Journal of Physics B: Atomic, Molecular and Optical Physics

Highlights of 2012

Journal of Physics B: Atomic, Molecular and Optical Physics presents to you the Highlights of 2012. The Highlights, selected by the journal's Editorial and Advisory Boards, give a taste of the outstanding and excellent research published.

2012 has seen many advances for the journal, with the launch of the first video abstract, publishing the 25th tutorial, as well as publishing the first open access paper submitted to the journal- to name a few.

Alongside the many excellent tutorials, topical reviews and popular special issues published in Journal of Physics B, it is your research which forms the core of the journal's success and for this, we would like to thank you.

View also the 2008, 2009, 2010 and 2011 Highlights, and access the 2012 brochure.

Isabelle Auffret-Babak,
Publisher

All selected Highlights are free to read until the 31 December 2013.

Atomic physics Show article list

Quantum phase-space analysis of electronic rescattering dynamics in intense few-cycle laser fields

Stefanie Gräfe et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 055002

View article, Quantum phase-space analysis of electronic rescattering dynamics in intense few-cycle laser fields PDF, Quantum phase-space analysis of electronic rescattering dynamics in intense few-cycle laser fields

We present a quantum phase-space analysis of the ionization and rescattering dynamics of a one-electron atom in an intense few-cycle laser field. Snapshots of the Wigner function W(x, p) will be analysed for both soft-core Coulomb and short-ranged potentials tuned to yield identical ground state energies and similar energies for the first lowest electronic states. The influence of the long-range Coulomb potential on the rescattering wavepacket in the continuum can be disentangled from that of the induced bound-state polarization. Short- and long-range atomic potentials entail differences in the rescattering dynamics, manifesting themselves in high-order harmonic generation spectra.

https://doi.org/10.1088/0953-4075/45/5/055002

Calculated and measured angular correlation between photoelectrons and Auger electrons from K-shell ionization

F Robicheaux et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 175001

View article, Calculated and measured angular correlation between photoelectrons and Auger electrons from K-shell ionization PDF, Calculated and measured angular correlation between photoelectrons and Auger electrons from K-shell ionization

We have applied a recently developed computational method to an experimental puzzle that involves a slow outgoing electron that is scattered by a high-energy Auger electron. Although the experiment seemed to be in a regime accurately described by classical mechanics, such classical calculations could not accurately model the angular distribution of the electron pair. Using the wavefunction from our calculations to generate the energy and angular distributions of the two electrons, we have compared our results to measurements performed at the Advanced Light Source. We have obtained good agreement between the experiment and our quantum results, attributing the poor classical result to the small number of angular momenta in the wavefunction. We have included predictions on how measurements depend on the Auger energy and/or the photoelectron energy.

https://doi.org/10.1088/0953-4075/45/17/175001

Long-range Rydberg–Rydberg interactions in calcium, strontium and ytterbium

C L Vaillant et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 135004

View article, Long-range Rydberg–Rydberg interactions in calcium, strontium and ytterbium PDF, Long-range Rydberg–Rydberg interactions in calcium, strontium and ytterbium

Long-range dipole–dipole and quadrupole–quadrupole interactions between pairs of Rydberg atoms are calculated perturbatively for calcium, strontium and ytterbium within the Coulomb approximation. Quantum defects, obtained by fitting existing laser spectroscopic data, are provided for all S, P, D and F series of strontium and for the ³P₂ series of calcium. The results show qualitative differences with the alkali metal atoms, including isotropically attractive interactions of the strontium ¹S₀ states and a greater rarity of Förster resonances. Only two such resonances are identified, both in triplet series of strontium. The angular dependence of the long-range interaction is briefly discussed.

https://doi.org/10.1088/0953-4075/45/13/135004

Analysis of tungsten laser produced plasmas in the extreme ultraviolet (EUV) spectral region

Colm S Harte et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 205002

View article, Analysis of tungsten laser produced plasmas in the extreme ultraviolet (EUV) spectral region PDF, Analysis of tungsten laser produced plasmas in the extreme ultraviolet (EUV) spectral region

Tungsten will be used as a wall material in ITER and therefore will be present as an intrinsic plasma impurity with the resulting emission having the potential to be used as a plasma diagnostic. We have recorded spectra of tungsten laser produced plasmas in the 1–7 nm region using Nd:YAG lasers operating at a range of power densities. We have analysed these spectra, giving special attention to the unresolved transition arrays in the 3 nm region that appear at the highest laser power densities. We compare our results to those from previous work and also use new atomic structure calculations to identify a number of new features.

https://doi.org/10.1088/0953-4075/45/20/205002

Investigation of the electromagnetically induced transparency in the era of cosmological hydrogen recombination

D Solovyev et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 215001

View article, Investigation of the electromagnetically induced transparency in the era of cosmological hydrogen recombination PDF, Investigation of the electromagnetically induced transparency in the era of cosmological hydrogen recombination

Investigation of cosmic microwave background formation processes is one of the most compelling problems at the present time. In this paper we analyse the response of the hydrogen atom to external photon fields. Field characteristics are defined via conditions corresponding to the recombination era of the universe. Approximation of the three-level atom is used to describe the 'atom–field' interaction. It is found that the phenomena of the electromagnetically induced transparency (EIT) take place in this case. Consideration of EIT phenomena makes it necessary to update the astrophysical description of the processes of cosmic microwave background formation and, in particular, the Sobolev escape probability. Additional terms to the optical depth entering in the Sobolev escape probability are found to contribute on the level about 1%.

https://doi.org/10.1088/0953-4075/45/21/215001

Molecular and cluster structure, properties and dynamics Show article list

Orientation-dependent ionization yields from strong-field ionization of fixed-in-space linear and asymmetric top molecules

Jonas L Hansen et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 015101

View article, Orientation-dependent ionization yields from strong-field ionization of fixed-in-space linear and asymmetric top molecules PDF, Orientation-dependent ionization yields from strong-field ionization of fixed-in-space linear and asymmetric top molecules

The yield of strong-field ionization, by a linearly polarized probe pulse, is studied experimentally and theoretically as a function of the relative orientation between the laser field and the molecule. Experimentally, carbonyl sulphide (OCS), benzonitrile and naphthalene molecules are aligned in one or three dimensions before being singly ionized by a 30 fs laser pulse centred at 800 nm. Theoretically, we address the behaviour of these three molecules. We consider the degree of alignment and orientation and model the angular dependence of the total ionization yield by molecular tunnelling theory accounting for the Stark shift of the energy level of the ionizing orbital. For naphthalene and benzonitrile, the orientational dependence of the ionization yield agrees well with the calculated results, in particular, we observe that ionization is maximized when the probe laser is polarized along the most polarizable axis. For OCS the observation of the maximum ionization yield when the probe is perpendicular to the internuclear axis contrasts the theoretical results.

https://doi.org/10.1088/0953-4075/45/1/015101

Ionization dynamics in expanding clusters studied by XUV pump–probe spectroscopy

M Krikunova et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 105101

View article, Ionization dynamics in expanding clusters studied by XUV pump–probe spectroscopy PDF, Ionization dynamics in expanding clusters studied by XUV pump–probe spectroscopy

The expansion and disintegration dynamics of xenon clusters initiated by the ionization with femtosecond soft x-ray extreme ultraviolet (XUV) pulses were studied with pump–probe spectroscopy using the autocorrelator setup of the Free-Electron LASer in Hamburg (FLASH) facility. The ionization by the first XUV pulse of 92 eV photon energy (8 × 10¹² W cm⁻²) leads to the generation of a large number of quasi-free electrons trapped by the space charge of the cluster ions. A temporally delayed, more intense probe (4 × 10¹³ W cm⁻²) pulse substantially increases a population of nanoplasma electrons providing a way of probing plasma states in the expanding cluster by tracing the average charge of fragment ions. The results of the study reveal a timescale for cluster expansion and disintegration, which depends essentially on the initial cluster size. The average charge state of fragment ions, and thus the cluster plasma changes significantly on a timescale of 1–3 ps.

https://doi.org/10.1088/0953-4075/45/10/105101

Light-induced conical intersections for short and long laser pulses: Floquet and rotating wave approximations versus numerical exact results

Gábor J Halász et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 135101

View article, Light-induced conical intersections for short and long laser pulses: Floquet and rotating wave approximations versus numerical exact results PDF, Light-induced conical intersections for short and long laser pulses: Floquet and rotating wave approximations versus numerical exact results

It has recently been shown that dressing of diatomic molecules by standing or by running laser waves can give rise to the appearance of the so-called light-induced conical intersections (LICIs). The effect of these LICIs on different physical properties of the diatomic molecules has been demonstrated in several publications [1–6]. In the majority of these works, the sodium dimer was chosen as an explicit showcase example and the Floquet picture was used to describe the nuclear Hamiltonian. This representation of the Hamiltonian is very illustrative and helps to understand the essence of the light-induced nonadiabatic effects. However, the natural question arises: what are the limits of the Floquet approximation? In this paper, the performance of the 2×2 Floquet Hamiltonian in the space of the ground and resonantly excited molecular electronic states is compared to that of the time-dependent exact Hamiltonian in the same space. For the latter case, we also present results employing the popular rotating wave approximation. To carry out the comparisons, different physical properties—-autocorrelation function, excited state diabatic populations and molecular alignment—have been computed.

https://doi.org/10.1088/0953-4075/45/13/135101

Probing the dynamics of dissociation of methane following core ionization using three-dimensional molecular-frame photoelectron angular distributions

J B Williams et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 194003

View article, Probing the dynamics of dissociation of methane following core ionization using three-dimensional molecular-frame photoelectron angular distributions PDF, Probing the dynamics of dissociation of methane following core ionization using three-dimensional molecular-frame photoelectron angular distributions

We present experimental measurements and theoretical calculations for the photoionization of CH₄ at the carbon K-edge. Measurements performed using cold target recoil ion momentum spectroscopy (COLTRIMS) combined with complex Kohn variational calculations of the photoelectron in the molecular frame demonstrate the surprising result that the low energy photoelectrons effectively image the molecule by emerging along the bond axes. Furthermore, we observe a dynamic breakdown of axial recoil behaviour in one of the dissociation pathways of the intermediate dication, which we interpret using electronic structure calculations.

https://doi.org/10.1088/0953-4075/45/19/194003

Atomic and molecular collisions Show article list

Investigation of slow collisions for (quasi) symmetric heavy systems: what can be extracted from high resolution x-ray spectra

M Trassinelli et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 085202

View article, Investigation of slow collisions for (quasi) symmetric heavy systems: what can be extracted from high resolution x-ray spectra PDF, Investigation of slow collisions for (quasi) symmetric heavy systems: what can be extracted from high resolution x-ray spectra

We present a new experiment on (quasi) symmetric collision systems at low velocity, namely Ar^{17 +} ions (v = 0.53 au) on gaseous Ar and N₂ targets, using low- and high-resolution x-ray spectroscopy. Thanks to an accurate efficiency calibration of the spectrometers, we extract absolute x-ray emission cross sections combining low-resolution x-ray spectroscopy and a complete determination of the ion beam–gas jet target overlap. Values with improved uncertainty are found in agreement with previous results (Tawara et al 2001 Phys. Rev. A 64 042712). Resolving the whole He-like Ar^{16 +} Lyman series from n = 2–10 with our crystal spectrometer enables us to determine precisely the distribution $\lbrace \mathcal {P}_n\rbrace$ of the electron capture probability and the preferential n_pref level of the selective single-electron capture. Evaluation of cross sections for this process as well as for the contribution of multiple-capture is carried out. Their sensitivity to the ℓ-distribution of n levels populated by single-electron capture is clearly demonstrated, providing a stringent benchmark for theories. In addition, the hardness ratio is extracted and the influence of the decay of the metastable 1s2s ³S₁ state on this ratio is discussed.

https://doi.org/10.1088/0953-4075/45/8/085202

Interference in the molecular photoionization and Young's double-slit experiment

A S Baltenkov et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 035202

View article, Interference in the molecular photoionization and Young's double-slit experiment PDF, Interference in the molecular photoionization and Young's double-slit experiment

The photoabsorption by an electron bound by a two-centre potential has been investigated within the framework of the zero-range potential model. Expressions for total photoabsorption cross sections and for the photoelectron angular distributions have been derived for fixed-in-space and randomly oriented targets. The analytical formulae for gerade and ungerade molecular states have been used to analyse separately the molecular effects due to the two-centre ground state of quasi-molecule and diffraction effects that are connected with the spherical waves in the molecular continuum. It is shown that the interference of these waves significantly influences the magnitude of the cross sections near threshold but does not significantly distort the shape of the photoelectron angular distribution and it depends rather weakly on the character of the forces acting between the electron and molecular residue: Coulomb forces for neutral molecular photoionization or the short-range forces in the case of photodetachment of molecular negative ions. It is shown that despite the fact that the photoionization of diatomic molecules is reminiscent of Young's double-slit experiment, the similarity between these processes has been grossly exaggerated. This is confirmed by comparing the results of the classical interference of an electron scattered by two spatially separated centres with molecular photoelectron angular distributions.

https://doi.org/10.1088/0953-4075/45/3/035202

Modelling single positron tracks in Ar

R P McEachran et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 045207

View article, Modelling single positron tracks in Ar PDF, Modelling single positron tracks in Ar

In this study, we present a complete set of positron interaction cross sections for scattering from Ar, for incident energies ranging from 0 to 10 keV. Experimental data have been critically reviewed from previous experiments performed at the Australian National University and University College London. Differential and integral cross sections, including the effect of positronium formation, have been calculated by using two different optical potential methods. The results of these calculations, in combination with experimental cross sections and experimental energy-loss spectra, have been established as input parameters for an event-by-event Monte Carlo simulation procedure to generate single positron tracks in argon. The reliability of this method to obtain energy deposition models at the nano-scale is also discussed.

https://doi.org/10.1088/0953-4075/45/4/045207

Kicking electrons

Martin Gerlach et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 235204

View article, Kicking electrons PDF, Kicking electrons

The concept of dominant interaction Hamiltonians is introduced. It is applied as a test case to classical planar electron–atom scattering. Each trajectory is governed in different time intervals by two variants of a separable approximate Hamiltonian. Switching between them results in exchange of energy between the two electrons. A second mechanism condenses the electron–electron interaction to instants in time and leads to an exchange of energy and angular momentum among the two electrons in the form of kicks. We calculate the approximate and full classical deflection functions and show that the latter can be interpreted in terms of the switching sequences of the approximate one. Finally, we demonstrate that the quantum results agree better with the approximate classical dynamical results than with the full ones.

https://doi.org/10.1088/0953-4075/45/23/235204

Cold matter Show article list

Robust states of ultracold bosons in tilted optical lattices

Moritz Hiller et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 095301

View article, Robust states of ultracold bosons in tilted optical lattices PDF, Robust states of ultracold bosons in tilted optical lattices

We identify regular structures in the globally chaotic spectra of an interacting bosonic quantum gas in tilted periodic potentials. The associated eigenstates exhibit strong localization properties on the lattice and are dynamically robust against external perturbations.

https://doi.org/10.1088/0953-4075/45/9/095301

A gradient and offset compensated Ioffe–Pritchard trap for Bose–Einstein condensation experiments

Vasiliki Bolpasi et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 235301

View article, A gradient and offset compensated Ioffe–Pritchard trap for Bose–Einstein condensation experiments PDF, A gradient and offset compensated Ioffe–Pritchard trap for Bose–Einstein condensation experiments

The Ioffe–Pritchard trap is the workhorse of modern cold atom physics. Here, we present a novel Ioffe–Pritchard trap coil configuration based purely on circular coils. By eliminating the traditional Ioffe bars one can increase the gradient and thus the radial trapping frequency by almost a factor 2. We also present a method to achieve minimal coupling between the gradient, curvature and offset fields of the trap, thus facilitating the dynamic control of the trapping frequencies and aspect ratio.

https://doi.org/10.1088/0953-4075/45/23/235301

Two-dimensional dipolar Bose–Einstein condensate bright and vortex solitons on a one-dimensional optical lattice

S K Adhikari and P Muruganandam 2012 J. Phys. B: At. Mol. Opt. Phys. 45 045301

View article, Two-dimensional dipolar Bose–Einstein condensate bright and vortex solitons on a one-dimensional optical lattice PDF, Two-dimensional dipolar Bose–Einstein condensate bright and vortex solitons on a one-dimensional optical lattice

By solving the three-dimensional Gross–Pitaevskii equation, we generate two-dimensional axially symmetric and anisotropic dipolar Bose–Einstein condensate bright solitons for repulsive atomic interaction, stabilized by only a weak one-dimensional optical lattice (OL) aligned along and perpendicular, respectively, to the dipole polarization direction. In the former case, vortex solitons can also be created. We show that it is possible to make a stable array of small interacting axially symmetric dipolar solitons put on alternate OL sites. Further, we demonstrate the elastic nature of the collision of two such solitons.

https://doi.org/10.1088/0953-4075/45/4/045301

Recombinative dissociation and the evolution of a molecular ultracold plasma

Nicolas Saquet et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 175302

View article, Recombinative dissociation and the evolution of a molecular ultracold plasma PDF, Recombinative dissociation and the evolution of a molecular ultracold plasma

We present a kinetic model based on coupled rate-equation simulations that accounts for state-detailed rate processes in the evolution of a molecular Rydberg gas to plasma. Calculations support a steady-state picture of Rydberg predissociation and its contribution to plasma dissipation. Inelastic collisions of electrons with Rydberg states efficiently transfer energy from and to the thermal bath of free electrons, giving rise to quasi equilibria of relatively high temperature. Dissipative effects that remove low-n population change the time course of this heating. Including statistically sampled channels of Rydberg predissociation with n- and ℓ-dependent lifetimes slows the relaxation to quasi equilibrium and retards the initial electron temperature increase. At later times, the dissociative loss of low-n population displaces this quasi equilibrium leading to continuous electron heating. Dissociation products appear at a rate that is controlled initially by the rate of Rydberg relaxation to quasi equilibrium. Thereafter, the overall rate at which the total dissociation consumes neutral molecules determines the decay rate for all levels. Simulations predict a total predissociation rate that is smaller than the initial rate of direct electron–ion dissociative recombination. At later times, residual predissociation displays an apparent first-order decay with a rate constant that fits well with experimental observations.

https://doi.org/10.1088/0953-4075/45/17/175302

Quantum optics, information and control Show article list

Adiabatic elimination of a nearly resonant quantum state

Boyan T Torosov and Nikolay V Vitanov 2012 J. Phys. B: At. Mol. Opt. Phys. 45 135502

View article, Adiabatic elimination of a nearly resonant quantum state PDF, Adiabatic elimination of a nearly resonant quantum state

Quantum systems with multiple states are often reduced to simpler systems by adiabatic elimination of far-off-resonant states. This method provides accurate coupling and Stark shifts in the reduced system for very large values of the detunings only, far greater than that of the relevant couplings. We introduce two alternative techniques for exclusion of an off-resonant state based on the adiabatic and superadiabatic approximations, which provide far more accurate expressions for the coupling and the Stark shifts in the reduced system, and which are valid in much broader ranges of interaction parameters than the traditional adiabatic elimination method. The reason is that the conditions for the adiabatic and superadiabatic approximations only demand, for smooth pulse shapes, the detuning to be greater than the Fourier width of the pulse; hence, these approximations can be used even when the couplings exceed the detunings, i.e. when the excluded states can be described as 'nearly resonant'. The error of the adiabatic and superadiabatic approximations can be easily suppressed below the quantum computing benchmark of 10⁻⁴, an objective that is hard to achieve with the traditional adiabatic elimination method.

https://doi.org/10.1088/0953-4075/45/13/135502

Bloch–Zener quantum walk

S Longhi 2012 J. Phys. B: At. Mol. Opt. Phys. 45 225504

View article, Bloch–Zener quantum walk PDF, Bloch–Zener quantum walk

Coherent transport of quantum particles in binary superlattices driven by an ac force can realize a kind of quantum walk (QW) that can be referred to as Bloch–Zener QW. In this regime, the particle wavepacket undergoes a sequence of Zener tunnelling events that mimic a discrete QW on a lattice, provided that the impulse of the ac force over one semi-cycle of oscillation is properly adjusted. In the Bloch–Zener QW, the coin states are related to the particle occupation of the two minibands, and thus it does not require any internal degrees of freedom of the particle.

https://doi.org/10.1088/0953-4075/45/22/225504

Quantum-correlated motion and heralded entanglement of distant optomechanically coupled objects

Wolfgang Niedenzu et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 245501

View article, Quantum-correlated motion and heralded entanglement of distant optomechanically coupled objects PDF, Quantum-correlated motion and heralded entanglement of distant optomechanically coupled objects

The motion of two distant trapped particles or mechanical oscillators can be strongly coupled by light modes in a high finesse optical resonator. In a two mode ring cavity geometry, trapping, cooling and coupling is implemented by the same modes. While the cosine mode provides for trapping, the sine mode facilitates ground state cooling and mediates non-local interactions. For classical point particles the centre-of-mass mode is strongly damped and the individual momenta get anti-correlated. Surprisingly, quantum fluctuations induce the opposite effect of positively-correlated particle motion, which close to zero temperature generates entanglement. The non-classical correlations and entanglement are dissipation-induced and particularly strong after detection of a scattered photon in the sine mode. This allows for heralded entanglement by post-selection. Entanglement is concurrent with squeezing of the particle distance and relative momenta, while the centre-of-mass observables acquire larger uncertainties.

https://doi.org/10.1088/0953-4075/45/24/245501

Towards a coherent picture of excitonic coherence in the Fenna–Matthews–Olson complex

Andrew F Fidler et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 154013

View article, Towards a coherent picture of excitonic coherence in the Fenna–Matthews–Olson complex PDF, Towards a coherent picture of excitonic coherence in the Fenna–Matthews–Olson complex

Observations of long-lived coherence between excited states in several photosynthetic antenna complexes has motivated interest in developing a more detailed understanding of the role of the protein matrix in guiding the underlying dynamics of the system. These experiments suggest that classical rate laws may not provide an adequate description of the energy transfer process and that quantum effects must be taken into account to describe the near unity transfer efficiency in these systems. Recently, it has been shown that coherences between different pairs of excitons dephase at different rates. These details should provide some insight about the underlying electronic structure of the complex and its coupling to the protein bath. Here we show that a simple model can account for the different dephasing rates as well as the most current available experimental evidence of excitonic coherences in the Fenna–Matthews–Olson complex. The differences in dephasing rates can be understood as arising largely from differences in the delocalization and shared character between the underlying electronic states. We also suggest that the anomalously low dephasing rate of the exciton 1–2 coherence is enhanced by non-secular effects.

https://doi.org/10.1088/0953-4075/45/15/154013

Optical and laser physics Show article list

Controllable behaviours of rogue wave triplets in the nonautonomous nonlinear and dispersive system

Chao-Qing Dai et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 085401

View article, Controllable behaviours of rogue wave triplets in the nonautonomous nonlinear and dispersive system PDF, Controllable behaviours of rogue wave triplets in the nonautonomous nonlinear and dispersive system

A similarity transformation connecting the variable coefficient nonlinear Schrödinger equation with the standard nonlinear Schrödinger equation is constructed. The self-similar rogue wave triplet solutions (rational solutions) are analytically obtained for the nonautonomous nonlinear and dispersive system. The controllable behaviours of rogue wave triplets in two typical soliton management systems are discussed. In the exponential dispersion decreasing fibre, three kinds of rogue wave triplets with controllable behaviours are analysed. In the periodic distributed system, the rogue wave triplets recur periodically in the form of a cluster.

https://doi.org/10.1088/0953-4075/45/8/085401

Frequency uncertainty estimation for the ⁴⁰CaH⁺ vibrational transition frequencies observed by Raman excitation

Masatoshi Kajita and Minori Abe 2012 J. Phys. B: At. Mol. Opt. Phys. 45 185401

View article, Frequency uncertainty estimation for the 40CaH+ vibrational transition frequencies observed by Raman excitation PDF, Frequency uncertainty estimation for the 40CaH+ vibrational transition frequencies observed by Raman excitation

This paper gives estimates of the attainable frequency uncertainty in the ⁴⁰CaH⁺ X¹Σ(v, J) = (0, 0) → (v_u, 0)(v_u = 1, 2, 3, 4) transition frequencies when these transitions are observed by Raman excitation. Using the Raman lasers in the far-off resonant frequency area (<15 000 cm⁻¹), the (0, 0) → (1, 0) transition is much more advantageous to measure with small Stark shift than overtone vibrational transitions. In the quasi-resonant area, solutions of Raman laser frequencies exist where the Stark shift is nulled (magic Raman frequencies). The vibrational transition frequency can be measured with uncertainty lower than 10⁻¹⁶, and can be used to test the variation in the proton-to-electron mass ratio.

https://doi.org/10.1088/0953-4075/45/18/185401

High harmonic generation with long-wavelength few-cycle laser pulses

Bruno E Schmidt et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 074008

View article, High harmonic generation with long-wavelength few-cycle laser pulses PDF, High harmonic generation with long-wavelength few-cycle laser pulses

We report the extension of hollow-core fibre pulse compression to longer wavelengths. High-energy multi-cycle infrared pulses are generated via optical parametric amplification and subsequently broadened in the fibre. 2.5-cycle pulses at the Signal wavelength (1.4 µm) and 1.6-cycle pulses at the Idler wavelength (1.8 µm) in the sub-millijoule regime have been generated. New compression schemes can be applied at 1.8 µm and beyond. In this manner, 1.6-cycle carrier envelope phase stable pulses were generated by linear propagation in the anomalous dispersion regime of bulk glass which surprisingly enables compression below its third-order dispersion limit. Furthermore, a dispersion-free way of controlling the carrier envelope phase is demonstrated. Moreover, we experimentally confirm the increase in high-harmonic cut-off energy with driving laser wavelength and demonstrate the beneficial effect of few-cycle pulses which enable higher saturation intensities on target compared to multi-cycle pulses. It will be an ideal tool for future synthesis of isolated attosecond pulses in the sub-keV regime. With this laser source, we revealed for the first time multi-electron effects in high harmonic generation in xenon.

https://doi.org/10.1088/0953-4075/45/7/074008

High-order harmonic generation with μJ laser pulses at high repetition rates

C M Heyl et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 074020

View article, High-order harmonic generation with μJ laser pulses at high repetition rates PDF, High-order harmonic generation with μJ laser pulses at high repetition rates

We investigate the generation of high-order harmonics using laser pulse energies in the few-μJ range at high repetition rates. We analyse how the conversion efficiency is influenced by the tight focusing geometry required for the generation of high-order harmonics under these conditions. A generalized phase-matching model allows us to discuss macroscopic phase effects independent of focal length. We present experimental results using the example of a 100 kHz laser system to generate harmonics up to the 27th order in Ar with a photon flux up to 3 × 10⁹ photons s⁻¹ into one harmonic order. High-repetition-rate femtosecond or even attosecond light sources open new possibilities for a broad range of applications such as time-resolved photoelectron spectroscopy and microscopy in the extreme ultraviolet regime.

https://doi.org/10.1088/0953-4075/45/7/074020

Ultrafast, high-field, and x-ray physics Show article list

Molecular frame Auger electron energy spectrum from N₂

J P Cryan et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 055601

View article, Molecular frame Auger electron energy spectrum from N2 PDF, Molecular frame Auger electron energy spectrum from N2

Here we present the first angle-resolved, non-resonant (normal) Auger spectra for impulsively aligned nitrogen molecules. We have measured the angular pattern of Auger electron emission following K-shell photoionization by 1.1 keV photons from the Linac Coherent Light Source (LCLS). Using strong-field-induced molecular alignment to make molecular frame measurements is equally effective for both repulsive and quasi-bound final states. The capability to resolve Auger emission angular distributions in the molecular frame of reference provides a new tool for spectral assignments in congested Auger electron spectra that takes advantage of the symmetries of the final diction states. Based on our experimental results and theoretical predictions, we propose the assignment of the spectral features in the Auger electron spectrum.

https://doi.org/10.1088/0953-4075/45/5/055601

Dichroism in the above-threshold two-colour photoionization of singly charged neon

V Richardson et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 085601

View article, Dichroism in the above-threshold two-colour photoionization of singly charged neon PDF, Dichroism in the above-threshold two-colour photoionization of singly charged neon

In this work we report on the ionization of a gaseous neon target by combining extreme ultraviolet (XUV) radiation from the Free Electron Laser in Hamburg with an intense synchronized optical laser. Applying energy-resolved photoelectron spectroscopy, the dependence of the electrons ejected from singly charged neon (Ne⁺) on the relative polarization of the XUV and optical laser fields was investigated. The electron spectra exhibit a strong dependence on the degree of the relative orientation of the linear polarization vectors. A quantitative analysis was undertaken using multichannel time-dependent perturbation theory in order to reproduce the magnitude of the ejected low kinetic energy electrons as a function of the relative polarization directions and explicitly taking into account the influence of the residual Ne^{2 +} cores (¹D,³P). It is concluded that the variation of the photoelectron spectrum with the deviation of XUV and optical field directions is mainly related to the different responses of the magnetic substates of the ejected electrons in their interaction with the laser field.

https://doi.org/10.1088/0953-4075/45/8/085601

A novel estimate of the two-photon double-ionization cross section of helium

L Malegat et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 175601

View article, A novel estimate of the two-photon double-ionization cross section of helium PDF, A novel estimate of the two-photon double-ionization cross section of helium

In a previous publication, a procedure was proposed for unambiguously extracting the cross sections for double ionization and single ionization from a time-propagated wavepacket, and it was tested on the well-known case of one-photon double ionization of helium successfully. Here, we apply it to the two-photon process for which the numerically predicted double ionization cross section is not completely stabilized yet. Our results confirm the value obtained for this cross section by all but two active groups in the field, they definitely exonerate electron correlations in the final state from any responsibility in this splitting of the published data into two sets, they emphasize the need for a more careful account of reflection effects and propose a tentative explanation for an overestimation of the cross section in the J-matrix method. They also demonstrate the conceptual and computational advantages of the method proposed.

https://doi.org/10.1088/0953-4075/45/17/175601

Photoelectron kinetic and angular distributions for the ionization of aligned molecules using a HHG source

Arnaud Rouzée et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 074016

View article, Photoelectron kinetic and angular distributions for the ionization of aligned molecules using a HHG source PDF, Photoelectron kinetic and angular distributions for the ionization of aligned molecules using a HHG source

We present an experimental and theoretical investigation of the angular distributions of electrons ejected in aligned molecules by extreme ultra-violet ionization using a high harmonic generation (HHG) source. Impulsive alignment in O₂, N₂ and CO molecules was achieved using a near-IR laser pulse and the photoelectron angular distribution after ionization by a fs harmonic comb composed of harmonic H11 to H29 (17.5–46 eV) was recorded at the maximum of both alignment and anti-alignment. The experiment reveals signatures that are specific for the electronic orbitals that are ionized as well as the onset of the influence of the molecular structure and is well reproduced by theoretical calculations based on the multichannel Schwinger configuration interaction method.

https://doi.org/10.1088/0953-4075/45/7/074016

Astrophysics and plasma physics Show article list

Time-dependent calculations of electron energy distribution functions for cold argon gas in the presence of intense black-body radiation

Joseph Abdallah Jr and James Colgan 2012 J. Phys. B: At. Mol. Opt. Phys. 45 035701

View article, Time-dependent calculations of electron energy distribution functions for cold argon gas in the presence of intense black-body radiation PDF, Time-dependent calculations of electron energy distribution functions for cold argon gas in the presence of intense black-body radiation

Boltzmann electron kinetic simulations are performed to study the time development of the electron energy distribution in a plasma resulting from a cold argon gas subject to a black-body radiation source (100–300 eV). The study provides insight into the role of ionized electrons on the kinetics during a period of irradiation. The simulations are performed without any assumptions of electron temperature. The distributions are calculated as a function of time through 1 ps using Boltzmann kinetics, including the appropriate processes that alter state populations and electron energy. The processes included in the electron and atomic kinetics are collisional excitation/de-excitation, photo-excitation/decay, photo-ionization/radiative-recombination, collisional ionization and auto-ionization/di-electronic capture. In addition, terms are included in the electron kinetics to account for electron–electron interactions and for free–free radiation absorption and emission. Results are presented that follow the evolution of the ionization state, effective temperature as well as the electron energy distribution function. The role of inelastic electron collisions, photo-ionization, auto-ionization, resonant radiative excitation and electron–electron interactions is discussed.

https://doi.org/10.1088/0953-4075/45/3/035701

On the role of atomic metastability in the production of Balmer line radiation from 'cold' atomic hydrogen, deuterium and hydrogenic ion impurities in fusion edge plasmas

J D Hey 2012 J. Phys. B: At. Mol. Opt. Phys. 45 065701

View article, On the role of atomic metastability in the production of Balmer line radiation from 'cold' atomic hydrogen, deuterium and hydrogenic ion impurities in fusion edge plasmas PDF, On the role of atomic metastability in the production of Balmer line radiation from 'cold' atomic hydrogen, deuterium and hydrogenic ion impurities in fusion edge plasmas

Published arguments, which assign an important role to atomic metastability in the production of 'narrow' Zeeman component radiation from the boundary region of fusion plasmas, are examined critically in relation to l-redistribution by proton and electron collisions, and mixing of unperturbed atomic states by the ion microfield and microfield gradient. It is concluded that these important processes indeed severely constrain the contribution from 'metastable' states to the generation of the hydrogen Balmer spectra, for electron concentrations above 10¹² cm⁻³, as pointed out before by the present author (Hey et al 1999 J. Phys. B: At. Mol. Opt. Phys. 32 3555). The analysis of collision-induced l-redistribution represents an extension of that used previously (Hey et al 1996 Contrib. Plasma Phys. 36 583), applicable up to higher electron densities. For comparison purposes, we also consider the question of metastability of ionized helium in a low-temperature plasma, and that of some common hydrogenic impurities (C⁵⁺ and Ne⁹⁺) in a hydrogen (deuterium) fusion plasma. While for low nuclear charge Z the metastability of 2s_1/2 levels is quenched by the plasma environment, it is much reduced in high-Z ions owing to the rapid increase with Z of the two-photon electric dipole (2E1) and magnetic dipole (M1) spontaneous transition rates to the ground state, whereas the role of the plasma in these cases is less important. The main new principle elaborated in this work is the sensitivity of atomic line strengths, and hence collision strengths, to perturbation by the plasma environment for transitions between fine-structure sublevels of the same principal quantum number. As the plasma microfield strength grows, 'allowed' transitions diminish in strength, while 'forbidden' transitions grow. However, owing to violation of the parity selection rule, there is an overall loss of collision strength available to transitions, resulting from the appearance of significant 'self-strength' contributions, in accord with the sum rules for the line strengths, which remain valid over the range of fields considered. Thus, the relative effectiveness per perturber of both electron and ion collisions, for inducing population transfer between fine-structure sublevels, diminishes as the sublevels evolve from a fine-structure dominated to a Stark-effect-dominated regime. In the concluding discussion, we mention that this finding may have a bearing on discrepancies claimed between Stark broadening theory developed by Griem (1967 Astrophys. J. 148 547) and by Watson (2006 J. Phys.B: At. Mol. Opt. Phys. 39 1889), and the measurements of Bell and co-workers (2000 Publ. Astron. Soc. Pac. 112 1236; 2011 Astrophys. Space Sci. 335 451) for high-n radio recombination lines from galactic H II regions. In the absence of detailed modelling to test this suggestion, however, it would be premature to attempt to draw any firm conclusions along these lines.

https://doi.org/10.1088/0953-4075/45/6/065701

Fast Track Communications Show article list

Controlling core hole relaxation dynamics via intense optical fields

T Mazza et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 141001

View article, Controlling core hole relaxation dynamics via intense optical fields PDF, Controlling core hole relaxation dynamics via intense optical fields

The influence of an intense optical laser on the electronic relaxation of the 3d–5p resonance in atomic Kr has been studied experimentally and theoretically. The resonance profile undergoes a strong modification, observed as a shift and a broadening of the excitation spectrum, as a function of the optical intensity. The theoretical treatment of the process demonstrates the importance of strongly time-dependent dynamics as the origin of the observed phenomena resulting in the ponderomotive shift of the resonance position as well as a competition between resonant and normal Auger decay. The ionization of the excited 5p electron by the optical laser provides the possibility to change the ratio between singly and doubly charged final states by controlling the relaxation of the resonant core hole state via resonant or normal Auger decay.

https://doi.org/10.1088/0953-4075/45/14/141001

Tuning the internuclear distance in ionization of H₂

A Senftleben et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 021001

View article, Tuning the internuclear distance in ionization of H2 PDF, Tuning the internuclear distance in ionization of H2

The dependence of triply differential cross sections on the internuclear distance R is explored for ionization of molecular hydrogen by 200 eV electrons for two distinct values R = 1.1 and 1.4 au in comparison with helium as an isoelectronic system with a single nucleus (R = 0). A clear and systematic trend is observed with an increasing recoil-peak contribution with decreasing R. BBK-type model calculations indicate that the inclusion of the passive electron in the target wavefunction is the key ingredient for explaining the data rather than the two-centre character of the system.

https://doi.org/10.1088/0953-4075/45/2/021001

Resonant phenomena in laser-assisted radiative attachment or recombination

A N Zheltukhin et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 081001

View article, Resonant phenomena in laser-assisted radiative attachment or recombination PDF, Resonant phenomena in laser-assisted radiative attachment or recombination

Resonant enhancements are predicted in cross sections σ_n for laser-assisted radiative attachment or electron–ion recombination accompanied by absorption of n laser photons. These enhancements occur for incoming electron energies at which the electron can be attached or recombined by emitting μ laser photons followed by emission of a spontaneous photon upon absorbing n + μ laser photons. The close similarity between rescattering plateaus in spectra of resonant attachment/recombination and of high-order harmonic generation is shown based on a general parametrization for σ_n and on numerical results for e − H attachment.

https://doi.org/10.1088/0953-4075/45/8/081001

Breaking a tetrahedral molecular ion with electrons: study of NH⁺₄

N Douguet et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 051001

View article, Breaking a tetrahedral molecular ion with electrons: study of NH+4 PDF, Breaking a tetrahedral molecular ion with electrons: study of NH+4

We apply a general theoretical model to study the dissociative recombination of the polyatomic ion NH⁺₄. The high symmetry of the molecule, represented by the tetrahedral group, leads to complex vibronic couplings responsible for dissociative recombination. By applying multi-channel quantum defect theory and using symmetry considerations, we treat the doubly and triply degenerate modes and electronic states of NH⁺₄ to calculate a theoretical cross section. The cross section agrees well with existing experimental data.

https://doi.org/10.1088/0953-4075/45/5/051001

'Fast light' effect in experiments with cryogenic resonators

E Ivanov et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 221001

View article, 'Fast light' effect in experiments with cryogenic resonators PDF, 'Fast light' effect in experiments with cryogenic resonators

The 'fast light' effect has been observed when studying interaction of modulated microwave signals with a cryogenic resonator. The effect manifests itself as a time advance of the peak of the reflected pulse relative to that of the modulation pulse. The effect is much more pronounced for the phase-modulated signals, as compared to the amplitude-modulated ones. We show that 'fast light' is a pulse-shaping phenomenon, which depends on parameters of the modulation signal, and is not related to superluminal pulse propagation. We also discuss possible applications of the 'fast light' to pulse control and generation of complex waveforms at microwave and optical frequencies.

https://doi.org/10.1088/0953-4075/45/22/221001

Topical Reviews Show article list

Foundations and measures of quantum non-Markovianity

Heinz-Peter Breuer 2012 J. Phys. B: At. Mol. Opt. Phys. 45 154001

View article, Foundations and measures of quantum non-Markovianity PDF, Foundations and measures of quantum non-Markovianity

The basic features of the dynamics of open quantum systems, such as the dissipation of energy, the decay of coherences, the relaxation to an equilibrium or non-equilibrium stationary state, and the transport of excitations in complex structures are of central importance in many applications of quantum mechanics. The theoretical description, analysis and control of non-Markovian quantum processes play an important role in this context. While in a Markovian process an open system irretrievably loses information to its surroundings, non-Markovian processes feature a flow of information from the environment back to the open system, which implies the presence of memory effects and represents the key property of non-Markovian quantum behaviour. Here, we review recent ideas developing a general mathematical definition for non-Markovianity in the quantum regime and a measure for the degree of memory effects in the dynamics of open systems, which are based on the exchange of information between system and environment. We further study the dynamical effects induced by the presence of system–environment correlations in the total initial state and design suitable methods to detect such correlations through local measurements on the open system.

https://doi.org/10.1088/0953-4075/45/15/154001

Review of attosecond resolved measurement and control via carrier–envelope phase tagging with above-threshold ionization

T Rathje et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 074003

View article, Review of attosecond resolved measurement and control via carrier–envelope phase tagging with above-threshold ionization PDF, Review of attosecond resolved measurement and control via carrier–envelope phase tagging with above-threshold ionization

A precise, real-time, single-shot carrier–envelope phase (CEP) tagging technique for few-cycle pulses was developed and combined with cold-target recoil-ion momentum spectroscopy and velocity-map imaging to investigate and control CEP-dependent processes with attosecond resolution. The stability and precision of these new techniques have allowed for the study of intense, few-cycle, laser-matter dynamics with unprecedented detail. Moreover, the same stereo above-threshold ionization (ATI) measurement was expanded to multi-cycle pulses and allows for CEP locking and pulse-length determination. Here we review these techniques and their first applications to waveform characterization and control, non-sequential double ionization of argon, ATI of xenon and electron emission from SiO₂ nanospheres.

https://doi.org/10.1088/0953-4075/45/7/074003

XUV frequency combs via femtosecond enhancement cavities

Arthur K Mills et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 142001

View article, XUV frequency combs via femtosecond enhancement cavities PDF, XUV frequency combs via femtosecond enhancement cavities

We review the current state of tabletop extreme ultraviolet (XUV) sources based on high harmonic generation (HHG) in femtosecond enhancement cavities (fsEC). Recent developments have enabled generation of high photon flux (10¹⁴ photons s⁻¹) in the XUV, at high repetition rates (>50 MHz) and spanning the spectral region from 40 to 120 nm. This level of performance has enabled precision spectroscopy with XUV frequency combs and promises further applications in XUV spectroscopic and photoemission studies. We discuss the theory of operation and experimental details of the fsEC and XUV generation based on HHG, including current technical challenges to increasing the photon flux and maximum photon energy produced by this type of system. Current and future applications for these sources are also discussed.

https://doi.org/10.1088/0953-4075/45/14/142001

Quantum optics with ultracold quantum gases: towards the full quantum regime of the light–matter interaction

Igor B Mekhov and Helmut Ritsch 2012 J. Phys. B: At. Mol. Opt. Phys. 45 102001

View article, Quantum optics with ultracold quantum gases: towards the full quantum regime of the light–matter interaction PDF, Quantum optics with ultracold quantum gases: towards the full quantum regime of the light–matter interaction

Although the study of ultracold quantum gases trapped by light is a prominent direction of modern research, the quantum properties of light were widely neglected in this field. Quantum optics with quantum gases closes this gap and addresses phenomena where the quantum statistical natures of both light and ultracold matter play equally important roles. First, light can serve as a quantum nondemolition probe of the quantum dynamics of various ultracold particles from ultracold atomic and molecular gases to nanoparticles and nanomechanical systems. Second, due to the dynamic light–matter entanglement, projective measurement-based preparation of the many-body states is possible, where the class of emerging atomic states can be designed via optical geometry. Light scattering constitutes such a quantum measurement with controllable measurement back-action. As in cavity-based spin squeezing, the atom number squeezed and Schrödinger cat states can be prepared. Third, trapping atoms inside an optical cavity, one creates optical potentials and forces, which are not prescribed but quantized and dynamical variables themselves. Ultimately, cavity quantum electrodynamics with quantum gases requires a self-consistent solution for light and particles, which enriches the picture of quantum many-body states of atoms trapped in quantum potentials. This will allow quantum simulations of phenomena related to the physics of phonons, polarons, polaritons and other quantum quasiparticles.

https://doi.org/10.1088/0953-4075/45/10/102001

Photoionization dynamics of excited Ne, Ar, Kr and Xe atoms near threshold

V L Sukhorukov et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 092001

View article, Photoionization dynamics of excited Ne, Ar, Kr and Xe atoms near threshold PDF, Photoionization dynamics of excited Ne, Ar, Kr and Xe atoms near threshold

A review of experimental and theoretical studies of the threshold photoionization of the heavier rare-gas atoms is presented, with particular emphasis on the autoionization resonances in the spectral region between the lowest two ionization thresholds ²P_3/2 and ²P_1/2, accessed from the ground or excited states. Observed trends in the positions, widths and shapes of the autoionization resonances depending on the atomic number, the principal quantum number n, the orbital angular momentum quantum number ℓ and further quantum numbers specifying the fine- and hyperfine-structure levels are summarized and discussed in the light of ab initio and multichannel quantum defect theory calculations. The dependence of the photoionization spectra on the initially prepared neutral state are also discussed, including results on the photoionization cross sections and photoelectron angular distributions of polarized excited states. The effects of various approximations in the theoretical treatment of photoionization in these systems are analysed. The very large diversity of observed phenomena and the numerous anomalies in spectral structures associated with the threshold ionization of the rare-gas atoms can be described in terms of a limited set of interactions and dynamical processes. Examples are provided illustrating characteristic aspects of the photoionization, and sets of recommended parameters describing the energy-level structure and photoionization dynamics of the rare-gas atoms are presented which were extracted in a critical analysis of the very large body of experimental and theoretical data available on these systems in the literature.

https://doi.org/10.1088/0953-4075/45/9/092001

Circularly polarized attosecond pulses from molecular high-order harmonic generation by ultrashort intense bichromatic circularly and linearly polarized laser pulses

Kai-Jun Yuan and André D Bandrauk 2012 J. Phys. B: At. Mol. Opt. Phys. 45 074001

View article, Circularly polarized attosecond pulses from molecular high-order harmonic generation by ultrashort intense bichromatic circularly and linearly polarized laser pulses PDF, Circularly polarized attosecond pulses from molecular high-order harmonic generation by ultrashort intense bichromatic circularly and linearly polarized laser pulses

We describe the generation of high-order elliptically and circularly polarized harmonic spectra in an aligned H⁺₂ molecule ion by a combination of two-colour ultrashort intense laser fields from numerical solutions of the corresponding time-dependent Schrödinger equation (TDSE). In intense bichromatic circularly and linearly or circularly polarized laser pulses with intensity I₀ and angular frequencies ω₀ and 2ω₀, it is found that maximum molecular high-order harmonic generation (MHOHG) energies are functions of the molecular internuclear distance. Based on a classical model of laser-induced electron collisions with neighbouring ions, the optimal values of the pulse relative carrier envelope phase ϕ, the molecular internuclear distance R and the angle ϑ of molecular alignment to the laser polarization axis are obtained for efficiently producing MHOHG spectra with the maximum harmonic energy I_p + 13.5U_p, where I_p is the ionization potential of the molecule and U_p = I₀/4m_eω²₀ is the ponderomotive energy of the continuum electron at intensity I₀ and frequency ω₀ of the laser pulse. The results have been confirmed from corresponding TDSE nonperturbative numerical simulations. The polarization property of the generated harmonics is also presented. The mechanism of MHOHG is further characterized with a Gabor time frequency analysis. It is confirmed that a single collision trajectory of the continuum electron with neighbouring ions dominates in the MHOHG processes. The high efficiency of the proposed MHOHG scheme provides a possible source for production of elliptically and/or circularly polarized attosecond extreme ultraviolet pulses. Circularly polarized attosecond pulses can also be generated by using intense ultrashort circularly polarized laser pulses in combination with static electric fields of comparable intensity for H⁺₂ at equilibrium. A time frequency analysis also confirms the role of single recollisions as the dominant mechanism of the generation of circularly polarized harmonics.

https://doi.org/10.1088/0953-4075/45/7/074001

Tutorials Show article list

Structure determination through correlated fluctuations in x-ray scattering

Richard A Kirian 2012 J. Phys. B: At. Mol. Opt. Phys. 45 223001

View article, Structure determination through correlated fluctuations in x-ray scattering PDF, Structure determination through correlated fluctuations in x-ray scattering

It was suggested more than three decades ago that the three-dimensional structure of one particle may be determined using the simultaneous x-ray scattering from many randomly oriented copies ab initio, without modelling of a priori information. This may be possible, provided sufficiently brief and intense x-ray pulses that can 'outrun' the effects of radiation damage and simultaneously produce significant signal within 'snapshot' diffraction patterns. Because the ensemble of particles is static throughout the snapshot exposure, solution scattering patterns contain angular intensity fluctuations and thus differ from conventional isotropic scattering patterns. X-ray free-electron lasers may be able to provide the x-ray source properties that are required to make such experiments feasible. In this tutorial we discuss how structures might be determined through correlated x-ray scattering measurements, with an emphasis on dilute suspensions of identical bioparticles.

https://doi.org/10.1088/0953-4075/45/22/223001

Introduction to attosecond delays in photoionization

J M Dahlström et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 183001

View article, Introduction to attosecond delays in photoionization PDF, Introduction to attosecond delays in photoionization

This tutorial presents an introduction to the interaction of light and matter on the attosecond timescale. Our aim is to detail the theoretical description of ultra-short time delays and to relate these to the phase of extreme ultraviolet (XUV) light pulses and to the asymptotic phaseshifts of photoelectron wave packets. Special emphasis is laid on time-delay experiments, where attosecond XUV pulses are used to photoionize target atoms at well-defined times, followed by a probing process in real time by a phase-locked, infrared laser field. In this way, the laser field serves as a 'clock' to monitor the ionization event, but the observable delays do not correspond directly to the delay associated with single-photon ionization. Instead, a significant part of the observed delay originates from a measurement induced process, which obscures the single-photon ionization dynamics. This artefact is traced back to a phaseshift of the above-threshold ionization transition matrix element, which we call the continuum–continuum phase. It arises due to the laser-stimulated transitions between Coulomb continuum states. As we shall show here, these measurement-induced effects can be separated from the single-photon ionization process, using analytical expressions of universal character, so that eventually the attosecond time delays in photoionization can be accessed.

https://doi.org/10.1088/0953-4075/45/18/183001

The theory of Hawking radiation in laboratory analogues

Scott J Robertson 2012 J. Phys. B: At. Mol. Opt. Phys. 45 163001

View article, The theory of Hawking radiation in laboratory analogues PDF, The theory of Hawking radiation in laboratory analogues

Hawking radiation, despite being known to theoretical physics for nearly 40 years, remains elusive and undetected. It also suffers, in its original context of gravitational black holes, from practical and conceptual difficulties. Of particular note is the trans-Planckian problem, which is concerned with the apparent origin of the radiation in absurdly high frequencies. In order to gain better theoretical understanding and, it is hoped, experimental verification of Hawking radiation, much study is being devoted to laboratory systems which use moving media to model the spacetime geometry of black holes, and which, by analogy, are also thought to emit Hawking radiation. These analogue systems typically exhibit dispersion, which regularizes the wave behaviour at the horizon at the cost of a more complicated theoretical framework. This tutorial serves as an introduction to Hawking radiation and its analogues, developing the moving medium analogy for black holes and demonstrating how dispersion can be incorporated into this generalized framework.

https://doi.org/10.1088/0953-4075/45/16/163001

An experimental and theoretical guide to strongly interacting Rydberg gases

Robert Löw et al 2012 J. Phys. B: At. Mol. Opt. Phys. 45 113001

View article, An experimental and theoretical guide to strongly interacting Rydberg gases PDF, An experimental and theoretical guide to strongly interacting Rydberg gases

We review experimental and theoretical tools to excite, study and understand strongly interacting Rydberg gases. The focus lies on the excitation of dense ultracold atomic samples close to, or within quantum degeneracy, high-lying Rydberg states. The major part is dedicated to highly excited S-states of rubidium, which feature an isotropic van der Waals potential. Nevertheless, the setup and the methods presented are also applicable to other atomic species used in the field of laser cooling and atom trapping.

https://doi.org/10.1088/0953-4075/45/11/113001

Spin squeezing, entanglement and quantum metrology with Bose–Einstein condensates

Christian Gross 2012 J. Phys. B: At. Mol. Opt. Phys. 45 103001

View article, Spin squeezing, entanglement and quantum metrology with Bose–Einstein condensates PDF, Spin squeezing, entanglement and quantum metrology with Bose–Einstein condensates

Squeezed states, special kinds of entangled states, are known as a useful resource for quantum metrology. In interferometric sensors, they allow us to overcome the 'classical' projection noise limit stemming from the independent nature of the individual photons or atoms within the interferometer. Motivated by the potential impact on metrology as well as by fundamental questions in the context of entanglement, a lot of theoretical and experimental effort has been made to study squeezed states. The first squeezed states useful for quantum-enhanced metrology have been proposed and generated in quantum optics, where the squeezed variables are the coherences of the light field. In this tutorial, we focus on spin squeezing in atomic systems. We give an introduction to its concepts and discuss its generation in Bose–Einstein condensates. We discuss in detail the experimental requirements necessary for the generation and direct detection of coherent spin squeezing. Two exemplary experiments demonstrating adiabatically prepared spin squeezing based on motional degrees of freedom and diabatically realized spin squeezing based on internal hyperfine degrees of freedom are discussed.

https://doi.org/10.1088/0953-4075/45/10/103001

Highlights of 2012

Highlights of 2012

Journal links