Lifetime of adsorbate vibrations: The role of anharmonicity

Abstract

We discuss calculations of the anharmonic damping of adsorbate vibrations, with application to species (S, O, CO) adsorbed on the Ni (100) and the Ni(111) surfaces. If the frequency of vibration of the adsorbate lies below twice the maximum phonon frequency of the substrate, then energy conservation allows it to be damped by emitting two substrate phonons. Also, in some cases, the adsorbate vibration can decay to a final state with one substrate phonon and one mode localized on the adsorbate. For the above-cited examples, we explore the magnitude and temperature variation of the linewidth of adsorbate modes from this mechanism. Near room temperature, the calculated linewidths exhibit a linear variation with temperature. For the C—Ni stretching mode of topbonded CO, our results are in accord with the data of Chiang, Tobin, Richards, and Thiel [Phys. Rev. Lett. 52, 648 (1984)]. We also simulate inhomogeneous broadening produced by disorder by considering the eigenfrequencies of infrared-active modes from pairs or small clusters of adsorbates. We find this source of broadening comparable to that provided by anharmonicity.