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Ammonidsilane mixtures have been studied by ion trap mass spectrometry, and variations of ion abundances with reaction time in 1 5 , 1:1, and 5 : 1 mixtures have been reported. Mechanisms of ion-molecule reactions have been elucidated by single and multiple isolation steps, and exact mass measurements of isobaric ions have been carried out by Fourier transform mass spectrometry. The SiH,+ (n = 0-3) primary ions give self-condensation processes in which Si,H,+ species are formed. These ions react with NH3 and give Si2-NH,+ (n = 3-6) and Si2N2Hn+ (n = 4-7) ions in successive steps with elimination of H2. In parallel processes, the SiH,+ (n = 0-3) primary ions react with NH3 to give ions belonging to the SiNH,+ (n = 2-4) and SiN2H,+ (n = 4-7) families, which do not react with Si& furtherly. A number of precursors give SiNH6+ and N&+ through different pathways, the last ion being the most abundant one also after short reaction times in all the mixtures examined. The rate constants of the gas-phase reactions of the most important ions have been determined by ion trap mass spectrometry, compared with calculated collision rate constants, and the efficiencies have been determined. Formation of Si2N2H,+ (n = 4-7) and SigN2Hn+ (n = 7, 8) ions, even if with rather low efficiencies, suggests that ionic species in addition to radicals can give a contribution to the deposition of solid silicon nitride from ammonidsilane mixtures by radiolytical methods.

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Results of gas-phase experiments and theoretical investigations are reported for ionic reactions in silane/ethene systems with the main interest in the formation and growth of species containing both silicon and carbon atoms. Ion/molecule reactions in different SiH4/C2H4 mixtures have been studied with an ion trap mass spectrometer, determining variation of ion abundances with reaction time, reaction paths starting from primary ions of both reagents and reaction rate constants of the main processes. The best yield in formation of new Si−C bonds occurs in mixtures with an excess of silane, through processes of silicon-containing ions with ethene molecules. Since reactions of SiH2 + with ethene have been observed to play a major role in this system, they have been investigated by high-level ab initio methods. Structures and energies of intermediates (SiC2H6 •+) and products (SiC2H5 +, SiC2H4 •+, SiCH3 +), as well as energy profiles of the pathways observed experimentally, have been determined. The initial step is formation of a SiC2H6 •+ adduct at −44 kcal mol-1 with respect to the reactants, followed by isomerization reactions to four different structures through viable paths. Hydrogen atom loss to give SiC2H5 + occurs through homolytic cleavage of a Si−H or C−H bond without energy barriers for the inverse process. Four different structures have been computed for SiC2H4 •+ ion species, but only three of them are attainable by H2 elimination from SiC2H6 •+ or by isomerization. Formation of SiCH3 + involves three isomerization steps of the SiC2H6 •+ adduct before the cleavage of a Si−C bond. Enthalpies of formation of all the structures have also been computed, and a good agreement with previously reported experimental data is generally observed for the most stable isomers.

Silicon clusters are of considerable interest for their importance in astrophysics and chemical vapour deposition processes, as well as from a fundamental point of view. Here, we present a quadrupole ion trap study of the self-condensation ion/molecule reactions of anions of silane. In the high-pressure regime, several ion clusters are formed with increasing size: the largest ions detected are Si5Hn- (n = 0-3). Selective ion isolation and storage allowed detection of the main reaction sequences occurring in the reacting system. The most frequent condensation step is followed by single or multiple dehydrogenation, this latter being particularly observed for the high-mass reactant ions. As a consequence, the most abundant ions in the mass spectra are those with a low content of hydrogen, namely Si2H-, Si3H-, and Si4H-. These results are discussed with reference to literature data on silicon cluster anions and related systems.