Local electronic structural effects and measurements on the adsorption of benzene on Ag(110)

The adsorption of benzene on the Ag(110) surface has been investigated using low-temperature scanning tunneling microscopy (STM). We found that benzene molecules preferentially adsorb above step edges at 66 K, leaving the terraces free of molecules. The preference for the step edge adsorption is attributed to the Smoluchowski effect enhancing the empty states to which charge is donated from the $\pi$ orbitals of the benzene. However, only the $\left[11¯0\right]$ step edges are decorated. We attribute this to the presence of the band gap that exists at the [001] step edge, reducing the Smoluchowski effect—the charge transfer across step edges—and thus the adsorption on these steps. We have also explored the deposition of additional benzene onto this surface at 4 K, where benzene populates a weak adsorbed state. A saturated monolayer of these species arranges hexagonally, commensurate along one direction with the Ag surface. These overlayers present interesting transparency effects in the STM images. The transparency is highly dependent upon changes in the tip-sample distance and is believed to be due to dielectric screening of the Ag orbitals by the benzene.