Journal of the American Chemical Society, Vol.117, No.2, 764-773, 1995
Reactions of Fe+, Co+, and Ni+ with Silane - Electronic-State Effects, Comparison to Reactions with Methane, and M(+)-Sihx (X=0-3) Bond-Energies
Guided ion beam techniques are used to measure cross sections as a function of kinetic energy for the reaction of SiH4 with M(+) = Fe+, Co+, and Ni+. Ionic products include MSiH(x)(+) (x = 0-3), as well as MH(+) and SiH3+. No structural information concerning the MSiH(x)(+) species is obtained in the present results. The major low-energy process in all three systems is formation of MSiH(2)(+) + H-2, while at higher energies, formation of MH(+) + SiH3 (M = Fe and Co) or SiH3+ + MH (M = Ni) dominates the reactivity. Variation of source conditions allows the effect of electronic excitation on the reactivity of Fe+ to be studied in detail. The a(4)F first excited state of Fe+ is more reactive by approximately an order of magnitude than the a(6)D ground state and has a different product distribution. The reactivity of Fe+ (a(4)F) is found to closely resemble that of ground state Co+ (a(3)F), which is approximately half as reactive as ground state Ni+ (a(2)D). The reactivity of these systems may be understood in terms of simple molecular orbital and spin conservation arguments. The thresholds for Fe+, Co+, and Ni+ reactions are evaluated to yield 0 K bond dissociation energies (BDEs) for M(+)-Si, M(+)-SiH, M(+)-SiH2, and M(+)-SiH3 of 2.87 +/- 0.09, 2.63 +/- 0.13, 1.88 +/- 0.09, and 1.90 +/- 0.09 eV, respectively, for M = Fe; 3.25 +/- 0.07, 3.03 +/- 0.16, 2.25 +/- 0.08, and 1.96 +/- 0.13 eV, respectively, for M = Co; and 3.34 +/- 0.07, 3.38 +/- 0.15, greater than or equal to 2.39 +/- 0.07, and 1.91 +/- 0.12 eV, respectively, for M = Ni. Evaluation of thresholds for SiH3+ + MH formation (M = Fe, Co, and Ni) is combined with previous studies in our laboratories to yield 0 K BDEs for Fe-H of 1.52 +/- 0.05 eV, for Co-H of 1.95 +/- 0.05 eV, and for Ni-H of 2.56 +/- O.11 eV.
Keywords:SEQUENTIAL CLUSTERING REACTIONS;HYDROGENATED SILICON PARTICLES;TRANSITION-METAL HYDRIDES;GAS-PHASE;ION-BEAM;MOLECULAR POLARIZABILITIES;REACTION-MECHANISMS;PERIODIC TRENDS;CROSS-SECTIONS;AB-INITIO