THE EFFECTS OF REACTANT VIBRATIONAL, FINE-STRUCTURE, AND COLLISION ENERGY ON THE REACTIONS OF OCS+ WITH C2H2 - COMPLEMENTARY STUDIES OF REACTIONS IN THE [C2H2+OCS]+ SYSTEM

Resonance-enhanced multiphoton ionization has been used to create state-selected OCS+ ions, which are then reacted with C2H2 in a guided-beam tandem mass spectrometer. OCS+ can be produced with excitation in all three of its vibrational modes, in either the upper or lower fine structure electronic state. Absolute cross sections for all product channels (C2H2+, C2H(n)S+ (n = 1,2), and S+) are reported as a function of collision energy and vibrational state in the range from 0.06-4.5 eV. Different modes of nuclear motion have markedly different effects on reactivity and branching ratios. Production of C2H2S+, is the major chemical reaction channel, and its formation is strongly inhibited by collision energy, but only weakly affected by vibrational and fine structure state. The cross section for charge transfer (CT) shows vibrational effects that change with collision energy. For collision energies below 0.3 eV, CT is enhanced by all forms of nuclear motion, while at higher energies CT is weakly enhanced by C-S stretching, strongly enhanced by C-O stretching, and inhibited by bending. Both C2HS+ and S+ are minor channels, which turn on at higher collision energies. They are weakly affected by vibrational energy and fine structure state. These results are compared with those from our complementary study [T.M. Orlando, B. Yang, Y. Chiu, and S.L. Anderson, J. Chem. Phys. 92 7356 (1990)] of the other charge state of the [C2H2 + OCS]+ system: reactions of C2H2+ with OCS. This allows comparison of the effects of 12 different reactant internal energy states on the same product channels.