DETAILED STUDY OF THE DYNAMICS OF THE O-4)+HCN REACTION - A CASE-STUDY OF ION-MOLECULE REACTIONS IN THE SPACECRAFT ENVIRONMENT((S)

Guided-ion beam (GIB) cross-section and time-of-flight (TOF) measureme nts are presented for the O+(S-4) + HCN reaction from 0.2-25 eV centre -of-mass (cm) collision energies. The reaction exhibits a charge-trans fer and three chemical reaction channels, NO+, CHO+ and CO+. Charge-tr ansfer predominates above ca. 1 eV. The GIB measurements are augmented with selected ion how tube (SIFT) rate constant measurements at 310 a nd 485 K. The total SIFT thermal reaction rate coefficients at 310 and 485 K are, 2.5 +/- 0.6 x 10(-9) cm(3) s(-1) and 2.3 +/- 0.6 x 10(-9) cm(3) s(-1). These values are compared to the 2.0 +/- 0.5 x 10(-9) cm( 3) s(-1) reaction rate coefficient derived from the 0.2 eV GIB total c ross-section. The analysis of the TOF spectra with the osculating comp lex model reveals that product ion formation is mediated by long-lived intermediate complexes at low collision energies, while a direct mech anism dominates for the majority of the product ions at high energy. C ontributions from both direct and complex-mediated mechanisms are obse rved at intermediate energies. The complex-mediated products show a ne ar-statistical partitioning of reaction energy which is substantiated by a phase-space-theory based calculation. The chemical reaction produ ct branching ratios are discussed in the context of recent ab initio [ O-H-C-N](+) hypersurface calculations (A. Luna, A. Mebel and K. Moroku ma, personal communication). The application of the measurements to an understanding of atmosphere-spacecraft interactions is also discussed .