“Early” and “Late” Barriers in Dissociative Attachment: Steering Surface Reaction

In gaseous or surface reactions, “early” barriers along the reaction coordinate are commonly surmounted by reagent translation, and “late” barriers are surmounted by vibration. Here, we link barrier location to the separation of the reaction products (2H or H + Cl) following dissociative attachment of H2 and HCl at silicon. Two markedly different approximations are used to represent Si(100)-2×1, a pair of silyl radicals and a cluster of 53 silicon atoms. In both approximations, the crest of the energy barrier is found to be located early along the reaction coordinate for attachment at neighboring silicon atoms and late for attachment at more widely separated silicons. This suggests a means for steering reaction by reagent translation or vibration, to give dissociative attachment at closer or further separations on a surface.