Dynamics of molecular adsorption and rotation on nonequilibrium sites

It is generally accepted that important events on surfaces such as diffusion and reactions can be adsorption site dependent. However, due to their short lifetime and low concentration in most systems, adsorbates on nonequilibrium adsorption sites remain largely understudied. Using low-temperature scanning tunneling microscopy, site-dependent adsorption is shown for the molecule butyl methyl sulfide, which is trapped in multiple metastable adsorption sites upon deposition onto a Au(111) surface at 5 K. As this molecule does not have enough energy to diffuse to its preferred adsorption site on the surface, it is possible to study the behavior of individual molecules in a variety of nonequilibrium sites. Here we present atomic-scale data of the same chemical species in three independent, metastable adsorption sites and equilibration to a single equilibrium site as a function of either electrical or thermal excitation. Butyl methyl sulfide exhibits distinctly different physical properties at all four adsorption sites, including rotational dynamics and appearance in scanning tunneling microscopy (STM) images. An energy profile is proposed for the adsorption and equilibration of these species, and a correlation is drawn between rotational barrier and adsorption energy.