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.

Non-neutral plasma manipulation techniques in development of a high-capacity positron trap

Preliminary experiments have been performed toward the development of a multi-cell Penning-Malmberg trap for the storage of large numbers of positrons (≥10¹⁰ e⁺). We introduce the master-cell test trap and the diagnostic tools for first experiments with electrons. The usage of a phosphor screen to measure the z-integrated plasma distribution and the number of confined particles is demonstrated, as well as the trap alignment to the magnetic field (B = 3.1 T) using the m = 1 diocotron mode. The plasma parameters and expansion are described along with the autoresonant excitation of the diocotron mode using rotating dipole fields and frequency chirped sinusoidal drive signals. We analyze the reproducibility of the excitation and use these findings to settle on the path for the next generation multi-cell test device.

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.

Lossless Positron Injection into a Magnetic Dipole Trap

The high-efficiency injection of a low-energy positron beam into the confinement volume of a magnetic dipole has been demonstrated experimentally. This was accomplished by tailoring the three-dimensional guiding-center drift orbits of positrons via optimization of electrostatic potentials applied to electrodes at the edge of the trap, thereby producing localized and essentially lossless cross-field particle transport by means of the E×B drift. The experimental findings are reproduced and elucidated by numerical simulations, enabling a comprehensive understanding of the process. These results answer key questions and establish methods for use in upcoming experiments to create an electron-positron plasma in a levitated dipole device.

Confinement of Positrons Exceeding 1 s in a Supported Magnetic Dipole Trap

An ensemble of low-energy positrons injected into a supported magnetic dipole trap can remain trapped for more than a second. Trapping experiments with and without a positive magnet bias yield confinement times up to τ_{A}=(1.5±0.1) and τ_{B}=(0.28±0.04)  s, respectively. Supported by single-particle simulations, we conclude that the dominant mechanism limiting the confinement in this trap is scattering off of neutrals, which can lead to both radial transport and parallel losses onto the magnet surface. These results provide encouragement for plans to confine an electron-positron plasma in a levitated dipole trap.

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.

Interaction of strong single-cycle terahertz pulses with semiconductor quantum wells

An experiment-theory comparison is presented to demonstrate terahertz-induced extreme-nonlinear transients in a GaAs/AlGaAs quantum-well system. The terahertz-pump and optical-probe experiments show pronounced spectral modulations of the light- and heavy-hole excitonic resonances. Excellent agreement with the results of microscopic many-body calculations is obtained, identifying clear ponderomotive contributions and the generation of terahertz harmonics.

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.

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.

Measurement of Landau damping and the evolution to a BGK equilibrium

Linear Landau damping and nonlinear wave-particle trapping oscillations are observed with standing plasma waves in a trapped pure electron plasma. For low wave amplitudes, the measured linear damping rate agrees quantitatively with linear Landau damping theory. At larger amplitudes, the wave initially damps at the Landau rate, then regrows and oscillates, approaching a steady state, as predicted by O'Neil in 1965 [Phys. Fluids 8, 2255 (1965)]]. This BGK equilibrium is observed to decay slowly due to external dissipation.

Thermally excited modes in a pure electron plasma

Thermally excited plasma modes are observed in near-thermal-equilibrium pure electron plasmas over a temperature range of 0.05<kT<5 eV. The measured thermal emission spectra together with a plasma-antenna coupling calibration uniquely determine the temperature. This calibration is obtained from the spectra themselves when absorption of the receiver-generated noise is significant, or from kinetic theory. This nondestructive temperature diagnostic agrees well with standard diagnostics and may be useful for expensive species such as antimatter.

Upper extremity electrical injury

Electrical injuries to the upper extremity are far ranging in extent and vary in magnitude. Proper management requires an appreciation for the pathophysiology, clinical manifestations, and therapeutic options. Persistent neurologic and psychiatric problems further impact the high incidence of disability in the electrical injury patient. Loss is significant for victims, their families, and employers, in part because of the relative youth of those injured.

Characterization of a chemotactic defect in patients with Kartagener syndrome

Kartagener syndrome (KS) is an autosomally inherited recessive condition characterized by situs inversus, bronchiectasis, and chronic sinusitis. Ciliary dynein, the mechanochemical force generator in ciliary movement, is deficient in patients with KS. We examined blood samples from two patients and tissue biopsy specimens from five patients and found: (1) no significant defect in neutrophil or monocyte chemotaxis in response to formylpeptide chemoattractant; (2) no alterations in centriolar structure, but significantly more centriole-associated microtubules in KS neutrophils and monocytes than in control leukocytes; and (3) a marked reduction in KS fibroblast chemotaxis in response to fibronectin compared with control fibroblasts. The significance of these cellular defects in KS is described.

Characterization of formylpeptide chemoattractant binding on neutrophils and monocytes from patients with head and neck cancer

Monocytes from patients with head and neck tumors display a marked chemotactic deficiency in response to formylpeptide chemoattractants. Normal leukocytes possess receptors known to bind to chemotactic formylpeptides such as the hexapeptide N-formyl-nor-leucyl-leucyl-phenylalanyl-nor-leucyl-tyrosyl-lysine, with high specificity and great avidity. Light microscopic autoradiography was used to visualize the binding and subsequent cellular processing of the formylhexapeptide N-formyl-nor-leucyl-leucyl-phenylalanyl-nor-leucyl-[125I]tyrosyl-lysine to polymorphonuclear leukocytes (PMN) and monocytes from patients with epidermoid tumors of the head and neck. Surface-adherent leukocytes were exposed to 5 nM 125I-labeled hexapeptide for 15 minutes at 4 degrees C and were either fixed immediately or rinsed and then further incubated for 20 minutes at 37 degrees C prior to fixation. Cells underwent autoradiography, and silver grains associated with 400 cells were counted. There were no significant differences between the numbers of formylpeptide receptors on control and cancer patient PMN. However, there were significantly larger numbers of formylpeptide receptors (P less than .02) on cancer patient monocytes than on control monocytes. Cells that were allowed to process bound hexapeptide at 37 degrees C prior to fixation showed large decreases in the number of cell-associated silver grains. However, the amount of hexapeptide associated with PMN and monocytes treated in this way was the same for both control and tumor patients. In addition, histograms were plotted showing the amount of hexapeptide bound versus the frequency of cells at each level of binding. Examination of these histograms revealed that two major peaks or subpopulations occurred in each for neutrophils and monocytes from control patients. Histograms of grain counts for neutrophils and monocytes from tumor patients did not display two separate peaks of formylhexapeptide binding; instead, only one broad peak was evident. These observations indicated that: Tumor patient PMN expressed as many formylpeptide receptors as did control PMN, and tumor patient monocytes possessed significantly more receptors than did control monocytes. The disposition of formylpeptide initially bound to control and tumor patient leukocytes appeared to be essentially the same. Subpopulations existed in each control neutrophil and monocyte sample, one exhibiting approximately 30% more formylpeptide binding than the other. However, in the leukocytes obtained from tumor patients, the distinction between these two subpopulations had become obscured such that only one cell population was evident.

The use of uniform simulated models in the reconstruction of craniopagus twins

A unique approach to the planning of the separation and subsequent reconstruction of craniopagus twins is presented. In consultation with medical artists and prosthetists, exact models of our patients were fabricated preoperatively. Models were made of a hard acrylic to simulate bone and silicone rubber to simulate skin. In this fashion, the surgical teams were given an opportunity to preplan the bony separation of the heads, as well as to design the best possible coverage for reconstruction of the resulting defects. Physiological, mechanical, and aesthetic considerations are discussed. Two surgical delay procedures were performed without complication prior to neurosurgical separation. A broad segment of inextricably interconnected cerebellar tissue and a shared sagittal sinus could not be overcome at the time of final separation and proved to be fatal for both twins. Owing to the untimely deaths, we can only postulate that the delayed compound musculocutaneous flaps would have produced the best possible results and physiological protection for the extensive bony defects in both skulls resulting from this separation.

Defective monocyte chemotaxis in patients with epidermoid tumors of the head and neck

The function of the immune surveillance system has been studied extensively in patients with neoplastic disease. Defective monocyte chemotaxis has been demonstrated in a variety of neoplastic conditions. We have assayed chemotaxis in monocytic and polymorphonuclear leukocytes taken from patients with epidermoid tumors of the head and neck. Multiwell chemotaxis chambers and the chemoattractant, formyl-methionyl-leucyl-phenylalanine, were used. Polymorphonuclear leukocytes responded normally, but monocytes from all tumor patients studied displayed marked reductions in random migration (86% inhibition) and in chemotaxis (77% inhibition) toward formylpeptide.

Characterization of antibody-forming cells adjacent to laryngeal carcinoma

Plasma cells are often found in the stroma adjacent to squamous carcinoma of the larynx, although the class of immunoglobulin produced by these cells is unknown. Using an immunofluorescence technique with counterstaining by homidium bromide, we determined the proportion of plasmacytic cells producing IgG, IgA, IgM, IgD, IgE, kappa-light chain, and lambda-light chain immunoglobulins adjacent to tumor. IgG was found in 80% to 90% of the plasma cells at primary and metastatic sites. IgA and IgM were found in lesser percentages. IgA was often found in small clusters. IgE was found in rare isolated cells adjacent to tumor, but no IgD-containing cells were detected. Approximately equal numbers of kappa- and lambda-bearing cells were found. These findings indicate an IgG humoral immune response commonly occurs adjacent to squamous carcinoma of the larynx.