Aspects of positronium formation in polyatomic molecules

Using a new simple experimental method for measurement of the positronium (Ps) formation cross-sections, the cross-sections, QPs, for many polyatomic molecules have been studied and are presented at 2 eV above the threshold energy of Ps formation EPs. The experimental method for the QPs employed here is based on use of the stronger magnetic field of 31 G (compared to the 9 G used for our total cross-sections) to increase forward scattering effects, and thereby enable determination of QPs. The QPs results are discussed in relation to the corresponding total cross sections (Qt) for each molecule, i.e. as QPs/Qt, for about 50 different molecular gases. As for some of the characteristics of the ratio values observed (i) the ratios for alkane molecules decrease with increasing number of valence electrons, (ii) the ratios for fluoro-alkane molecules are notably lower than those for alkane molecules and (iii) the ratios for polar molecules are very low compared to other cases above.