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Role of molecular electronic structure in IETS: the case of O_2 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 molecules on Ag(110). Semilocal DFT calculations predict a spinless adsorbed molecule, however the inclusion of the U leads to the polarization of the molecule by shifting a spin-polarized molecular orbital towards the Fermi level. 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. This allows us to reproduce the experimental data in: (i) the negative conductance variation observed in the vibrational spectra of O_2 along the [001] direction, (ii) the spatial distribution of the conductance changes recorded over the O_2 molecule for 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 and vibrational symmetries at play. The vibrational frequencies need to go beyond the harmonic approximation in order to be compared with the experimental ones, hence we present a Morse-potential fitting of the potential energy surface in order to evaluate accurate vibrational frequencies. 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. This ensemble of results reveals that the IETS of O_2 cannot be ascribed to the effect of a single orbital molecular resonance.