The single ionization cross sections of He, Ne and Ar by positron impact are re-examined. The new results differ from earlier published data for impact energies from the threshold to about 100 eV with the new cross sections being smaller.
Twdimensional angular correlation distributions have been measured for the first time for positron and positronium annihilation in lowdensity helium, neon, argon, krypton and xenon, and in a range of helium-xenon mixtures. The full widths of the (cylindrically averaged) free positron components are in reasonable agreement with theoretical values, The intensity of the mixed-state positronium (Ps) component (mean vacuum lifetime 9.7 ns in a magnetic field of 0.8 T) in helium is consistent with the Ps fraction previously d e t a i n e d in lifetime measurements; the widths of the Ps component in helium. neon and argon are consistent with a Ps-atom scattering cross Section below 6.8 eV in all three gases of approximately (9-0.5E) nu:, where E is the Ps energy in eV.
We report the formation of positronium hydride (PsH) in collisions between positrons and methane. A preliminary value of 1.1 ±0.2 eV for the binding energy of PsH is obtained. PACS numbers: 36.IO.-k, 35.20.Gs Since the existence of the positronium atom (Ps) was predicted [1] and later experimentally discovered [2], there has been much theoretical and experimental interest in studying the possibilities of forming Ps-containing molecules. The simplest of these are the polyleptons Ps 2 and Ps". Beside these, PsH, PsF, PsCl, PsBr, and PsI have all been predicted to be chemically stable from quantal calculations [3-6]. Many other, more complicated, Ps-containing molecules are thought to be stable [7]. _ Ps has been observed by Mills [8] whereas none of the other simple molecules have been observed in vacuum. However, several of the halogen compounds have been produced and identified in aqueous solutions [9] and in graphite [10]. Much less convincing experimental results suggest the formation of PsH in various kinds of condensed matter [11,12]. Below, we report an experiment in which PsH is formed in positron collisions with CH 4 . The stability of the PsH molecule was first established by Ore [5] who found it to be bound by at least 0.0683 eV. Later, many other theoretical studies of PsH have confirmed its stability with the most accurate calculation by Ho [13] yielding a binding energy, #p S H, of 1.0598 eV.
Positron lifetime measurements are reported as a function of temperature for bis [rn-(m-phenoxyphenoxy)phenyl] ether and m-phenoxyphenyl-m-(m-phenoxyphen0xy)phenyl ether. The decay curves were analysed in terms of three lifetime components ; two short lifetimes of typically 200 and 500 ps associated with the decay of para-positronium(p-Ps) and positrons and a longer component attributed to decay of the ortho-positronium (o-Ps). The o-Ps lifetime is nearly independent of temperature in the region from 213 to 233 K, above this it becomes strongly temperature dependent. The change in the temperature dependence at w 233 K is associated with the glass transition for these liquids. A further change in the temperature dependence of the o-Ps lifetime was observed above 313 K where it becomes once more only weakly temperature dependent. Below the glass transition (Td Ps is probably trapped in preexisting holes, which may be slightly enlarged due to the presence of Ps. Above 313 K Ps is apparently localized in a thermal equilibrated bubble. The size of the bubble is probably independent of the size of the pre-existing holes, as described by the " bubble model ". The transition between those two cases occurs between T ' and 313 K, reflecting the changes in the free volume and the thermal motion of the molecules as the temperature is increased. The results are correlated to viscosity data and a molecular interpretation is presented.
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