Reaction Dynamics of Collisionally activated Ammonia Cations.

In the framework of a research on the collisional mechanisms involved in tandem mass spectrometry, the collisional activation processes of NH3+ with different target gases have been investigated. The experiments have been performed on an upgraded AEI-MS9 sector instrument equipped with two collision cells and include daughter ion spectra, ion kinetic energy spectra, determination of kinetic energy release distributions (KERD) and appearance energy measurements. We have shown that collisionally activated dissociation (CAD) of NH3+ into NH++[H,H] depends upon the internal energy of NH3+. This internal energy effect is also observed for the NH2++H channel, when Xe is the target gas; with He, such an effect could not be detected. The analysis of the KERD shows that production of NH2+ upon CAD takes place from the NH3+ ground electronic state by rotational predissociation; two components are observed, one which corresponds to a near-threshold process (mean kinetic energy release <T>=0.13 eV), and one characterized by <T>=0.56 ev corresponding to an excess energy of 2.1 eV above the dissociation threshold. NH+ fragments can be formed by two processes: the first one occurs at threshold and leads to NH+(X2Pi)+H2(X1Sigma+g); the second one is characterized by large KER values (up to 7 eV) and leads to either electronically excited NH+ or dissociation of H2.