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 CH(3)Br(ad) as covalently bound Br-Si(s) at Si(111)-7x7. Here we show that the thermal surface bromination reaction by CH(3)Br(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, E(a) approximately 0.2 eV, for the thermal bromination of Si(111) by CH(3)Br(ad), and also for the desorption of intact CH(3)Br(g) (E(des) approximately 0.2 eV). Two physisorbed states of CH(3)Br(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 images simulated by DFT. At the elevated temperature of 80 K, a markedly displaced physisorbed state (III) appears in the STM image, indicated by DFT to have a configuration encountered along the reaction path immediately prior to the transition state. The electron-induced bromination of Si(111) by CH(3)Br(ad), and also electron-induced molecular desorption, are examined as a function of the energy of the incident electron, giving for both processes a threshold energy of E(e) approximately 1.8 eV in accord with ab initio theory, and a substantial yield of 10(-6) to 10(-5) Br-Si(s)/electron.