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Density functional theory (DFT) simulations corrected by the intramolecularCoulomb repulsion U, are performed to evaluate the vibrational inelasticelectron tunneling spectroscopy (IETS) of O_2 molecules on Ag(110). SemilocalDFT calculations predict a spinless adsorbed molecule, however the inclusion ofthe U leads to the polarization of the molecule by shifting a spin-polarizedmolecular orbital towards the Fermi level. A molecular resonance at the Fermilevel can imply a decrease in conductance while in the off-resonance case, anincrease in conductance is the expected IETS signal. We use the lowest-orderexpansion on the electron-vibration coupling, in order to evaluate themagnitude and spatial distribution of the inelastic signal. This allows us toreproduce the experimental data in: (i) the negative conductance variationobserved in the vibrational spectra of O_2 along the [001] direction, (ii) thespatial distribution of the conductance changes recorded over the O_2 moleculefor the O--O stretch and the antisymmetric O_2--Ag stretch vibrations, (iii)the absence of signal for the center-of-mass and hindered rotations modes, and(iv) the lack of IETS signal for the molecule chemisorbed along the [1-10]direction. Moreover, our results give us insight of the electronic andvibrational symmetries at play. The vibrational frequencies need to go beyondthe harmonic approximation in order to be compared with the experimental ones,hence we present a Morse-potential fitting of the potential energy surface inorder to evaluate accurate vibrational frequencies. The final IET spectra areevaluated with the help of the self-consistent Born Approximation and theeffect of temperature and modulation-voltage broadening are explored. Thisensemble of results reveals that the IETS of O_2 cannot be ascribed to theeffect of a single orbital molecular resonance.

Role of molecular electronic structure in IETS: the case of O_2 on Ag(110)