Atomic-scale chemistry:: Desorption of ammonia from Cu(111) induced by tunneling electrons

We report on excitation experiments on individual ammonia molecules adsorbed on Cu(lll) using a low-temperature scanning tunneling microscope. Multiple electronic excitation of the ammonia-substrate bond can lead to the desorption of molecules from the substrate and their transfer to the STM tip apex. The dependency of the desorption yield on the tunneling current at different biases shows that the order of the desorption process correlates directly with the minimum number of electrons necessary to overcome the binding energy. In contrast to previous experiments, excitation with either polarity, i.e., electron and hole attachment, can cause desorption. Hartree-Fock calculations allow us to deduce from spectroscopical data that the desorption process is mediated by an ammonia modified Cu4s state near the Fermi level. (C) 1999 Elsevier Science B.V. All rights reserved.