Excited states at metal surfaces and their nonradiative relaxation

Abstract

We present a general overview of the spectroscopy and relaxation dynamics of vibrational and electronic excitations of molecules and atoms adsorbed on metal surfaces. We discuss briefly some experimental tools, in particular, electron energy loss spectroscopy, and present experimental illustrations of rotational, vibrational, and electronic excitations of adsorbates. For excitations at surfaces, new effective nonradiative decay paths are opened involving, for example, excitation of electron-hole pairs and phonons. For excited molecules not in direct contact with the metal substrate (or where this overlap can be neglected), the nonradiative quenching can only result from the interaction between the oscillating electric field of the excited molecule and the metal. We first discuss the "classical theory" of this coupling and point out its limitations. We then present an improved theory and compare its predictions with experimental data. For excited molecules in direct contact with the substrate, electron transfer between the molecule and the metal can often occur, and this leads in general to strongly nonadiabatic processes. We illustrate this type of decay path for both vibrational and electronic excitations. Finally, multiphonon and phase relaxation of vibrations at surfaces are briefly discussed. © 1984 American Chemical Society.