Recoil-ion and electron momentum spectroscopy is a rapidly developing technique that allows one to measure the vector momenta of several ions and electrons resulting from atomic or molecular fragmentation. In a unique combination, large solid angles close to 4π and superior momentum resolutions around a few per cent of an atomic unit (a.u.) are typically reached in state-of-the art machines, so-called reaction-microscopes. Evolving from recoil-ion and cold target recoil-ion momentum spectroscopy (COLTRIMS), reaction-microscopes—the `bubble chambers of atomic physics'—mark the decisive step forward to investigate many-particle quantum-dynamics occurring when atomic and molecular systems or even surfaces and solids are exposed to time-dependent external electromagnetic fields. This paper concentrates on just these latest technical developments and on at least four new classes of fragmentation experiments that have emerged within about the last five years. First, multi-dimensional images in momentum space brought unprecedented information on the dynamics of single-photon induced fragmentation of fixed-in-space molecules and on their structure. Second, a break-through in the investigation of high-intensity short-pulse laser induced fragmentation of atoms and molecules has been achieved by using reaction-microscopes. Third, for electron and ion-impact, the investigation of two-electron reactions has matured to a state such that the first fully differential cross sections (FDCSs) are reported. Fourth, comprehensive sets of FDCSs for single ionization of atoms by ion-impact, the most basic atomic fragmentation reaction, brought new insight, a couple of surprises and unexpected challenges to theory at keV to GeV collision energies. In addition, a brief summary on the kinematics is provided at the beginning. Finally, the rich future potential of the method is briefly envisaged.

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Recoil-ion and electron momentum spectroscopy: reaction-microscopes

The observable universe could be a 1 + 3-surface (the “brane”) embedded in a 1 + 3 + d-dimensional spacetime (the “bulk”), with Standard Model particles and fields trapped on the brane while gravity is free to access the bulk. At least one of the d extra spatial dimensions could be very large relative to the Planck scale, which lowers the fundamental gravity scale, possibly even down to the electroweak (∼ TeV) level. This revolutionary picture arises in the framework of recent developments in M theory. The 1 + 10-dimensional M theory encompasses the known 1 + 9-dimensional superstring theories, and is widely considered to be a promising potential route to quantum gravity. General relativity cannot describe gravity at high enough energies and must be replaced by a quantum gravity theory, picking up significant corrections as the fundamental energy scale is approached. At low energies, gravity is localized at the brane and general relativity is recovered, but at high energies gravity “leaks” into the bulk, behaving in a truly higher-dimensional way. This introduces significant changes to gravitational dynamics and perturbations, with interesting and potentially testable implications for high-energy astrophysics, black holes, and cosmology. Brane-world models offer a phenomenological way to test some of the novel predictions and corrections to general relativity that are implied by M theory. This review discusses the geometry, dynamics and perturbations of simple brane-world models for cosmology and astrophysics, mainly focusing on warped 5-dimensional brane-worlds based on the Randall-Sundrum models.

Brane-World Gravity

Light–matter interactions are inherently slow as the wavelengths of optical and electronic states differ greatly. Surface plasmon polaritons — electromagnetic excitations at metal–dielectric interfaces — have generated significant interest because their spatial scale is decoupled from the vacuum wavelength, promising accelerated light–matter interactions. Although recent reports suggest the possibility of accelerated dynamics in surface plasmon lasers, this remains to be verified. Here, we report the observation of pulses shorter than 800 fs from hybrid plasmonic zinc oxide (ZnO) nanowire lasers. Operating at room temperature, ZnO excitons lie near the surface plasmon frequency in such silver-based plasmonic lasers, leading to accelerated spontaneous recombination, gain switching and gain recovery compared with conventional ZnO nanowire lasers. Surprisingly, the laser dynamics can be as fast as gain thermalization in ZnO, which precludes lasing in the thinnest nanowires (diameter less than 120 nm). The capability to combine surface plasmon localization with ultrafast amplification provides the means for generating extremely intense optical fields, with applications in sensing, nonlinear optical switching, as well as in the physics of strong-field phenomena. Electron scattering limits the optical excitations produced by metal-based lasers to femtosecond timescales. But sub-picosecond pulsing can be achieved in a plasmonic nanowire laser by operating near the surface plasmon frequency.

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Ultrafast plasmonic nanowire lasers near the surface plasmon frequency

Electronic states of highly charged ions show magnified fine-structure, Lamb shift, and hyperfine effects making them sensitive probes of bound-state quantum electrodynamics and nuclear physics. Being also impervious to external perturbations renders them ideal candidates for precision spectroscopy and accurate clocks that could test physics beyond the standard model. This review discusses how a variety of ion species and transitions may optimally be used to target such new applications, and presents routes to handle them in the laboratory.

Highly charged ions: Optical clocks and applications in fundamental physics

This paper reviews some of the fundamental properties of highly charged ions, the methods of producing them (with particular emphasis on table-top devices), and their use as a tool for both basic science and applied technology. Topics discussed include: charge dependence and scaling laws along isoelectronic or isonuclear sequences (for wavefunction size or Bohr radius, ionization energy, dipole transition energy, relativistic fine structure, hyperfine structure, Zeeman effect, Stark effect, line intensities, linewidths, strength of parity violation, etc), changes in angular momentum coupling schemes, selection rules, interactions with surfaces, electron-impact ionization, the electron beam ion trap (EBIT), ion accelerators, atomic reference data, cosmic chronometers, laboratory x-ray astrophysics, vacuum polarization, solar flares, ion implantation, ion lithography, ion microprobes (SIMS and x-ray microscope), nuclear fusion diagnostics, nanotechnology, quantum computing, cancer therapy and biotechnology.

https://iopscience.iop.org/article/10.1088/0953-4075/34/19/201/pdf

Highly charged ions

Double ionization of helium by 6 MeV proton impact has been explored in a kinematically complete experiment using a "reaction microscope." For the first time, fully differential cross sections for positively charged projectiles have been obtained and compared with data from 2 keV electron impact. The significant differences observed in the angular distribution of the ejected electrons are attributed to the charge sign of the projectile, resulting in different dynamics of the four-particle Coulomb system, which is not considered in the first Born approximation.

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Projectile-Charge Sign Dependence of Four-Particle Dynamics in Helium Double Ionization

An electron beam ion trap (EBIT) is under construction at the University of Freiburg. It will be operated both as an ion trap and as an ion source, and its design specifications will allow production of hydrogenic ions from even the heaviest elements. It will be possible to extract and accelerate the ions to energies up to 350 kV/q.

The Freiburg Electron Beam Ion Trap/Source Project FreEBIT

Dissociation of CO{sub 2}{sup 3+} formed in fast, heavy-ion-induced ionization of CO{sub 2}, using 5 MeV u{sup -1} Si{sup 12+} ions as projectiles, has been studied by the technique of multi-ion coincidence momentum imaging. From the momentum correlation between the ejected ions, we conclude that dissociation of CO{sub 2}{sup 3+} into C{sup +}+O{sup +}+O{sup +} is a concerted process involving linear and bent conformations of CO{sub 2}{sup 3+}. The kinetic-energy-release spectrum of the dissociation channel CO{sub 2}{sup 3+}{yields}C{sup +}+O{sup +}+O{sup +} is measured and the possible excited states of CO{sub 2}{sup 3+} are computed ab initio.

Ion-induced triple fragmentation of CO 2 3 + into C + + O + + O +

In an experiment involving detection of a photoelectron and up to three photoions from CO{sub 2} in coincidence, we observe the triple ion coincidence C{sup +}:O{sup +}:O{sup +}. Moreover, we observe double coincidences between doubly charged cations and singly charged cation pairs C{sup 2+}:O{sup +}, O{sup 2+}:C{sup +}, O{sup 2+}:O{sup +}. These ion triplets and pairs arise from fragmentation of the triply charged molecular ion CO{sub 2}{sup 3+}. Other ion pairs--viz., C{sup +}:O{sup +}, O{sup +}:O{sup +}, O{sup +}:CO{sup +}--arising from the doubly charged molecular ion CO{sub 2}{sup 2+}, are also observed. From an analysis of the coincidence pattern we postulate four decay modes of the CO{sub 2}{sup 3+} ion. Kinetic energy release in the channel leading to C{sup +}:O{sup +}:O{sup +} is measured, and its distribution is postulated to have four contributing precursor states.

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B. Bapat

Papers

Projectile-Charge Sign Dependence of Four-Particle Dynamics in Helium Double Ionization

The Freiburg Electron Beam Ion Trap/Source Project FreEBIT

Ion-induced triple fragmentation of CO 2 3 + into C + + O + + O +

Triply charged carbon dioxide molecular ion: Formation and fragmentation

Fragmentation dynamics of the methanol dication