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 assume...

Positron scattering from atoms and molecules is studied at low values of incident positron energy. The experiments use a cold magnetized positron beam formed in a positron accumulator. A discussion of positron scattering in a magnetic ®eld is presented along with di€erential cross-sections (DCS) for positron±atom collisions and positron± molecule total vibrational excitation cross-sections. Abs...

The development of cold trap-based positron beams and new scattering techniques has recently enabled the first measurements of state-resolved positron-impact vibrational excitation cross sections. These measurements revealed a number of features worth further consideration, such as relatively sharp increases near threshold. This paper describes a comparison of the magnitudes and shapes of these...

axial distributions of plasma and floating potentials, electron number density and the electron energy distribution function (eedf) have been measured in the double layer (dl) region of an he discharge constriction. excitation processes in the constriction region are studied. using a linearization method for the nonstationary balance equations and choosing appropriate discharge conditions, the ...

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...

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) has been observing gammaray lines from the Sun and the Galaxy since its launch in February 2002. Here I summarize the status of RHESSI observations of solar lines (nuclear de-excitation, neutron capture, and positron annihilation), the lines of Al and Fe from the inner Galaxy, and the search for positron annihilation in novae.

Plasma-wakefield excitation by positron beams is examined in a regime for which the plasma dynamics are highly nonlinear. Three dimensional particle-in-cell simulations and physical models are presented. In the nonlinear wake regime known as the blowout regime for electrons, positron wakes exhibit an analogous "suck-in" behavior. Although analogous, the two wakefield cases are quite different i...

1,1-difluoroethylene (1,1-C2H2F2) molecules have been studied for the first time experimentally and theoretically by electron and positron impact. 0.4-1000 eV electron and 0.2-1000 eV positron impact total cross sections (TCSs) were measured using a retarding potential time-of-flight apparatus. In order to probe the resonances observed in the electron TCSs, a crossed-beam method was used to inv...

The control of ferromagnetism by light at room temperature is essential for the development of some optical-magnetic coupling devices, data storage and quantum computation techniques. In the present work, we demonstrate that the ferromagnetism of a semiconducting ZnO film on Pt substrate can be controlled by nonpolarized ultraviolet or violet light. The illumination of light with sufficiently h...