Molecular dynamics of localized reaction, experiment and theory: methyl bromide on Si(111)-7x7.

An earlier experimental study by scanning tunneling microscopy (STM) from this laboratory described the use of “localized reaction” as a means to the electron- or photon-imprinting of self-assembled patterns of CH3Br(ad) as covalently bound Br−Si(s) at Si(111)-7×7. Here we show that the thermal surface bromination reaction by CH3Br(ad) is also highly localized, and present a detailed ab initio dynamical model for the reaction, using DFT. Localization is seen to be due to the coexistence in the reactive transition-state of the neighboring bonds being broken (C−Br) and formed (Br−Si). Both experiment and theory are consistent with a low energy-barrier, Ea ≈ 0.2 eV, for the thermal bromination of Si(111) by CH3Br(ad), and also for the desorption of intact CH3Br(g) (Edes ≈ 0.2 eV). Two physisorbed states of CH3Br(ad)/Si(111) (I and II) are distinguishable by STM at 50 K by their differing displacement from the underlying Si adatom. These states can be identified with similarly displaced states in the STM imag...