Low-temperature STM investigation of acetylene on Pd(1Â 1Â 1)

The adsorption of individual acetylene molecules at a Pd(1 1 1) surface has been studied with scanning tunneling microscopy (STM) at 4.7 K. The adsorbed acetylene molecules appear in the STM images as a combination of a protrusion and a pair of depressions. To determine the adsorption site and configuration of the molecule, we performed rotational manipulation of the molecule by injecting tunneling electrons into the molecule as well as performing atomically resolved STM imaging. The STM images revealed that the molecules have a total of six equivalent adsorption orientations on Pd(1 1 1), indicating three equivalent rotational states on two (both fcc and hcp) 3-fold hollow adsorption sites. From the atomically resolved STM images of the surface with a monatomic step edge, we have distinguished the two types of 3-fold hollow sites and confirmed that the fcc site is more stable than the hcp site at a low temperature.