Two-electron processes of desorption of a single ammonia molecule from Cu(1Â 0Â 0)

Novel two-electron processes are proposed for the desorption of a single ammonia molecule (NH3) chemisorbed on a Cu(1 0 0) surface as induced by an inelastic tunneling current with the scanning tunneling microscope. According to density functional theory calculations, it is found that a single excitation of the high-frequency (HF) symmetric N-H stretch mode (408 meV) can induce the lateral translation of NH3 via the transfer of energy to the frustrated-translation mode. In order to overcome the 600 meV desorption barrier, a double excitation of the symmetric N-H stretch mode is needed. A rate equation approach is employed to study different elementary processes for NH3 desorption following the double excitation of the N-H mode by two tunneling electrons. After the double excitation of the HF mode, two possible reaction paths are possible: either the HF mode de-excites once creating and intermediate state with excited HF and reaction-coordinate (RC) modes that after decay leads to desorption, or the HF mode completely de-excites in the RC mode leading to desorption. In the first scenario, when the vibrational damping rate of the RC mode is lower than that of the HF mode, the intermode coupling from the excited state activates the RC mode above the barrier with help of energy transfer from the HF mode. Both, mode decay and intermode coupling, are made possible by energy transfer to the continuum of electronic excitations of the metallic surface: the electron-hole pair excitations.