Gas phase ion/molecule reactions in phosphine/methylsilane mixtures

Phosphine/methylsilane mixtures investigated by ion trap mass spectrometry and reaction pathways together with the rate constants of the main reactions are reported. Mechanisms of ion/molecule reactions have been elucidated by single and multiple isolation steps. The SiHn+ (n = 0-3) ions react with phosphine to give mainly the SiPHn+ (n = 1-4) ion family. These ions further react with phosphine or methylsilane to yield SiP2Hn+ (n = 2, 4) and Si2CPHn+ (n = 5-8) ions respectively. Mixed SiPHn+ (n = 3, 4) ions also originate from the PHn+ (n = 1, 2) phosphine primary ions. Hydride abstraction from methylsilane is a very common process, which takes place from a great number of ionic precursors, including the PHn+ (n = 0-2) ions, yielding the SiCH5+ species. Trends in relative abundance, respectively summed for the SiCPHn+ (n = 5, 6), Si2CPHn+ (n = 5-8) and Si2C2PHn+ (n = 7-12) families of ions as a function of reaction time for different PH3/CH3SiH3 pressure ratios, show that the nucleation of mixed Si-P ions proceeds to a lower extent with excess phosphine and is not substantially influenced by excess methylsilane.