GUIDED ION-BEAM STUDIES OF THE REACTIONS OF GROUP-3 METAL-IONS (SC- ELECTRONIC-STATE EFFECTS, COMPARISON TO REACTIONS WITH METHANE, AND M(+)-SIHX (X=0-3) BOND-ENERGIES(, Y+, LA+, AND LU+) WITH SILANE )

Guided ion beam techniques are used to measure cross sections as a fun ction of kinetic energy for the reactions of silane with M(+) = Sc+, Y +, La+, and Lu+. Ionic products include MH(+) and MH(2)(+), as well as MSiH(x)(+) (x = 0-3). The major low-energy process in all four system s is formation of MSiH(2)(+) + H-2, while at higher energies, formatio n of MH(+) + SiH3 and MH(2)(+) + SiH2 dominates the reactivity. Variat ion of source conditions allows the effect of electronic excitation on the reactivity of Sc+ and Y+ to be studied in detail. The Sc+ (a(3)D) ground state and the Y+ (a(3)D) first excited state are approximately an order of magnitude less reactive than the Sc+ (a(1)D, a(3)F) excit ed and the Y+ (a(1)S) ground states. Formation of ScH2+ + SiH2 is obse rved only for reaction of silane with Sc+ (a(1)D). The reactivity of t hese systems may be understood in terms of simple molecular orbital an d spin conservation concepts. The thresholds for Sc+, Y+, La+, and Lu reactions are evaluated to yield 0 K bond dissociation energies (BDEs ) for M(+)-Si, M(+)-SiH, M(+)-SiH2, and M(+)-SiH3 of 2.51 +/- 0.11, 2. 33 +/- 0.11, 2.17 +/- 0.08, and 1.76 +/- 0.16 eV, respectively, for M = Sc; 2.52 +/- 0.13, 2.82 +/- 0.16, greater than or equal to 2.39 +/- 0.07, and 2.13 +/- 0.16 eV, respectively, for M = Y; and 2.87 +/- 0.10 , 2.76 +/- 0.25, greater than or equal to 2.39 +/- 0.07, and 2.00 +/- 0.28 eV, respectively, for M = La. In the case of Lu+, the M(+)-Si and M(+)-SiH2 BDEs are 1.11 +/- 0.14 and 0.98 +/- 0.10 eV, respectively. Values determined in the present study for D-0(M(+) - H) and D-0(M(+)- 2H) on the basis of state-specific information indicate that a reexami nation of previously determined values may be warranted.