Atomic Hydrogen Reactions of Alkanethiols on Au(111): Phase Transitions at Elevated Temperatures

Scanning tunneling microscopy (STM) was used to observe the chemical transformations of an octanethiolate monolayer on Au(111) exposed to gas-phase atomic hydrogen at 27 °C. Reaction begins with the steady conversion of the close-packed ϕ phase to the high-density, liquid-like ϵ phase. Following this step is a rapid event in which both ϕ and ϵ disappear from the surface and are replaced with low-density striped-phase structures. The ϵ phase, which does not form at room temperature, leads to this more complex reaction behavior. Experimental observations are reproduced by a simple set of kinetic equations describing the transformations between phases and the relative reactivity of molecules in each phase. Reaction kinetics had previously been attributed to enhanced reactivity near defect sites; with the phase transformation model, no such enhancement is necessary. The reaction rate of monolayers formed from 8-mercapto-1-octanol confirms this proposal, as the much higher density of defects in these monolayers does not cause significant changes in the reaction.