Electron-mediated phonon-phonon coupling drives the vibrational relaxation of CO on Cu(100)

We bring forth a consistent theory for the electron-mediated vibrational intermode coupling that clarifies the microscopic mechanism behind the vibrational relaxation of adsorbates on metal surfaces. Our analysis points out the inability of state-of-the-art nonadiabatic theories to quantitatively reproduce the experimental linewidth of the CO internal stretch mode on Cu(100) and it emphasizes the crucial role of the electron-mediated phonon-phonon coupling in this regard. The results demonstrate a strong electron-mediated coupling between the internal stretch and low-energy CO modes, but also a significant role of surface motion. Our nonadiabatic theory is also able to explain the temperature dependence of the internal stretch phonon linewidth, thus far considered a sign of the direct anharmonic coupling.Financial support by Donostia International Physics Center (DIPC) during various stages of this work is highly acknowledged. M. A. and J. I. J. acknowledge the Spanish Ministerio de Economía, Industria y Competitividad Grant No. FIS2016-76471-P.Peer Reviewe