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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.

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.

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