Structure-dependent change of desorption species from n-alkanethiol monolayers adsorbed on Au(111): Desorption of thiolate radicals from low-density structures

The coverage dependence of desorption behavior of n-alkanethiol monolayers on Au(111) has been studied using thermal desorption spectroscopy (TDS) together with scanning tunneling microscopy (STM) to investigate the molecule - substrate interactions. STM observations indicated four different phases during the growth. We present for the first time evidence for a drastic coverage-dependent change of desorption species, which is parallel with a structure change from a low-density phase to a denser phase: while the thiolates in the latter phase bimolecularly desorb as disulfides, those in the former phase unimolecularly desorb as thiolate radicals.