Controlling Surface Dynamics by Vibrational Excitation of a Single Molecule

—Acetylene Molecules on Pd(111)—

Abstract

We report the control of surface dynamics of individual acetylene molecules on Pd(111) at 4.7 K, which involves (1) determination of adsorption sites of the molecules, and (2) inducing rotational and hopping motions of acetylene by excitation of specific vibrational modes through inelastic electron tunneling process. Topographic STM images of an acetylene molecule on the Pd(111) surface revealed that the molecule occupies the 3-fold hollow site as the most stable adsorption site. When the tunneling electrons were injected, the molecule rotated among three equivalent orientations with two threshold voltages at ∼150 mV (∼150 mV) and at ∼370 mV (∼280 mV) for C₂H₂ (C₂D₂), which correspond to C-C and C-H stretch modes. On the other hand, when the higher electron energy was applied, the molecule showed lateral hopping motion to the adjacent site in addition to rotational motion. The threshold was obtained at ∼370 mV (∼280 mV) for C₂H₂ (C₂D₂). The apparent energy dependence of the efficiency of the dynamic motions and the strong isotopic shift indicate that we can control the rotational and hopping motions by the excitation of specific vibrational modes of a single acetylene molecule.