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.

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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

The measurements of Crandall et al. [Phys. Rev. A 10, 141 (1974)] for the polarization of light emitted during electron-impact excitation of ${\mathrm{Ba}}^{+}$ are in significant disagreement with relativistic distorted wave scattering theory calculations of Sharma et al. [Phys. Rev. A 83, 062701 (2011)]. The relativistic convergent close-coupling (RCCC) method is applied to the problem and the discrepancy between theory and experiment is resolved across the full range of energies measured. Furthermore, the oscillations in the polarization fraction measurements at low energies near threshold are reproduced by the RCCC calculations and linked to the formation of autoionizing states of neutral Ba.

Calculation of the polarization of light emitted during electron-impact excitation of Ba+

A recently developed fully quantal convergent-close-coupling (CCC) approach (Abdurakhmanov et al 2011 J Phys B: At Mol Opt Phys 44 075204) to ion–atom collisions is extended to differential ionization studies An important feature of the method is that it does not have classical limitations on the relative motion of participating particles The approach is applied to calculate fully differential, as well as various doubly and singly differential cross sections of ionization in antiproton collisions with atomic hydrogen The CCC results for various differential cross sections agree reasonably well with the results of the semiclassical CC and the continuum-distorted-wave-eikonal-initial-state approaches, particularly at high energies However, some discrepancies exist at low energies, where the final state interactions among colliding particles are found to be very important

Differential ionization in antiproton–hydrogen collisions within the convergent-close-coupling approach

We apply the fully quantum-mechanical convergent close-coupling method to the calculation of antiproton scattering on the ground state of helium. The helium target is treated as a three-body Coulomb system using frozen-core and multiconfiguration approximations. The electron-electron correlation of the target is fully treated in both cases. Though both calculations yield generally good agreement with experiment for the total ionization cross sections, the multiconfiguration results are substantially higher at the lower energies than the frozen-core ones. Calculated longitudinal ejected electron and recoil-ion momentum distributions for the single ionization of helium are in good agreement with the experiment.

Convergent close-coupling calculations of helium single ionization by antiproton impact

Following absorption of a single photon, angles of simultaneous emission of two electrons from a He(n 1S) atom become more correlated with increasing n. We find that the strength of this correlation is due to the two-electron continuum of the electron-impact ionization of the He+(ns) ion. The strength is determined by the width of the momentum profile of the ionic ns state but not the strength of the electron correlation in the He initial state. This can explain the increasing (over He) angular correlation strength found in double photoionization of targets such as Be, Ne, and H2.

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Angular correlation in the two-electron continuum

Angular correlation parameters, lambda , R, the circular polarization parameter I and the differential cross section are measured for the electron impact excitation of hydrogen atoms to the 32Pj states at an incident electron energy of 54.42 eV and for electron scattering angles up to 35 degrees . Comparison of the measurements with the convergent close-coupling calculations of Bray and Stelbovics (1992) shows excellent agreement with experiment.

Igor Bray

Papers

Calculation of the polarization of light emitted during electron-impact excitation of Ba+

Differential ionization in antiproton–hydrogen collisions within the convergent-close-coupling approach

Convergent close-coupling calculations of helium single ionization by antiproton impact

Angular correlation in the two-electron continuum

Angular and polarization correlation measurements for the 32P states of atomic hydrogen