Probing Franck-Condon-like Excitations in Anchoring of Phthalocyanine Molecules on Au(111)

Abstract

The nuclear motions coupled with electronic excitations of reactants play an essential role in electron-induced chemical reactions. Here, we study the vibrational-electronic (vibronic) coupling effects in the anchoring of Ni-phthalocyanine molecules (NiPCs) on Au(111) using scanning tunneling microscopy. The anchoring occurs through the dehydrogenation of a C-H bond in NiPC by tunneling electrons. By counting the number of anchored molecules, we measure the reaction rate as a function of the bias voltage. We find an unexpected dip feature in the reaction rate near the bias voltage of 4.8 V. To understand this, we employ density functional theory calculations to study atomic force exerted on a NiPC by Franck-Condon-like excitations. We find the molecule anchoring is enhanced when the C-H bonds are stretched by the induced force, which is lacking for the bias voltage near 4.8 V and thus responsible for the anomalous dip in the reaction rate.