IONIZATION OF AMMONIA MOLECULES BY COLLISION WITH METASTABLE NEON ATOMS M.Ben Arfa$^\ast$, B.Brunetti$^\sharp$, P.Candori$^\sharp$, M.Cherid$^\ast$, S.Falcinelli$^\circ$, B.Lescop$^\ast$, D.Malfatti$^\sharp$, F.Vecchiocattivi$^\circ$ $^\ast$Laboratoire de Physique des Collisions -Faculte' des Sciences, Universite' des Brest, 29285 Brest, France $^\sharp$Dipartimento di Chimica - Universita' di Perugia, 06123 Perugia, Italy $^\circ$Istituto Tecnologie Chimiche, Facolta' di Ingegneria, 06123 Perugia, Italy The ionization of ammonia by collision with metastable neon atoms has been studied by measuring the energy spectrum of emitted electrons and the mass spectrum of product ions. Nascent NH$_3^+$ ions are formed in the ground, X$^2$A', and in the first excited, A$^2$E, state. The latter then gives rise to NH$_2^+$ fragments. Total ionization cross sections and branching fractions, NH$_2^+$/NH$_3^+$, have been measured as a function of collision energy in the 0.03-0.2 eV energy range . At higher collision energy the NH&_2^+$/NH$_3^+$ ratio increases.