Multiple pathways of dissociative attachment: CH3Br on Si(100)-2×1.

We describe the dissociative attachment (DA) of methyl bromide to form chemisorbed CH3 and Br on a Si(100)-2×1 surface at 270 K. The patterns of DA were studied experimentally by ultra-high vacuum scanning tunneling microscopy (STM) and interpreted by ab initio theory. The parent molecules were found to dissociate thermally by breaking the C–Br bond, attaching the resulting fragments CH3 and Br at adjacent Si-atom sites. The observed DA resulted in three distinct attachment geometries: inter-row (IR, 88%), inter-dimer (ID, 11%), and on-dimer (OD, 1%). Ab initio computation agreed in predicting these three DA reaction pathways, with yields decreasing down the series, in accord with experiment. The three computed physisorption geometries, each of which correlated with a preferred outcome, IR, ID, or OD, exhibited similar heats of adsorption, the choice of pathway being governed by the energy barriers to DA chemisorption predicted to increase along the series: EIR = 0.48 eV, EID = 0.57 eV, and EOD = 0.63 eV.