Crossed beams and theoretical studies of the O(3P)+CH4-->H+OCH3 reaction excitation function

The excitation function for the reaction, O(3P)+CH4-->H+OCH3, has been measured in a crossed molecular beams experiment and determined with direct dynamics calculations that use the quasiclassical trajectory method in conjunction with a recently developed semiempirical Hamiltonian. Good agreement is found between experiment and theory, enabling us to address two fundamental issues for the O(3P)+CH4 reaction that arise for all O(3P)+saturated hydrocarbon reactions: (1) the importance of triplet excited states that correlate adiabatically to ground-state reactants and products and (2) the importance of intersystem crossing processes involving the lowest singlet surface [corresponding to reaction with O(1D)]. Our results indicate that the first excited triplet surface contributes substantially to the cross section when the collision energy exceeds the reaction barrier (approximately 2 eV) by more than 0.5 eV. Although triplet-singlet crossings may occur at all energies, we have found that their effect on the excitation function is negligible for the collision energies studied-up to 1.5 eV above threshold.