Kinetic-energy dependence of competitive spin-allowed and spin-forbidden reactions:: V++CS2

The kinetic-energy dependence of the V+ + CS2 reaction is examined using guided ion-beam mass spectrometry. Several different ion sources are used to systematically vary the V+ electronic state distributions and elucidate the reactivities of both the ground and excited state V+ cation. The cross section for VS+ formation from ground state V+(D-5) exhibits two endothermic features corresponding to the formation of ground state VS+((3)Sigma(-)) and excited state VS+((5)Pi). The thresholds for these two processes are in good agreement with theoretically determined excitation energies. The cross section for spin-forbidden formation of ground state VS+((3)Sigma(-)) exhibits an unusual variation with kinetic energy that is attributed to the energy dependence of the surface-crossing probability. From the thresholds associated with the formation of VS+ and V(CS)(+), D-0(V+-S)=3.72+/-0.09 eV and D-0(V+-CS)=1.70+/-0.08 eV are derived. Further, circumstantial evidence for formation of a high-energy isomer of V(CS)(+) is obtained. (C) 1999 American Institute of Physics. [S0021-9606(99)01116-2].