GAS-PHASE IONIZATION OF POLYATOMIC-MOLECULES VIA INTERACTIONS WITH POSITRONS

Positrons, electron anti-particles, can ionize isolated gaseous molecu les by a variety of mechanisms, The dominant mechanism is determined p rimarily by positron kinetic energy, Fast positrons appear to ionize i n direct analogy with ionization by fast electrons, At positron energi es below the ionization energy of the molecule, ionization can take pl ace in conjunction with the formation of positronium, the bound electr on-positron species, The resulting mass spectra can be interpreted on the basis of a two-step mechanism involving electron transfer to the p ositron followed by unimolecular decomposition of the molecular ion, A t kinetic energies below the threshold for postironium formation, ioni zation can proceed via an apparent positron capture process, This mech anism is far more selective than the mechanisms operative at higher ki netic energies and can result in much more extensive fragmentation tha n is observed with either impact ionization or positronium formation.