Visualization of Covalent Bonding between NO Molecules on Cu(110)

Abstract

Using STM at 6 K, the valence states of NO molecules adsorbed monomerically on Cu(110) upon deposition at about 15 K were investigated. The NO monomer was found to be bonded at the short-bridge site in an upright configuration. An STM image of the monomer appears as a dumbbell-shaped protrusion, corresponding to the shape of the $2\pi ^*$ orbital aligned in the [1–10] direction. In contrast, the resonance state of the $2\pi ^*$ orbital in the [001] direction is located about 0.4 eV above the Fermi level. Although the double degeneracy of the NO $2\pi ^*$ orbital is lifted by the interaction with the anisotropic surface, the mixing of the NO $2\pi ^*$ and the Cu d band is relatively weak and the two orthogonal $2\pi ^*$ valence states are still localized on the molecule. Two isolated NO molecules on the surface were manipulated to approach each other closely along the [1–10] direction, and, at the separation less than 5.12 Å, the resonance states of the $2\pi ^*$ orbital in the [1–10] direction split, modifying the shape of the STM image. This result demonstrates the covalent interactions between two NO molecules are controlled by manipulating the overlap of their “active” $2\pi ^*$ orbitals.