Resonant Annihilation and Positron Bound States in Benzene

Positrons attach to molecules in vibrationally resonant two-body collisions that result in greatly enhanced annihilation rates. Measurements of annihilation as a function of positron energy are presented for benzene using a cryogenic, trap-based beam. They establish a positron binding energy of 132±3  meV to test state-of-the-art theoretical calculations, and they exhibit many unexpected resonances, likely due to combination and overtone vibrational modes. The relationship of these results to the unique π-bonded structure of benzene is discussed.

Enhanced Resonant Positron Annihilation due to Nonfundamental Modes in Molecules

Positrons attach to most molecules through Feshbach resonant excitation of fundamental vibrational modes, and this leads to greatly enhanced annihilation rates. In all but the smallest molecules, vibrational energy transfer further enhances these annihilation rates. Evidence is presented that in alkane and cycloalkane molecules, this can occur by the excitation of other than fundamental vibrations and produce roughly comparable annihilation rates. These features are compared to infrared absorption spectra. A possible mechanism is discussed that involves combination and overtone vibrations.

Vibrational Feshbach Resonances Mediated by Nondipole Positron-Molecule Interactions

Measurements of energy-resolved positron-molecule annihilation show the existence of positron binding and vibrational Feshbach resonances. The existing theory describes this phenomenon successfully for the case of infrared-active vibrational modes that allow dipole coupling between the incident positron and the vibrational motion. Presented here are measurements of positron-molecule annihilation made using a recently developed cryogenic positron beam capable of significantly improved energy resolution. The results provide evidence of resonances associated with infrared-inactive vibrational modes, indicating that positron-molecule bound states may be populated by nondipole interactions. The anticipated ingredients for a theoretical description of such interactions are discussed.

Evolution of a Vortex in a Strain Flow

Experiments and vortex-in-cell simulations are used to study an initially axisymmetric, spatially distributed vortex subject to an externally imposed strain flow. The experiments use a magnetized pure electron plasma to model an inviscid two-dimensional fluid. The results are compared to a theory assuming an elliptical region of constant vorticity. For relatively flat vorticity profiles, the dynamics and stability threshold are in close quantitative agreement with the theory. Physics beyond the constant-vorticity model, such as vortex stripping, is investigated by studying the behavior of nonflat vorticity profiles.

Electron plasma orbits from competing diocotron drifts

The perpendicular dynamics of a pure electron plasma column are investigated when the plasma spans two Penning-Malmberg traps with noncoinciding axes. The plasma executes noncircular orbits described by competing image-charge electric-field (diocotron) drifts from the two traps. A simple model is presented that predicts a set of nested orbits in agreement with observed plasma trajectories.

Role of vibrational dynamics in resonant positron annihilation on molecules

Vibrational Feshbach resonances are dominant features of positron annihilation for incident positron energies in the range of the molecular vibrations. Studies in relatively small molecules are described that elucidate the role of intramolecular vibrational energy redistribution into near-resonant multimode states, and the subsequent coupling of these modes to the positron continuum, in suppressing or enhancing these resonances. The implications for annihilation in other molecular species, and the necessary ingredients of a more complete theory of resonant positron annihilation, are discussed.

Comparisons of positron and electron binding to molecules

Positron binding to molecules is compared to the analogous electron-molecule bound states. For both, the bound lepton density is diffuse and remains outside the valence shell. Positron binding energies are found to be one to two orders of magnitude larger than those of the negative ions due to two effects: the orientation of the molecular dipole moment allows the positron to approach it more closely and, for positrons, lepton correlations (e.g., via dipole polarizability) contribute more strongly.

Ubiquitous nature of multimode vibrational resonances in positron-molecule annihilation

Positron annihilation on many molecules occurs via positron capture into vibrational Feshbach resonances, with annihilation rates often further enhanced by energy transfer to vibrational excitations weakly coupled to the positron continuum. Data presented here uncover another scenario in which the positron couples directly to a quasicontinuum of multimode vibrational states. A model that assumes excitation and escape from a statistically complete ensemble of multimode vibrations is presented that reproduces key features of the data.

Note: Electrostatic beams from a 5 T Penning-Malmberg trap

A procedure is described to extract beams from specially tailored electron plasmas in a Penning-Malmberg trap in a 4.8 T field. Transport to 1 mT is followed by extraction from the magnetic field and electrostatic focusing. Potential applications to positron beams are discussed.

Dipole enhancement of positron binding to molecules

Measurements of positron-molecule binding energies are made for molecules with large permanent dipole moments (>2.7  D), by studying vibrational-Feshbach-mediated annihilation resonances as a function of incident positron energy. The binding energies are relatively large (e.g., ≥90  meV) as compared to those for similar sized molecules studied previously and analogous weakly bound electron-molecule (negative ion) states. Comparisons with existing theoretical predictions are discussed.

High-density fixed point for radially compressed single-component plasmas

Rotating electric fields are used to compress electron plasmas confined in a Penning-Malmberg trap. Bifurcation and hysteresis are observed between low-density and high-density steady states as a function of the applied electric field amplitude and frequency. These observations are explained in terms of torque-balanced fixed points using a simple model of the torques on the plasma. Perturbation experiments near the high-density fixed point are used to determine the magnitude, frequency, and voltage dependence of the drive torque. The broader implications of these results are discussed.

Role of binding energy in Feshbach-resonant positron-molecule annihilation

Measurements of positron-on-molecule annihilation have established that positrons bind to a variety of molecules via vibrational Feshbach resonances. Data for deeply bound states in benzene and 1-chlorohexane and for positronically excited (i.e., second) bound states in alkanes are used to establish the dependence of annihilation rates on the binding energy and incident positron energy. With this dependence removed, annihilation rates for a broad class of molecules lie on a universal curve as a function of the number of molecular vibrational degrees of freedom. The implications of these results for theoretical models are discussed.

Torque-balanced high-density steady states of single-component plasmas

Electron plasmas in a Penning-Malmberg trap are compressed radially using a rotating electric field (the "rotating-wall technique"). For large electric fields, plasmas can be compressed over a broad range of frequencies. This permits access to a novel high-density regime in which outward transport is insensitive to plasma density. The limiting density occurs when the plasma rotation frequency equals the rotating-wall frequency. Characteristics of the resulting torque-balanced steady states are described, and implications for high-density electron and positron plasma confinement are discussed.

Vibrational-resonance enhancement of positron annihilation in molecules

The rate of annihilation of low-energy positrons in many molecular gases is orders of magnitude larger than can be explained on the basis of simple collisions. Developments in positron beam technology have enabled the first energy-resolved measurements of this annihilation process. The results of these experiments provide direct evidence that the large observed values of annihilation rate are due to the excitation of long-lived vibrational resonances of the positron-molecule complex. These results are generally consistent with a recent theoretical model of resonant annihilation.

Excitation of electronic states of Ar, H(2), and N(2) by positron impact

We have measured the first state-resolved, absolute cross sections for positron excitation of electronic states of an atom or molecule using a high resolution (Delta E approximately 25 meV FWHM) beam of positrons from a Penning-Malmberg trap. We present cross sections for the excitation of the low-lying levels of Ar, H(2), and N(2) for incident positron energies between threshold and 30 eV. For Ar and H2, comparison can be made with theoretical calculations, and, in the case of H(2), the results resolve a significant discrepancy between the only two available calculations.

Effects of lateral boundaries on traveling-wave dynamics in binary fluid convection

The global dynamics of traveling-wave patterns in convection in a mixture of ethanol in water is studied in different cell geometries: circular, rectangular, and stadium-shaped cells. The dynamics in these cells differ greatly, changing from a globally rotating state in the circular cell, to one large domain of locally parallel traveling waves in the rectangular cell, to a continually chaotic state in the stadium cell. In all three cases, the patterns can be described in terms of the phase of the complex order parameter. Disorder in the patterns is quantified in terms of topological defects in the phase field. While the numbers, net charge, and dynamics of defects differ greatly in the patterns in the three cells, the local dynamics of the defects, as measured by the defect-defect correlation functions, are similar.

Excitation of molecular vibrations by positron impact

Absolute cross sections for the vibrational excitation of CO, CO2, and H2 by positron impact are presented for incident positron energies from 0.5 eV to several electron volts. The measurements use a novel technique that exploits the adiabatic motion of a positron beam in a strong magnetic field. This work is the first systematic experimental study of vibrational excitation by positron impact, and extends to energies where positron measurements have traditionally been difficult. The measured cross sections are compared with available theoretical calculations.

Inward transport and compression of a positron plasma by a rotating electric field

Inward transport of a magnetized pure positron plasma confined in a Penning-Malmberg trap is produced by applying a rotating electric field to the plasma. Compression is observed over a broad range of frequencies. Compression factors up to 20 in central density were obtained. Positron collisions with a neutral buffer gas are used to counteract the heating due to the rotating electric field. The results have implications for a variety of applications including the production of brightness enhanced positron beams, the study of electron-positron plasmas, and antihydrogen production.