and I A Vartanyants 2013 J. Phys. B: At. Mol. Opt. Phys. 46 164013
Single-particle diffraction imaging experiments at free-electron lasers (FELs) have a great potential for the structure determination of reproducible biological specimens that cannot be crystallized. One of the challenges in processing the data from such an experiment is to determine the correct orientation of each diffraction pattern from samples randomly injected in the FEL beam. We propose an algorithm (Yefanov et al 2010 Photon Science—HASYLAB Annual Report) that can solve this problem and can be applied to samples from tens of nanometres to microns in size, measured with sub-nanometre resolution in the presence of noise. This is achieved by the simultaneous analysis of a large number of diffraction patterns corresponding to different orientations of the particles. The algorithm's efficiency is demonstrated for two biological samples, an artificial protein structure without any symmetry and a virus with icosahedral symmetry. Both structures are a few tens of nanometres in size and consist of more than 100 000 non-hydrogen atoms. More than 10 000 diffraction patterns with Poisson noise were simulated and analysed for each structure. Our simulations indicate the possibility of achieving resolution of about 3.3 Å at 3 Å wavelength and incoming flux of 1012 photons per pulse focused to 100×100 nm2.
Antibunching photons in a cavity coupled to an optomechanical system
Xun-Wei Xu and Yuan-Jie Li 2013 J. Phys. B: At. Mol. Opt. Phys. 46 035502
We study the photon statistics of a cavity linearly coupled to an optomechanical system via second-order correlation functions. Our calculations show that the cavity can exhibit strong photon antibunching even when optomechanical interaction in the optomechanical system is weak. The cooperation between the weak optomechanical interaction and the destructive interference between different paths for two-photon excitation leads to the efficient antibunching effect. Compared with the standard optomechanical system, the coupling between a cavity and an optomechanical system provides a method to relax the constraints to obtain a single photon by optomechanical interaction.
Unitary quantum phase operators for bosons and fermions: a model study on the quantum phases of interacting particles in a symmetric double-well potential
Biswajit Das et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 035501
We introduce unitary quantum phase operators for material particles. We carry out a model study on quantum phases of interacting bosons in a symmetric double-well potential in terms of unitary and commonly used non-unitary phase operators and compare the results for a different number of bosons. We find that the results for unitary quantum phase operators are significantly different from those for non-unitary ones especially in the case of a low number of bosons. We introduce unitary operators corresponding to the quantum phase-difference between two single-particle states of fermions. As an application of fermionic phase operators, we study a simple model of a pair of interacting two-component fermions in a symmetric double-well potential. We also investigate quantum phase and number fluctuations to ascertain number-phase uncertainty in terms of unitary phase operators.
The minimum-uncertainty squeezed states for atoms and photons in a cavity
Sergey I Kryuchkov et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 104007
We describe a multi-parameter family of the minimum-uncertainty squeezed states for the harmonic oscillator in nonrelativistic quantum mechanics. They are derived by the action of the corresponding maximal kinematical invariance group on the standard ground state solution. We show that the product of the variances attains the required minimum value 1/4 only at the instances that one variance is a minimum and the other is a maximum, when the squeezing of one of the variances occurs. The generalized coherent states are explicitly constructed and their Wigner function is studied. The overlap coefficients between the squeezed, or generalized harmonic, and the Fock states are explicitly evaluated in terms of hypergeometric functions and the corresponding photon statistics are discussed. Some applications to quantum optics, cavity quantum electrodynamics and superfocusing in channelling scattering are mentioned. Explicit solutions of the Heisenberg equations for radiation field operators with squeezing are found.
Optical bistability in strong-coupling cavity QED with a few atoms
A Dombi et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 224010
We present exact numerical solutions of the damped-driven Jaynes–Cummings model adapted to describe absorptive optical bistability in the limit of a few atoms strongly coupled to a high-finesse resonator. We show that the simplifying semiclassical result for many physical quantities of interest is well reproduced by the quantum model including even with only a few atoms in the strongly coupled system. Non-trivial atom–field quantum cross-correlations show up in the strong-driving limit.
Comparison of different approaches to the longitudinal momentum spread after tunnel ionization
C Hofmann et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 125601
We introduce a method to investigate the longitudinal momentum spread resulting from strong-field tunnel ionization of helium which, unlike other methods, is valid for all ellipticities of laser pulse. Semiclassical models consisting of tunnel ionization followed by classical propagation in the combined ion and laser field reproduce the experimental results if an initial longitudinal spread at the tunnel exit is included. The values for this spread are found to be of the order of twice the transverse momentum spread.
Attosecond synchronization of terahertz wave and high-harmonics
Zhihui Lü et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 155602
Argon atom ionizing in the ultrafast two-colour field has been used to investigate the physical mechanism of terahertz (THz) generation. We measured simultaneously pulse energies of THz and high-harmonics as the phase delay between the fundamental and its second harmonic was varied. The optimal phase delay of THz generation is determined according to the inherent attochirp of the emitted high-harmonics. The dynamic analysis of the tunnelling electron wave packet driven by the Coulomb–laser coupling shows that laser-assisted soft collisions of the electron wave packet with the atomic core play a key role. It is demonstrated that the rescattering process, being indispensable in high-harmonic generation processes, dominates THz wave generation as well in a more elaborate way.
Influence of the partial temporal coherence of short FEL pulses on two-colour photoionization and photoinduced Auger decay of atoms
A K Kazansky et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 025601
The influence of the partial temporal coherence of free electron laser (FEL) radiation on the sidebands arising in the electron spectra of laser-assisted photoionization and photoinduced Auger decay of atoms is theoretically analysed. A simple model is developed which describes the inner-shell photoionization by a short (femtosecond) FEL pulse and the following Auger decay in a strong field of an infrared laser. The model is based on the time-dependent approach and uses the strong field approximation for both photo- and Auger electrons. Particular calculations have been carried out for Ne 1s photoionization and KLL Auger emission. We demonstrate that the temporal coherence of FEL pulses influences the line widths in the photoelectron spectrum. For a small coherence time the sidebands in this spectrum cannot be resolved. On the other hand, our calculations show that in the Auger electron spectrum the sidebands are practically independent of the coherence time of the ionizing pulse.
Carrier–envelope phase effects on ionization dynamics of atomic clusters irradiated by intense laser pulses of a few cycles
Gaurav Mishra and N K Gupta 2013 J. Phys. B: At. Mol. Opt. Phys. 46 125602
A three-dimensional molecular dynamic approach is employed to investigate the ionization dynamics of small Xe400 clusters irradiated by intense lasers (I = 1016W cm−2) in the near infrared wavelength region (λ = 800 nm). The pulse duration of the incident laser is varied from a few cycles (τ = 2T0 with T0 as one laser time cycle) to many cycles (τ = 8T0). In the case of pulse durations of a few cycles, the carrier–envelope (CE) phase of the incident laser electric field is found to be an important parameter that affects the ionization dynamics of Xe clusters. The fractions of various ionized Xe species are observed to be different for the two values of the CE phases (ϕ = 0 and ϕ = π/2) in the case of the shorter laser pulse duration of τ = 2T0. The nature of the instantaneous electric field (the rising or falling edge of the field) at the time of birth of the electron due to ionization decides the extent of ionization. The difference in time-evolution of the electric field for the two values of the CE phase leads to an observable disparity in the yield of various ionic species. For the case of a pulse duration of many cycles (τ = 8T0), these differences average out and we do not observe any change in the yield of various ionic species for the two values of the CE phase.
Does atomic polarizability play a role in hydrogen radio recombination spectra from Galactic H II regions?*
J D Hey 2013 J. Phys. B: At. Mol. Opt. Phys. 46 175702
Since highly excited atoms, which contribute to the radio recombination spectra from Galactic H II regions, possess large polarizabilities, their lifetimes are influenced by ion (proton)–induced dipole collisions. It is shown that, while these ion–radiator collisional processes, if acting alone, would effectively limit the upper principal quantum number attainable for given plasma parameters, their influence is small relative to that of electron impacts within the framework of line broadening theory. The present work suggests that ion–permanent dipole interactions (Hey et al 2004 J. Phys. B: At. Mol. Opt. Phys. 37 2543) would also be of minor importance in limiting the occupation of highly excited states. On the other hand, the ion–induced dipole collisions are essential for ensuring equipartition of energy between atomic and electron kinetic distributions (Hey et al 1999 J. Phys. B: At. Mol. Opt. Phys. 32 3555; 2005 J. Phys. B: At. Mol. Opt. Phys. 38 3517), without which Voigt profile analysis to extract impact broadening widths would not be possible. Electron densities deduced from electron impact broadening of individual lines (Griem 1967 Astrophys. J. 148 547; Watson 2006 J. Phys. B: At. Mol. Opt. Phys. 39 1889) may be used to check the significance of the constraints arising from the present analysis. The spectra of Bell et al (2000 Publ. Astron. Soc. Pac. 112 1236; 2011 Astrophys. Space Sci. 333 377; 2011 Astrophys. Space Sci. 335 451) for Orion A and W51 in the vicinity of 6.0 and 17.6 GHz are examined in this context, and also in terms of a possible role of the background ion microfield in reducing the near-elastic contributions to the electron impact broadening below the predictions of theory (Hey 2012 J. Phys. B: At. Mol. Opt. Phys. 45 065701). These spectra are analysed, subject to the constraint that calculated relative intensities of lines, arising from upper states in collisional–radiative equilibrium, should be consistent with those obtained from Voigt profile analysis. It is shown that the experimental technique yields an excellent temperature diagnostic for the H II regions. On the other hand, strong evidence is not obtained, from those spectra which satisfy the above constraint on intensity, to indicate that the electron impact broadening theory requires substantial correction. The main grounds for attempting a revision of theory to allow for the influence of the ion microfield during the scattering processes on the upper and lower states of each line thus still appear to have a stronger theoretical (Hey 2007 J. Phys. B: At. Mol. Opt. Phys. 40 4077) than experimental basis.
K-shell photoionization of Be-like and Li-like ions of atomic nitrogen: experiment and theory
M M Al Shorman et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 195701
Absolute cross sections for the K-shell photoionization of Be-like and Li-like atomic nitrogen ions were measured by employing the ion–photon merged-beam technique at the SOLEIL synchrotron radiation facility in Saint-Aubin, France. High-resolution spectroscopy at nominal resolutions of 38, 56, 111, 133 meV full width at half maximum (FWHM) for Be-like and 125 meV FWHM for Li-like atomic nitrogen ions was achieved for the photon energies ranging from 410 up to 460 eV. The experimental measurements are compared with theoretical estimates from the multi-configuration Dirac–Fock, R-matrix and an empirical method. The interplay between experiment and theory enabled the identification and characterization of the strong 1s → 2p resonances features observed in the K-shell spectra of each ion and the region around 460 eV for the 1s → 3p resonance of the N3 + ion yielding suitable agreement with experiment.
Ne IX line G-ratio in a non-Maxwellian and anisotropic plasma
A K Ferouani et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 085701
We have theoretically studied how the presence of a small proportion of energetic beam electrons mixed to a bulk of Maxwellian electrons in a hot plasma affects the temperature-dependent intensity ratio G = (x + y + z)/w of the helium-like triplet intercombination (x, y) and forbidden (z) lines to the singlet resonance line (w). By modelling the electron distribution function as a combination of a Maxwellian isotropic component and a monoenergetic beam component, detailed calculations of the G ratio of the Ne8 + lines have been performed for temperatures Te of the Maxwellian component and kinetic energies e0 of the beam component in the ranges 106–107 K and 1.5–25 keV, respectively. A magnetic sublevel-to-magnetic sublevel collisional-radiative model has been used for determining the populations of the upper magnetic sublevels of the four lines at an electron density below 1013 cm−3. Excitations from the ground 1s2 1S0 and metastable 1s2s 3S1 magnetic sublevels to the 1snl (n = 2–4) magnetic sublevels as well as the inner-shell ionization of the lithium-like ion in its ground level were taken into account. All basic atomic data, including the radiative transition probabilities and the collisional excitation and ionization cross sections, were computed using the flexible atomic code. It is found that the contribution of a 5% fraction of the beam component can reduce the G ratio by a factor of 30 at Te = 106 K and of 2.4 at Te = 3 × 106 K. Our calculations also indicate that the effect of directionality of the beam component on G is negligible for e0 above ∼10 keV and that for a given Te, G is practically insensitive to variations in e0 above ∼7 keV.
Macroscopic quantum mechanics: theory and experimental concepts of optomechanics
Yanbei Chen 2013 J. Phys. B: At. Mol. Opt. Phys. 46 104001
Rapid experimental progress has recently allowed the use of light to prepare macroscopic mechanical objects into nearly pure quantum states. This research field of quantum optomechanics opens new doors towards testing quantum mechanics, and possibly other laws of physics, in new regimes. In the first part of this article, I will review a set of techniques of quantum measurement theory that are often used to analyse quantum optomechanical systems. Some of these techniques were originally designed to analyse how a classical driving force passes through a quantum system, and can eventually be detected with an optimal signal-to-noise ratio—while others focus more on the quantum-state evolution of a mechanical object under continuous monitoring. In the second part of this article, I will review a set of experimental concepts that will demonstrate quantum mechanical behaviour of macroscopic objects—quantum entanglement, quantum teleportation and the quantum Zeno effect. Taking the interplay between gravity and quantum mechanics as an example, I will review a set of speculations on how quantum mechanics can be modified for macroscopic objects, and how these speculations—and their generalizations—might be tested by optomechanics.
The B-spline R-matrix method for atomic processes: application to atomic structure, electron collisions and photoionization
Oleg Zatsarinny and Klaus Bartschat 2013 J. Phys. B: At. Mol. Opt. Phys. 46 112001
The basic ideas of the B-spline R-matrix (BSR) approach are reviewed, and the use of the method is illustrated with a variety of applications to atomic structure, electron–atom collisions and photo-induced processes. Special emphasis is placed on complex, open-shell targets, for which the method has proven very successful in reproducing, for example, a wealth of near-threshold resonance structures. Recent extensions to a fully relativistic framework and intermediate energies have allowed for an accurate treatment of heavy targets as well as a fully nonperturbative scheme for electron-impact ionization. Finally, field-free BSR Hamiltonian and electric dipole matrices can be employed in the time-dependent treatment of intense short-pulse laser–atom interactions.
Unconventional states of bosons with the synthetic spin–orbit coupling
Xiangfa Zhou et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 134001
The spin–orbit coupling with bosons gives rise to novel properties that are absent in usual bosonic systems. Under very general conditions, the conventional ground state wavefunctions of bosons are constrained by the 'no-node' theorem to be positive definite. In contrast, the linear dependence of the spin–orbit coupling leads to complex-valued condensate wavefunctions beyond this theorem. In this paper, we review the study of this class of unconventional Bose–Einstein condensations focusing on their topological properties. Both the 2D Rashba and 3D -type Weyl spin–orbit couplings give rise to Landau-level-like quantization of single-particle levels in the harmonic trap. Interacting condensates develop the half-quantum vortex structure spontaneously breaking the time-reversal symmetry and exhibit topological spin textures of the skyrmion type. In particular, the 3D Weyl coupling generates topological defects in the quaternionic phase space as an SU(2) generalization of the usual U(1) vortices. Rotating spin–orbit-coupled condensates exhibit rich vortex structures due to the interplay between vorticity and spin texture. In the Mott-insulating states in optical lattices, quantum magnetism is characterized by the Dzyaloshinskii–Moriya-type exchange interactions.
Accelerator- and laser-based sources of high-field terahertz pulses
Nikola Stojanovic and Markus Drescher 2013 J. Phys. B: At. Mol. Opt. Phys. 46 192001
At present we are witnessing a rapid development of sources for terahertz (THz) pulses with very strong electromagnetic fields. These pulses are reaching a stage where they can be used to not only probe, but also uniquely control a variety of processes that range from fundamental dynamics in individual atoms and molecules, through phase transitions in solids to a wealth of interactions in biological materials. In this review, we are presenting an overview of two major directions in the generation of such radiation. Large-scale accelerator-based sources offer unprecedented pulse energies coupled with a wide tuning range and extreme repetition rates. Laser-based sources, on the other hand, are laboratory-scale instruments and thus are very attractive in their availability to the wide scientific community. The capabilities of different variants of these THz sources are evaluated and compared with each other. In addition, powerful techniques for the temporal characterization of THz pulses are discussed.
Atoms and molecules in intense laser fields: gauge invariance of theory and models
A D Bandrauk et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 153001
Gauge invariance was discovered in the development of classical electromagnetism and was required when the latter was formulated in terms of the scalar and vector potentials. It is now considered to be a fundamental principle of nature, stating that different forms of these potentials yield the same physical description: they describe the same electromagnetic field as long as they are related to each other by gauge transformations. Gauge invariance can also be included into the quantum description of matter interacting with an electromagnetic field by assuming that the wavefunction transforms under a given local unitary transformation. The result of this procedure is a quantum theory describing the coupling of electrons, nuclei and photons. Therefore, it is a very important concept: it is used in almost every field of physics and it has been generalized to describe electroweak and strong interactions in the standard model of particles. A review of quantum mechanical gauge invariance and general unitary transformations is presented for atoms and molecules in interaction with intense short laser pulses, spanning the perturbative to highly nonlinear non-perturbative interaction regimes. Various unitary transformations for a single spinless particle time-dependent Schrödinger equation (TDSE) are shown to correspond to different time-dependent Hamiltonians and wavefunctions. Accuracy of approximation methods involved in solutions of TDSEs such as perturbation theory and popular numerical methods depend on gauge or representation choices which can be more convenient due to faster convergence criteria. We focus on three main representations: length and velocity gauges, in addition to the acceleration form which is not a gauge, to describe perturbative and non-perturbative radiative interactions. Numerical schemes for solving TDSEs in different representations are also discussed. A final brief discussion of these issues for the relativistic time-dependent Dirac equation for future super-intense laser field problems is presented.
A quantum information approach to statistical mechanics
Gemma De las Cuevas 2013 J. Phys. B: At. Mol. Opt. Phys. 46 243001
We review some connections between quantum information and statistical mechanics. We focus on three sets of results for classical spin models. First, we show that the partition function of all classical spin models (including models in different dimensions, different types of many-body interactions, different symmetries, etc) can be mapped to the partition function of a single model. Second, we give efficient quantum algorithms to estimate the partition function of various classical spin models, such as the Ising or the Potts model. The proofs of these two results are based on a mapping from partition functions to quantum states and to quantum circuits, respectively. Finally, we show how classical spin models can be used to describe certain fluctuating lattices appearing in models of discrete quantum gravity.
Compact XFEL and AMO sciences: SACLA and SCSS
M Yabashi et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 164001
The concept, design and performance of Japan's compact free-electron laser (FEL) facilities, the SPring-8 Compact SASE Source test accelerator (SCSS) and SPring-8 Angstrom Compact free electron LAser (SACLA), and their applications in mainly atomic, molecular and optical science are reviewed. At SCSS, intense, ultrafast FEL pulses at extreme ultraviolet (EUV) wavelengths have been utilized for investigating various multi-photon processes in atoms, molecules and clusters by means of ion and electron spectroscopy. The quantum optical effect superfluorescence has been observed with EUV excitation. A pump–probe technique combining FEL pulses with near infrared laser pulses has been realized to study the ultrafast dynamics of atoms, molecules and clusters in the sub-picosecond regime. At SACLA, deep inner-shell multi-photon ionization by intense x-ray FEL pulses has been investigated. The development of seeded FEL sources for producing transversely and temporally coherent light, as well as the expected impact on advanced science are discussed.
AMO science at the FLASH and European XFEL free-electron laser facilities
J Feldhaus et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 164002
Present performance and future development of the free-electron lasers (FELs) in Hamburg—FLASH for the extreme ultraviolet and the European XFEL for the soft and hard x-ray regimes—are presented. As an illustration of the unprecedented characteristics of these sources a few recent examples of experiments performed in the area of atomic, molecular and optical (AMO) physics are described. The results highlight in particular the available high photon intensities, the short pulse durations and the coherence of the FEL beam. Nonlinear processes involving for the first time inner-shell electrons, time-resolved experiments on the few femtosecond timescales, and imaging experiments on small particles have been the focus of these studies, demonstrating the unique potential of short-wavelength FELs and pointing to numerous exciting future opportunities.
Ultra-fast and ultra-intense x-ray sciences: first results from the Linac Coherent Light Source free-electron laser
C Bostedt et al 2013 J. Phys. B: At. Mol. Opt. Phys. 46 164003
X-ray free-electron lasers (FELs) produce femtosecond x-ray pulses with unprecedented intensities that are uniquely suited for studying many phenomena in atomic, molecular, and optical (AMO) physics. A compilation of the current developments at the Linac Coherent Light Source (LCLS) and future plans for the LCLS-II and Next Generation Light Source (NGLS) are outlined. The AMO instrumentation at LCLS and its performance parameters are summarized. A few selected experiments representing the rapidly developing field of ultra-fast and peak intensity x-ray AMO sciences are discussed. These examples include fundamental aspects of intense x-ray interaction with atoms, nonlinear atomic physics in the x-ray regime, double core-hole spectroscopy, quantum control experiments with FELs and ultra-fast x-ray induced dynamics in clusters. These experiments illustrate the fundamental aspects of the interaction of intense short pulses of x-rays with atoms, molecules and clusters that are probed by electron and ion spectroscopies as well as ultra-fast x-ray scattering.
ions are extracted in the 15–30 eV energy range from electron spectra recorded in field-free-aligned CO2 molecules using a femtosecond pump–probe scheme. The angular distributions are found to exhibit a steeper increase as the scattering angle goes from 150° to 180° for molecular ions aligned with the incident electron momentum.
