Characteristic configuration of cis-2-butene molecule on Pd(110) determined by scanning tunneling microscopy

Adsorption of cis-2-butene molecules on the Pd (110) surface was studied by scanning tunneling microscopy (STM) at 4.7 K for the first time. In contrast to the fact in the previous work that trans-2-butene molecules are adsorbed at the on-top site of the Pd(110) surface with its C=C double bond being almost parallel to the [11̄0] axis, cis-2-butene molecules were found to be adsorbed slightly off the on-top site. In addition, the C=C bond was not parallel to the [11̄0] axis, but to four directions equivalent to the structure of the Pd(110) surface. Furthermore, at high bias voltages for the STM measurement, the cis-2-butene molecule became unstable and moved in two different manners around the palladium atom to which the molecule was bonded, suggesting the existence of two different mechanisms underlying the motion on the surface.