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.

The cross sections for direct and total dissociative and non-dissociative ionization of the nitrogen molecule by positron impact have been measured for projectile energies from threshold to 2000 eV. The results are compared with corresponding data for impact of electrons, protons and antiprotons. The comprehensive model which during recent years has been developed to explain the behaviour of the single-and multiple-ionization cross sections of atoms by charged particle impact was found to also apply for this case of a molecular target. In particular, the interference and factorization models which have been found to explain important features of the cross sections for double ionization of atoms apply also to the dissociative ionization of nitrogen molecules.

We present experimental results for impact ionization of molecular hydrogen by electrons and positrons for the range of impact energies from threshold to about 2 keV (0.4-2 keV for e-). Our electron data agree with the most recent ones measured by others. When we compare our positron results to earlier published single ionization cross sections we find significant differences for impact energies from threshold to about 100 eV with the new cross sections being substantially smaller. In the present study an effort has been made to discriminate against false signals caused by positronium formation and other effects.

Cross sections for direct and total non-dissociative ionization of CO, CO2 and CH4 by positron impact have been measured for projectile energies ranging from threshold to 2000 eV. For CO and CO2 direct and total dissociative cross sections for several different fragments have been measured. The non-dissociative cross sections are observed to behave similarly to the single ionization cross sections of the light noble gases. The dissociative cross sections resemble that seen recently for N2, and can be related to models for double ionization of the light noble gases.