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The scanning tunneling microscopy (STM) experimental technique provides opportunities to manipulate single adsorbed molecules and atoms on a metallic surface. The mechanism of lateral hopping caused by inelastic electron tunneling current is under question. Recent experiments on CO molecule adsorbed on a Pd(110) surface suggest the indirect excitation of a hindered translational mode which is responsible for the molecule hopping.
A theoretical model based on Newns-Anderson hamiltonian type is considered. A low-frequency hindered translation mode is anharmonically coupled with the high-frequency vibrational mode (associated with the formation of electron-hole pairs from charge carriers of the adsorbed molecule and the substrate). The latter is excited by an inelastic scattering of electrons in a tunneling electron current in the STM junction. The solution of the kinetic equations for the occupation numbers of the interacting vibrational modes is obtained within the framework of the Keldysh (NEGF) diagram technique. A dependence of the hopping rate on the bias voltage is obtained by using Pauli master equations. The peculiarities of this function are discussed. |
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