Role of molecular electronic structure in inelastic electron tunneling spectroscopy: O2 on Ag(110)

Density-functional theory (DFT) simulations corrected by the intramolecular Coulomb repulsion U are performed to evaluate the vibrational inelastic electron tunneling spectroscopy (IETS) of O-2 on Ag(110). In contrast to DFT calculations that predict a spinless adsorbed molecule, the inclusion of the U correction leads to the polarization of the molecule by shifting a spin-polarized molecular orbital toward the Fermi level. Hence, DFT+U characterizes O-2 on Ag(110) as a mixed-valent system. This has an important implication in IETS because a molecular resonance at the Fermi level can imply a decrease in conductance while in the off-resonance case, an increase in conductance is the expected IETS signal. We use the lowest-order expansion on the electron-vibration coupling in order to evaluate the magnitude and spatial distribution of the inelastic signal. The final IET spectra are evaluated with the help of the self-consistent Born approximation and the effect of temperature and modulation-voltage broadening are explored. Our simulations reproduce the experimental data of O-2 on Ag(110) [J. R. Hahn, H. J. Lee, and W. Ho, Phys. Rev. Lett. 85, 1914 (2000)] and give extra insight of the electronic and vibrational symmetries at play. This ensemble of results reveals that the IETS of O-2 is more complicated that a simple decrease in conductance and cannot be ascribed to the effect of a single molecular-orbital resonance.