Role of valence states of adsorbates in inelastic electron tunneling spectroscopy: A study of nitric oxide on Cu(110) and Cu(001)

We studied nitric oxide (NO) molecules on Cu(110) and Cu(001) surfaces with low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). NO monomers on the surfaces are characterized by STM images reflecting 2π∗ resonance states located at the Fermi level. NO is bonded vertically to the twofold short-bridge site on Cu(110) and to the fourfold hollow site on Cu(001). When NO molecules form dimers on the surfaces, the valence orbitals are modified due to the covalent bonding. We measured inelastic electron tunneling spectroscopy (IETS) for both NO monomers and dimers on the two surfaces, and detected characteristic structures assigned to frustrated rotation and translation modes by density functional theory simulations. Considering symmetries of valence orbitals and vibrational modes, we explain the intensity of the observed IETS signals in a qualitative manner.A.S. acknowledges the support by JSPS KAKENHI Grants No. JP12J00219 and No. JP15H06127. M.A. and T.F. acknowledge financial support by the Basque Departamento de Educacion, Universidades e Investigacin, the University of the Basque Country UPV/EHU (Grant No. IT-756-13) and by the Spanish Ministerio de Economia y Competitividad (FIS2013-48286-C2-2-P and MAT-2013-46593-C6-2-P, respectively). T.F. also acknowledges support by the European Union FP7-ICT project PAMS (Contract No. 610446).Peer reviewe