Gas Phase Ion−Molecule Reactions in Phosphine/Silane Mixtures

Phosphine/silane mixtures have been investigated by ion trap mass spectrometry, and reaction pathways together with rate constants of the main reactions are reported. Mechanisms of ion−molecule reactions have been elucidated by single and multiple isolation steps, and exact mass measurements have been performed by Fourier transform mass spectrometry. The SiH n + ( n = 0−3) ions react with phosphine to give SiPH n + ( n = 1−4) ions. These ions further react and yield SiP 2 H n + ( n = 2−5) and Si 2 PH n + ( n = 3−7) ions, which, in turn, react following different pathways with silane or phosphine to give Si 2 P 2 H n + ( n = 5, 6, 8) and Si 3 PH n + ( n = 5−7), respectively. Mixed SiPH n + ( n = 1−4) ions also originate from the PH n + ( n = 0, 1) phosphine primary ions, as well as from the P 2 H n + ( n = 0−3) secondary ions of phosphine. Protonation of phosphine from several ionic precursors is a very common process and yields the stable phosphonium ion, PH 4 + . Trends in total abundances of tertiary SiP 2 H n + ( n = 2−4) and Si 2 PH n + ( n = 3−7) ions as a function of reaction time for different PH 3 /SiH 4 pressure ratios show that excess of silane favors the nucleation of mixed Si−P ions. The mechanism and energetics of the reaction of Si + with PH 3 have been investigated by ab initio calculations, and the most stable structure of the SiPH + product, with a hydrogen bridge between silicon and phosphorus, has been identified.