Direct Observation of Knock-on in Surface Reactions at Zero Impact Parameter

Reaction dynamics examines molecular motions in reactive collisions. The aiming of reagents at one another has been achieved at selected miss distances (impact parameters, b) by using the corrugations on crystalline surfaces as collimator. Prior experimental work and ab initio calculation showed single atoms aimed at chemisorbed molecules with b = 0 gave knock-on of atomic reaction products through a linear transition state. Here we report a study of b = 0 collision between directed CF₂ and stationary chemisorbed CF₃. Experiments and ab initio calculations again show linear reaction with a linear transition state, despite the additional degrees of freedom for CF₂. The directed motion of CF₂ is conserved through this linear transition state. Conservation of directionality is evidenced experimentally by the observation of a knock-on chain reaction along a line of chemisorbed CF₃.