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

摘要

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++SiF3channel is only slightly endothermic, while the SiF+3+SiF and SiF+2+SiF2channels have much higher thresholds. The SiF+3cross section magnitude is about half that of SiF+, while the SiF+2cross section is an order of magnitude smaller than that of SiF+. A second feature which appears in the SiF+2cross section is due to dissociation of SiF+3. There is evidence that SiF+and SiF+3are 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++SiF3channel. Competition on this latter surface has an energy dependence which is consistent with the Landaundash;Zener model. Reaction thresholds are analyzed to yield 298 K heats of formation for SiFxand SiF+xspecies. From an evaluation of these and literature values, we recommend the following values: Dgr;H0f(SiF+) =170.4plusmn;2.2 kcal/mol, IP(SiF)=7.54plusmn;0.16 eV, Dgr;H0f(SiF3)=minus;258plusmn;3 kcal/mol, and IP(SiF3)=9.99plusmn;0.24 eV.