Energetics and dynamics in the reaction of Si+ with SiF4. Thermochemistry of SiFx and SiF+x (x=1, 2, 3)

Abstract

The title reaction is studied using guided ion beam mass spectrometry. Absolute reaction cross sections are measured as a function of kinetic energy from thermal to 40 eV, and three endothermic product channels are observed. The dominant SiF++SiF3 channel is only slightly endothermic, while the SiF+3 +SiF and SiF+2 +SiF2 channels have much higher thresholds. The SiF+3 cross section magnitude is about half that of SiF+, while the SiF+2 cross section is an order of magnitude smaller than that of SiF+. A second feature which appears in the SiF+2 cross section is due to dissociation of SiF+3 . There is evidence that SiF+ and SiF+3 are produced via a direct mechanism. Competition between these two channels is interpreted in terms of molecular orbital correlations and qualitative potential energy surfaces. One surface is found to correlate only with the SiF+3 +SiF channel, while another correlates diabatically with this channel and adiabatically with the SiF++SiF3 channel. Competition on this latter surface has an energy dependence which is consistent with the Landau–Zener model. Reaction thresholds are analyzed to yield 298 K heats of formation for SiFx and SiF+x species. From an evaluation of these and literature values, we recommend the following values: ΔH0f(SiF+) =170.4±2.2 kcal/mol, IP(SiF)=7.54±0.16 eV, ΔH0f (SiF3)=−258±3 kcal/mol, and IP(SiF3)=9.99±0.24 eV.