There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Annals of physics

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.

/pdf/recoil-ion-and-electron-momentum-spectroscopy-reaction-c7ai542e3l.pdf

Recoil-ion and electron momentum spectroscopy: reaction-microscopes

▪ Abstract Turbulence affects the structure and motions of nearly all temperature and density regimes in the interstellar gas. This two-part review summarizes the observations, theory, and simulations of interstellar turbulence and their implications for many fields of astrophysics. The first part begins with diagnostics for turbulence that have been applied to the cool interstellar medium and highlights their main results. The energy sources for interstellar turbulence are then summarized along with numerical estimates for their power input. Supernovae and superbubbles dominate the total power, but many other sources spanning a large range of scales, from swing-amplified gravitational instabilities to cosmic ray streaming, all contribute in some way. Turbulence theory is considered in detail, including the basic fluid equations, solenoidal and compressible modes, global inviscid quadratic invariants, scaling arguments for the power spectrum, phenomenological models for the scaling of higher-order structu...

http://www.ifa.hawaii.edu/%7Ereipurth/reviews/elmegreen1_araa.pdf

Interstellar Turbulence I: Observations and Processes

Cold Target Recoil Ion Momentum Spectroscopy: a &momentum microscope' to view atomic collision dynamics

We observe a two-electron interference effect in the symmetrized amplitude of the resonant double photoionization of Mg which was predicted theoretically, but never before observed experimentally. This observation is based on our experimental study of angle-resolved double photoionization of Mg at the photon energy of 55.49 eV (2p → 3d resonance) under equal and unequal energy sharing conditions.

https://iopscience.iop.org/article/10.1088/1742-6596/488/1/012023/pdf

Two electron interference in angular resolved double photoionization of Mg

We present results for the Lyman-α line of hydrogen in dense plasmas. Full line profiles are calculated within a quantum-statistical method, based on thermodynamic Green's functions. The contributions of plasma ions and electrons are considered separately. Linear and quadratic Stark effect as well as quadrupole effects are taken into account for ions. The model microfield method is used to include ion dynamics. The focus of this work lies on the contribution to broadening and shift by free electrons beyond the Born approximation. The effect of strong collisions can be identified as ladder-like diagrams of the electron-emitter propagator. In an effective two-particle approximation, the electronic self-energy is given in terms of scattering amplitudes, analogous to Baranger's expressions [Baranger, M 1958 Phys. Rev. 112 855]. We obtained scattering amplitudes from convergent close-coupling calculations including medium effects via Debye screening. Additionally, the electronic coupling between initial and final states is taken care of by a vertex correction. In our examples, the free electron density ranges between 1023 and 1025 m−3 at a plasma temperature of 1 and 2 eV, respectively.

https://iopscience.iop.org/article/10.1088/1742-6596/397/1/012021/pdf

Quantum-statistical line shape calculation for Lyman-α lines in dense H plasmas

Integrated total and state-selective electron-capture cross sections are calculated for bare beryllium ion scattering on atomic hydrogen initially configured in the $2\ensuremath{\ell}m$ states (where $n=2, \ensuremath{\ell}$, and $m$ are the principal, angular momentum, and magnetic quantum numbers, respectively). This is done using the wave-packet convergent close-coupling approach which solves the three-body Schr\"odinger equation by employing a two-center expansion for the total scattering wave function. These calculations are performed within a projectile-energy range of 1 to 500 keV/u. The results suggest that at low energies, collisions with hydrogen in each of the $2\ensuremath{\ell}m$ states produce a total electron-capture cross section approximately an order of magnitude larger than for scattering on the ground state. However, as projectile energy increases, the cross section for capture from the excited states falls well below the $\mathrm{H}(1s$) electron capture cross section. A possible reason for this observation could be related with the way the target electron radial densities are distributed in different initial states. The results obtained in this work are compared to previous calculations where available. In terms of the $n$-resolved charge-exchange cross sections, significant disagreement is found between our results and some preceding calculations available in the literature to warrant further investigations. 

Fully-stripped-beryllium-ion collisions with 
2ℓm
 states of atomic hydrogen: Total and state-selective electron-capture cross sections

The stopping power of hydrogen for protons and antiprotons

Electron-impact ionisation cross sections for the hydrogen- and helium-like ions of iron are calculated. The convergent close-coupling and distorted-wave methods in the relativistic and nonrelativistic formulations are used. The cross sections are in very good agreement with the results of the corresponding methods, with the relativistic formulation establishing the benchmarks to few percent accuracy.

https://iopscience.iop.org/article/10.1088/0953-4075/49/18/184001/pdf

Igor Bray

Papers

Two electron interference in angular resolved double photoionization of Mg

Quantum-statistical line shape calculation for Lyman-α lines in dense H plasmas

Fully-stripped-beryllium-ion collisions with 2ℓm states of atomic hydrogen: Total and state-selective electron-capture cross sections

The stopping power of hydrogen for protons and antiprotons

Calculations of electron-impact ionisation of and