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ABSTRACTTheoretical and experihental methods are applied to the study of the reaction dynamics in hyperthermal collisions of 0+ ( 4 S3t) with methane. Measurements of the absolute reaction' cross sections for the interaction of 0+ with CD 4 and CH4 were obtained at collision energies in the range from near-thermal to approximately 15 eV, using the guided-ion beam (GIB) technique. Product recoil velocity distributions, using the GIB timeof-flight (TOF) methods, were determined for several product ions at selected collision energies. The main reaction channel, charge transfer, proceeds via large impact parameter collisions. A number of minor channels, involving more intimate collisions, were also detected. Ab initio electronic structure calcu'lations have been performed with different levels of theory and basis sets, including high-level coupled-cluster calculations to determine the energies of reaction intermediatesand transition states for reaction. Several, reaction paths on both quartet and doublet electronic states of (O.CH4)+ are found, and these provide a reasonable qualitative .interpretation of the experiments. Although most of the products can be produced via spin-allowed pathways, the appearance of CH 3 + at low energies suggests that intersystem crossing plays some role. Chemistry, Northwestern University, Evanston, Illinois 60208-3113 Received: May 10, 2004; In Final Form: June 4, 2004 Theoretical and experimental methods are applied to the study of the reaction dynamics in hyperthermal collisions of 0+ ( 4 S 3 / 2 ) with methane. Measurements of the absolute reaction cross sections for the interaction of 0+ with CD 4 and CH 4 were obtained at collision energies in the range from near-thermal to approximately 15 eV, using the guided-ion beam (GIB) technique. Product recoil velocity distributions, using the GIB timeof-flight (TOF) methods, were determined for several product ions at selected collision energies. The main reaction channel...