This topical review addresses how Rydberg atoms can serve as building blocks for emerging quantum technologies. Whereas the fabrication of large numbers of artificial quantum systems with the uniformity required for the most attractive applications is difficult if not impossible, atoms provide stable quantum systems which, for the same species and isotope, are all identical. Whilst atomic ground states provide scalable quantum objects, their applications are limited by the range over which their properties can be varied. In contrast, Rydberg atoms offer strong and controllable atomic interactions that can be tuned by selecting states with different principal quantum number or orbital angular momentum. In addition Rydberg atoms are comparatively long-lived, and the large number of available energy levels and their separations allow coupling to electromagnetic fields spanning over 6 orders of magnitude in frequency. These features make Rydberg atoms highly desirable for developing new quantum technologies. After giving a brief introduction to how the properties of Rydberg atoms can be tuned, we give several examples of current areas where the unique advantages of Rydberg atom systems are being exploited to enable new applications in quantum computing, electromagnetic field sensing, and quantum optics.
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ISSN: 1361-6455
Journal of Physics B: Atomic, Molecular and Optical Physics covers the study of atoms, ions, molecules and clusters, and their structure and interactions with particles, photons or fields.
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C S Adams et al 2020 J. Phys. B: At. Mol. Opt. Phys. 53 012002
Marc Vrakking 2024 J. Phys. B: At. Mol. Opt. Phys. 57 090201
Attosecond physics is a novel research field that pursues a better understanding of electron dynamics in atoms, molecules and condensed matter by means of pump-probe experiments where the motion of electrons are tracked with attosecond (1 as = 10−18 s) time resolution. The 2023 Physics Nobel Prize was awarded to three experimental pioneers of the field, who developed the key methods to generate and characterize attosecond pulses.
Lucy Downes 2023 J. Phys. B: At. Mol. Opt. Phys. 56 223001
Understanding the interactions between atoms and light is at the heart of atomic physics. Being able to 'experiment' with various system parameters, produce plots of the results and interpret these is very useful, especially for those new to the field. This tutorial aims to provide an introduction to the equations governing near-resonant atom-light interactions and present examples of setting up and solving these equations in Python. Emphasis is placed on clarity and understanding by showing code snippets alongside relevant equations, and as such it is suitable for those without an excellent working knowledge of Python or the underlying physics. Hopefully the methods presented here can form the foundations on which more complex models and simulations can be built. All functions presented here and example codes can be found on GitHub.
Eric P Glasbrenner and Wolfgang P Schleich 2023 J. Phys. B: At. Mol. Opt. Phys. 56 104001
We employ the Markov approximation and the well-known Fresnel-integral to derive in 'one-line' the familiar expression for the Landau–Zener transition probability. Moreover, we provide numerical as well as analytical justifications for our approach, and identify three characteristic motions of the probability amplitude in the complex plane.
Takuya Hatomura 2024 J. Phys. B: At. Mol. Opt. Phys. 57 102001
Shortcuts to adiabaticity guide given systems to final destinations of adiabatic control via fast tracks. Various methods have been proposed as shortcuts to adiabaticity. The basic theory of shortcuts to adiabaticity was established in the 2010s, but it has still been developing and many fundamental findings have been reported. In this topical review, we give a pedagogical introduction to the theory of shortcuts to adiabaticity and revisit relations between different methods. Some versatile approximations in counterdiabatic driving, which is one of the methods of shortcuts to adiabaticity, will be explained in detail. We also summarize the recent progress in studies of shortcuts to adiabaticity.
Haoquan Fan et al 2015 J. Phys. B: At. Mol. Opt. Phys. 48 202001
Atom-based measurements of length, time, gravity, inertial forces and electromagnetic fields are receiving increasing attention. Atoms possess properties that suggest clear advantages as self calibrating platforms for measurements of these quantities. In this review, we describe work on a new method for measuring radio frequency (RF) electric fields based on quantum interference using either Cs or Rb atoms contained in a dielectric vapor cell. Using a bright resonance prepared within an electromagnetically induced transparency window it is possible to achieve high sensitivities, <1 μV cm−1 Hz−1/2, and detect small RF electric fields μV cm−1 with a modest setup. Some of the limitations of the sensitivity are addressed in the review. The method can be used to image RF electric fields and can be adapted to measure the vector electric field amplitude. Extensions of Rydberg atom-based electrometry for frequencies up to the terahertz regime are described.
C H Valahu et al 2022 J. Phys. B: At. Mol. Opt. Phys. 55 204003
A major obstacle in the way of practical quantum computing is achieving scalable and robust high-fidelity entangling gates. To this end, quantum control has become an essential tool, as it can make the entangling interaction resilient to sources of noise. Nevertheless, it may be difficult to identify an appropriate quantum control technique for a particular need given the breadth of work pertaining to robust entanglement. To this end, we attempt to consolidate the literature by providing a non-exhaustive summary and critical analysis. The quantum control methods are separated into two categories: schemes which extend the robustness to (i) spin or (ii) motional decoherence. We choose to focus on extensions of the σx ⊗ σx Mølmer–Sørensen interaction using microwaves and a static magnetic field gradient. Nevertheless, some of the techniques discussed here can be relevant to other trapped ion architectures or physical qubit implementations. Finally, we experimentally realize a proof-of-concept interaction with simultaneous robustness to spin and motional decoherence by combining several quantum control methods presented in this manuscript.
M Saffman 2016 J. Phys. B: At. Mol. Opt. Phys. 49 202001
We present a review of quantum computation with neutral atom qubits. After an overview of architectural options and approaches to preparing large qubit arrays we examine Rydberg mediated gate protocols and fidelity for two- and multi-qubit interactions. Quantum simulation and Rydberg dressing are alternatives to circuit based quantum computing for exploring many body quantum dynamics. We review the properties of the dressing interaction and provide a quantitative figure of merit for the complexity of the coherent dynamics that can be accessed with dressing. We conclude with a summary of the current status and an outlook for future progress.
Olivier Pfister 2020 J. Phys. B: At. Mol. Opt. Phys. 53 012001
This topical review introduces the theoretical and experimental advances in continuous-variable (CV)—i.e. qumode-based in lieu of qubit-based—large-scale, fault-tolerant quantum computing and quantum simulation. An introduction to the physics and mathematics of multipartite entangled CV cluster states is given, and their connection to experimental concepts is delineated. Paths toward fault tolerance are also presented. It is the hope of the author that this review attract more contributors to the field and promote its extension to the promising technology of integrated quantum photonics.
Nilakantha Meher 2024 J. Phys. B: At. Mol. Opt. Phys. 57 073001
We provide a MATLAB numerical guide at the beginner level to support students starting their research careers in theoretical quantum optics and related areas. These resources are also valuable for undergraduate and graduate students working on semester projects in similar fields.
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Bing-Bing Li et al 2024 J. Phys. B: At. Mol. Opt. Phys. 57 115003
Combining the multi-configuration Dirac–Hartree–Fock method and the model-quantum electrodynamics (QED) approach, the wave functions, transition energies and hyperfine structure constants for 2l, 3l states in Li-like Ne7+, Mg9+, Al10+, P12+, Ar15+ and Ca17+ ions are calculated. The effects of electron correlation, Breit interactions, nuclear recoil and QED effects are analyzed in detail. We find that the contribution of the non-diagonal elements of the self-energy is significant for the and transitions. However, for the transitions, the contribution of non-diagonal elements is small. The accuracy of the currently calculated hyperfine structure constants is expected to be within 1%.
K Ferentinou et al 2024 J. Phys. B: At. Mol. Opt. Phys. 57 115002
Slow (meV) photoelectron imaging spectroscopy is employed in the experimental study of near-threshold photoionization of strontium atoms in the presence of an external static electric field. Specifically, the study is devoted to the glory effect, that is, the appearance of an intense peak at the center of the recorded photoelectron images, when dealing with m= 0 final ionized Stark states (m denoting the magnetic quantum number). This critical effect is formally identical to that encountered in classical scattering theory, where, for a nonzero value of the impact parameter, the zero-crossing of the deflection function leads to a divergent classical differential cross section. By recording the magnitude variation of this glory peak as a function of electron excitation energy, we observe that, besides the traces of classical origin, it also exhibits intense quantum interference and beating phenomena, above and below the zero-static-field ionization threshold. We study both, single- and two-photon ionization of Sr, thus enabling a comparison not only between the different excitation schemes, but also with an earlier work devoted to two-photon ionization of Mg atom by Kalaitzis et al (2020 Phys. Rev. A 102 033101). Our recordings are analyzed within the framework of the Harmin–Fano frame transformation Stark effect theory that is applied to both the hydrogen atom and a non-hydrogenic one simulating Sr. We discuss the various aspects of the recorded and calculated glory interference and beating structures and their 'short time Fourier transforms' and classify them as either atom-specific or atom independent. In particular, we verify the 'universal' connection between the glory oscillations above the zero-field threshold and the differences between the origin-to-detector times of flight corresponding to pairs of classical electron trajectories that end up to the image center.
Djamel Benredjem and Jean-Christophe Pain 2024 J. Phys. B: At. Mol. Opt. Phys. 57 115001
We propose a semi-empirical formula for the cross section of ionization by electron impact. The formula involves adjustable parameters which are determined by comparison with measured or numerically calculated cross sections. In the latter case, the ions are perturbed by their environment which is a high-density plasma. As a consequence, the cross section is significantly modified. We investigate Be-like carbon, nitrogen and oxygen as well as aluminum ions. We also show that the formula is well-suited for interpolation and extrapolation. Knowing the cross section, we calculate the rate coefficient within the Boltzmann and Fermi–Dirac statistics. In the first case, the rate can be calculated analytically. In the second one, it can be expressed in terms of special functions, but the numerical evaluation is more convenient while providing accurate results. Our results are compared to experiment and to other calculations.
P B Blakie 2024 J. Phys. B: At. Mol. Opt. Phys. 57 115301
We investigate the superfluid fraction of crystalline stationary states within the framework of mean-field Gross–Pitaevskii theory. Our primary focus is on a two-dimensional Bose–Einstein condensate with a non-local soft-core interaction, where the superfluid fraction is described by a rank-2 tensor. We then calculate the superfluid fraction tensor for crystalline states exhibiting triangular, square, and stripe geometries across a broad range of interaction parameters. Factors leading to an anisotropic superfluid fraction tensor are also considered. We also refine the Leggett bounds for the superfluid fraction of the 2D system. We systematically compare these bounds to our full numerical results, and other results in the literature. This work is of direct relevance to other supersolid systems of current interest, such as supersolids produced using dipolar Bose–Einstein condensates.
Hugo J B Marroux et al 2024 J. Phys. B: At. Mol. Opt. Phys. 57 115401
We present an extreme ultraviolet (EUV) transient grating (TG) experiment of the spinel Co3O4 compound using tuneable incident energies across the Co M2,3-edge and a 395 nm probe pulse, detecting both the first and the second diffraction orders (SDOs). While the first diffraction order shows a monotonous behavior as a function of time, with a sharp response at t = 0, followed by a weak sub-picosecond component and a nearly constant signal thereafter, the time dependence of SDO varies dramatically with the incident energy as it is tuned across the Co M-edge, with the appearance of a component at t > 1 ps that grows with increasing energy. The results are rationalized in terms of the deviations of the initial grating from sinusoidal to non-sinusoidal, namely a flattening of the grating pattern, that introduces new Fourier components. These deviations are due to higher order, three-body terms in the population relaxation kinetics. The present results highlight the use of the SDO response in EUV TG as a tool to identify higher order terms in the population kinetics.
Open all abstracts, in this tab
Takuya Hatomura 2024 J. Phys. B: At. Mol. Opt. Phys. 57 102001
Shortcuts to adiabaticity guide given systems to final destinations of adiabatic control via fast tracks. Various methods have been proposed as shortcuts to adiabaticity. The basic theory of shortcuts to adiabaticity was established in the 2010s, but it has still been developing and many fundamental findings have been reported. In this topical review, we give a pedagogical introduction to the theory of shortcuts to adiabaticity and revisit relations between different methods. Some versatile approximations in counterdiabatic driving, which is one of the methods of shortcuts to adiabaticity, will be explained in detail. We also summarize the recent progress in studies of shortcuts to adiabaticity.
A Niggas et al 2024 J. Phys. B: At. Mol. Opt. Phys. 57 072001
Electron beam ion traps allow studies of slow highly charged ion transmission through freestanding 2D materials as an universal testbed for surface science under extreme conditions. Here we review recent studies on charge exchange of highly charged ions in 2D materials. Since the interaction time with these atomically thin materials is limited to only a few femtoseconds, an indirect timing information will be gained. We will therefore discuss the interaction separated in three participating time regimes: energy deposition (charge exchange), energy release (secondary particle emission), and energy retention (material modification).
Nilakantha Meher 2024 J. Phys. B: At. Mol. Opt. Phys. 57 073001
We provide a MATLAB numerical guide at the beginner level to support students starting their research careers in theoretical quantum optics and related areas. These resources are also valuable for undergraduate and graduate students working on semester projects in similar fields.
Annika Bande et al 2023 J. Phys. B: At. Mol. Opt. Phys. 56 232001
Inter-particle Coulombic electron capture (ICEC) is an environment-enabled electron capture process by means of which a free electron can be efficiently attached to a system (e.g. ion, atom, molecule, or quantum dot (QD)). The excess electron attachment energy is simultaneously transferred to a neighbouring system which concomitantly undergoes ionization (or excitation). ICEC has been theoretically predicted in van-der-Waals and in hydrogen-bonded systems as well as in QD arrays. The theoretical approaches employed in these works range from analytical models to electronic structure and (quantum) dynamical calculations. In this article, we provide a comprehensive review of the main theoretical approaches that have been developed and employed to investigate ICEC and summarize the main conclusions learned from these works. Since knowledge on ICEC is still in its early stage, we conclude this review with our own views and proposals on the future perspectives for the research in ICEC.
Lucy Downes 2023 J. Phys. B: At. Mol. Opt. Phys. 56 223001
Understanding the interactions between atoms and light is at the heart of atomic physics. Being able to 'experiment' with various system parameters, produce plots of the results and interpret these is very useful, especially for those new to the field. This tutorial aims to provide an introduction to the equations governing near-resonant atom-light interactions and present examples of setting up and solving these equations in Python. Emphasis is placed on clarity and understanding by showing code snippets alongside relevant equations, and as such it is suitable for those without an excellent working knowledge of Python or the underlying physics. Hopefully the methods presented here can form the foundations on which more complex models and simulations can be built. All functions presented here and example codes can be found on GitHub.
Open all abstracts, in this tab
zhang et al
We introduce a two-dimensional chirped finite energy Pearcey beams (FEPB) for the first time and investigate the propagation dynamics. First, we applied Huygens-Fresnel integral to derive an explicit analytical expression which is suitable for describing FEPB propagation in free space. It is interesting to find that FEPB will experience three typical propagation patterns, i.e., single-autofocusing case, dual-autofocusing case and non-autofocusing diffraction case, only depending on the value of the input asymmetric chirp. We further arrive at the critical condition of such three patterns analytically. However, another input symmetric chirp acts to strengthen or weaken the autofocusing intensity through changing the former sign, but does not affect the focal distance. Our findings denote that two-dimensional chirped FEPB can pave another prospective wave in controlling linear self-focusing and the optical particle manipulation, when compared with the corresponding Airy field or conventional Gaussian field.
Ansel et al
Quantum optimal control is a set of methods for designing time-varying electromagnetic fields to perform operations in quantum technologies. This tutorial paper introduces the basic elements of this theory based on the Pontryagin maximum principle, in a physicist-friendly way. An analogy with classical Lagrangian and Hamiltonian mechanics is proposed to present the main results used in this field. Emphasis is placed on the different numerical algorithms to solve a quantum optimal control problem. Several examples ranging from the control of two-level quantum systems to that of Bose-Einstein Condensates (BEC) in a one-dimensional optical lattice are studied in detail, using both analytical and numerical methods. Codes based on shooting method and gradient-based algorithms are provided. The connection between optimal processes and the quantum speed limit is also discussed in two-level quantum systems. In the case of BEC, the experimental implementation of optimal control protocols is described, both for two-level and many-level cases, with the current constraints and limitations of such platforms. This presentation is illustrated by the corresponding experimental results.
Zhao et al
To investigate the completeness of coupled channel (CC) Breit-Pauli R-Matrix (BPRM) calculations for opacities, we employ the relativistic distorted wave (RDW) method to complement ("top-up") and compare the BPRM photoionization cross sections for high-nℓ levels of both Fe xvii and Fe xviii . Good agreement is found in background photoionization cross sections using these two methods, which also ensures correct matching of bound level cross sections for completeness. In order to top-up the CC-BPRM calculations, bound-bound transitions involving additional bound levels, and a large number of doubly-excited quasi-bound levels corresponding to BPRM autoionizing resonances described in paper RMOPII, are calculated using the RDW method. Photoionization cross sections in the high energy region are also computed and compared up to about 500 Ry, and contributions from higher core level excitations than BPRM are considered. The effect of configuration interaction is investigated, which plays a significant role in correctly reproducing some background cross sections. Owing to the fact that the additional RDW levels correspond to high-nℓ bound levels that are negligibly populated according to the Mihalas-Hummer-Däppen equation-of-state (paper I), the effect on opacities is expected to be small.
Zhou et al
The nonlinear feedback between the gauge field and the material field can yield novel quantum
phenomena. Here, the interplay of a density-dependent artificial gauge field and Bose-Einstein con densates (BECs) trapped in optical lattice are studied. The energy spectrum and superfluidity
represented by energetic and dynamical stabilities of the system are systematically discussed. The
density-dependent artificial gauge field with a back-action between the BECs dynamics and the
gauge field induces an effective atomic interaction that depends on the quasi-momentum and den sity of the condensates, resulting in symmetry-broken energy spectrum and exotic stability phase
diagram, i.e., the system is stable only in a certain range of atoms density and under a limited lattice
strength. The density-dependent artificial gauge field changes the sequence for the emergence of
energetic and dynamical instability and the regimes of the energetic and dynamical instabilities are
significantly separated, offering an efficient way to study the energetic and dynamical instabilities
of superfluid separately. Particularly, the density-dependent artificial gauge field, as a mechanism
for transferring momentum to the fluid, results in dynamic instability of the condensates even in
free space. Our results provide a deep insight into the dynamical response of superfluid system to
the gauge field and have potential application for coherent control of exotic superfluid states.
Pradhan et al
An extended version of the R-matrix methodology is presented for calculation of radiative parameters for improved plasma opacities. Contrast and comparisons with existing methods primarily relying on the Distorted Wave (DW) approximation are discussed to verify accuracy and resolve outstanding issues, particularly with reference to the Opacity Project (OP). Among the improvements incorporated are: (i) large-scale Breit-Pauli R-matrix (BPRM) calculations for complex atomic systems including fine structure, (ii) convergent close coupling wave function expansions for the (e + ion) system to compute oscillator strengths and photoionization cross sections, (iii) open and closed shell iron ions of interest in astrophysics and experiments, (iv) a treatment for plasma broadening of autoionizing resonances as function of energy-temperature-density dependent cross sections, (v) a "top-up" procedure to compare convergence with R-matrix calculations for highly excited levels, and (vi) spectroscopic identification of resonances and bound (e + ion) levels. The present R-matrix monochromatic opacity spectra are fundamentally different from OP and lead to enhanced Rosseland and Planck mean opacities. An outline of the work reported in other papers in this series and those in progress is presented. Based on the present re-examination of the OP work, it is evident that opacities of heavy elements require revisions in high temperature-density plasma sources.
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K Ferentinou et al 2024 J. Phys. B: At. Mol. Opt. Phys. 57 115002
Slow (meV) photoelectron imaging spectroscopy is employed in the experimental study of near-threshold photoionization of strontium atoms in the presence of an external static electric field. Specifically, the study is devoted to the glory effect, that is, the appearance of an intense peak at the center of the recorded photoelectron images, when dealing with m= 0 final ionized Stark states (m denoting the magnetic quantum number). This critical effect is formally identical to that encountered in classical scattering theory, where, for a nonzero value of the impact parameter, the zero-crossing of the deflection function leads to a divergent classical differential cross section. By recording the magnitude variation of this glory peak as a function of electron excitation energy, we observe that, besides the traces of classical origin, it also exhibits intense quantum interference and beating phenomena, above and below the zero-static-field ionization threshold. We study both, single- and two-photon ionization of Sr, thus enabling a comparison not only between the different excitation schemes, but also with an earlier work devoted to two-photon ionization of Mg atom by Kalaitzis et al (2020 Phys. Rev. A 102 033101). Our recordings are analyzed within the framework of the Harmin–Fano frame transformation Stark effect theory that is applied to both the hydrogen atom and a non-hydrogenic one simulating Sr. We discuss the various aspects of the recorded and calculated glory interference and beating structures and their 'short time Fourier transforms' and classify them as either atom-specific or atom independent. In particular, we verify the 'universal' connection between the glory oscillations above the zero-field threshold and the differences between the origin-to-detector times of flight corresponding to pairs of classical electron trajectories that end up to the image center.
P B Blakie 2024 J. Phys. B: At. Mol. Opt. Phys. 57 115301
We investigate the superfluid fraction of crystalline stationary states within the framework of mean-field Gross–Pitaevskii theory. Our primary focus is on a two-dimensional Bose–Einstein condensate with a non-local soft-core interaction, where the superfluid fraction is described by a rank-2 tensor. We then calculate the superfluid fraction tensor for crystalline states exhibiting triangular, square, and stripe geometries across a broad range of interaction parameters. Factors leading to an anisotropic superfluid fraction tensor are also considered. We also refine the Leggett bounds for the superfluid fraction of the 2D system. We systematically compare these bounds to our full numerical results, and other results in the literature. This work is of direct relevance to other supersolid systems of current interest, such as supersolids produced using dipolar Bose–Einstein condensates.
Hugo J B Marroux et al 2024 J. Phys. B: At. Mol. Opt. Phys. 57 115401
We present an extreme ultraviolet (EUV) transient grating (TG) experiment of the spinel Co3O4 compound using tuneable incident energies across the Co M2,3-edge and a 395 nm probe pulse, detecting both the first and the second diffraction orders (SDOs). While the first diffraction order shows a monotonous behavior as a function of time, with a sharp response at t = 0, followed by a weak sub-picosecond component and a nearly constant signal thereafter, the time dependence of SDO varies dramatically with the incident energy as it is tuned across the Co M-edge, with the appearance of a component at t > 1 ps that grows with increasing energy. The results are rationalized in terms of the deviations of the initial grating from sinusoidal to non-sinusoidal, namely a flattening of the grating pattern, that introduces new Fourier components. These deviations are due to higher order, three-body terms in the population relaxation kinetics. The present results highlight the use of the SDO response in EUV TG as a tool to identify higher order terms in the population kinetics.
Lianshui Zhao et al 2024 J. Phys. B: At. Mol. Opt. Phys.
To investigate the completeness of coupled channel (CC) Breit-Pauli R-Matrix (BPRM) calculations for opacities, we employ the relativistic distorted wave (RDW) method to complement ("top-up") and compare the BPRM photoionization cross sections for high-nℓ levels of both Fe xvii and Fe xviii . Good agreement is found in background photoionization cross sections using these two methods, which also ensures correct matching of bound level cross sections for completeness. In order to top-up the CC-BPRM calculations, bound-bound transitions involving additional bound levels, and a large number of doubly-excited quasi-bound levels corresponding to BPRM autoionizing resonances described in paper RMOPII, are calculated using the RDW method. Photoionization cross sections in the high energy region are also computed and compared up to about 500 Ry, and contributions from higher core level excitations than BPRM are considered. The effect of configuration interaction is investigated, which plays a significant role in correctly reproducing some background cross sections. Owing to the fact that the additional RDW levels correspond to high-nℓ bound levels that are negligibly populated according to the Mihalas-Hummer-Däppen equation-of-state (paper I), the effect on opacities is expected to be small.
K A Beyer and N S Oreshkina 2024 J. Phys. B: At. Mol. Opt. Phys. 57 105003
In an attempt to address the long-standing fine-structure puzzle in heavy muonic atoms we investigate the magnetic interaction between a nucleus and its bound muon. A simple estimate shows that the effect is only noticeable for unrealistic nuclear parameters. A further investigation as to the relation of this effect to nuclear polarisation (NP) identifies the interaction as the magnetic dipole part of NP. Motivated by the relative closeness of this simple estimate to rigorous evaluations of NP, we extract effective values for the nuclear magnetic polarisability, a quantity otherwise unknown for all but the lightest nuclei.
Laura Carlini et al 2024 J. Phys. B: At. Mol. Opt. Phys. 57 105401
The fragmentation of three cyclic dipeptides (c-Glycil-Phenylalanine, c-Tryptophan-Tyrosine and c-Tryptophan-Tryptophan), characterized by an aromatic side chain, has been investigated by synchrotron radiation and photoelectron-photoion coincidence (PEPICO) experiments, assisted by atomistic simulations. The PEPICO experiments show that the charged moiety containing the aromatic side chain is the main fragment in the three samples. The theoretical exploration of the potential energy surfaces has allowed to identify the possible fragmentation paths leading to the formation of these fragments. Then, the analysis of the differences in the electronic density distributions of the neutral molecule and the cation and a molecular dynamics simulation provided an understanding of the preferred localization of the positive charge on the aromatic side chain of the cyclic dipeptide.
H B Ambalampitiya et al 2024 J. Phys. B: At. Mol. Opt. Phys. 57 10LT01
We consider positronium formation in collisions of positrons with excited hydrogen atoms H(n) in an infrared laser field theoretically. This process is assisted by the dipolar focusing effect: a positron moving in a superposition of a laser field and the dipolar field can approach the atomic target even if its trajectory starts with a very large impact parameter, leading to a significant enhancement of the Ps formation cross section. The classical trajectory Monte Carlo method, which is justified for , allows efficient calculation of this enhancement. A similar effect can occur in collisions of positrons with other atoms in excited states, which can lead to improvements in the efficiency of positronium formation.
A K Pradhan et al 2024 J. Phys. B: At. Mol. Opt. Phys.
An extended version of the R-matrix methodology is presented for calculation of radiative parameters for improved plasma opacities. Contrast and comparisons with existing methods primarily relying on the Distorted Wave (DW) approximation are discussed to verify accuracy and resolve outstanding issues, particularly with reference to the Opacity Project (OP). Among the improvements incorporated are: (i) large-scale Breit-Pauli R-matrix (BPRM) calculations for complex atomic systems including fine structure, (ii) convergent close coupling wave function expansions for the (e + ion) system to compute oscillator strengths and photoionization cross sections, (iii) open and closed shell iron ions of interest in astrophysics and experiments, (iv) a treatment for plasma broadening of autoionizing resonances as function of energy-temperature-density dependent cross sections, (v) a "top-up" procedure to compare convergence with R-matrix calculations for highly excited levels, and (vi) spectroscopic identification of resonances and bound (e + ion) levels. The present R-matrix monochromatic opacity spectra are fundamentally different from OP and lead to enhanced Rosseland and Planck mean opacities. An outline of the work reported in other papers in this series and those in progress is presented. Based on the present re-examination of the OP work, it is evident that opacities of heavy elements require revisions in high temperature-density plasma sources.
Anil Kumar Pradhan 2024 J. Phys. B: At. Mol. Opt. Phys.
A general formulation is employed to study and quantitatively ascertain the effect of plasma broadening of intrinsic autoionizing (AI) resonances in photoionization cross sections. In particular, R-matrix data for iron ions described in the previous paper in the RMOP series (RMOP-II, hereafter RMOP2) are used to demonstrate underlying physical mechanisms due to electron collisions, ion microfields (Stark), thermal Doppler effects, core excitations, and free-free transitions. Breit-Pauli R-matrix (BPRM) cross section for the large number of bound levels of Fe ions are considered, 454 levels of Fe XVII, 1,184 levels of Fe XVIII and 508 levels of Fe XIX. Following a description of theoretical and computational methods, a sample of results is presented to show significant broadening and shifting of AI resonances due to Extrinsic plasma broadening as a function of temperature and density. Redistribution of AI resonance strengths broadly preserves their integrated strengths as well as the naturally intrinsic asymmetric shapes of resonance complexes which are broadened, smeared and flattened, eventually dissolving into the bound-free continua.
Sultana N Nahar et al 2024 J. Phys. B: At. Mol. Opt. Phys.
Iron is the dominant heavy element that plays an important role in radiation transport in stellar interiors. Owing to its abundance and large number of bound levels and transitions, iron ions determine the opacity more than any other astrophysically abundant element. A few iron ions constitute the abundance and opacity of iron at the base of the convection zone (BCZ) at the boundary between the solar convection and radiative zones, and are the focus of the present study. Together, Fe xvii , Fe xviii and Fe xix contribute 85% of iron ion fractions 20%, 39% and 26% respectively, at the BCZ physical conditions of temperature T ∼ 2.11 × 106K and electron density Ne = 3.1 × 1022cc. We report heretofore the most extensive R-matrix atomic calculations for these ions for bound-bound and bound-free transitions, the two main processes of radiation absorption. We consider wavefunction expansions with 218 target or core ion fine structure levels of Fe xviii for Fe xvii , 276 levels of Fe xix for Fe xviii , in the Breit-Pauli R-matrix (BPRM) approximation, and 180 LS terms (equivalent to 415 fine structure levels) of Fe xx for Fe xix calculations. These large target expansions which includes core ion excitations to n=2,3,4 complexes enable accuracy and convergence of photoionization cross sections, as well as inclusion of high lying resonances. The resulting R-matrix datasets include 454 bound levels for Fe xvii , 1,174 levels for Fe xviii , and 1,626 for Fe xix up to n ≤ 10 and l=0 - 9. Corresponding datasets of oscillator strengths for photoabsorption are: 20,951 transitions for Fe xvii , 141,869 for Fe xviii , and 289,291 for Fe xix . Photoionization cross sections have obtained for all bound fine structure levels of Fe xvii and Fe xviii , and for 900 bound LS states of Fe xix . Selected results demonstrating prominent characteristic features of photoionization are presented, particularly the strong Seaton PEC (photoexcitation-of-core) resonances formed via high-lying core excitations with ∆n = 1 that significantly impact bound-free opacity.