Single-molecule surface reaction by tunneling electrons

Scanning tunneling microscope images of isolated molecules of trans-2-butene (C₄H₈) and 1,3-butadiene (C₄H₆) adsorbed on the Pd(1 1 0) surface are presented at 4.7 K. The former appears as a dumbbell-shaped protrusion perpendicular to the Pd row, while the latter appears as an elliptical protrusion parallel to the Pd row. By dosing tunneling electrons, the trans-2-butene molecule is transformed into a 1,3-butadiene molecule via dehydrogenation reaction. The threshold voltage (∼365 mV) is so small that the electronic excitation to the C-H antibonding state cannot be attributed to the mechanism. Instead, we consider that the multiple excitation of C-H stretching mode by inelastically tunneled electron is responsible for the dehydrogenation.