Theory of Core Hole Decay Dynamics of Adsorbate on Metal Surfaces

A comparative study of a core hole decay dynamics is made for an adsorbate on a metal surface. On the basis of the available experimental results for a CO molecule chemisorbed on Cu(llO) as a prototype system, we investigate the elementary processes of the core hole decay via various new Auger channels open for adsorbates, i.e., the crossed Auger transition and the participation of an electron in the initially unoccupied level of the adsorbate created by charge transfer from the substrate before core hole decay or resonant core excitation. We calCulate the valence Auger spectrum and deexcitation spectrum following resonant excita­ tion. These spectra are compared with the direct valence photoemission, in particular with a single valence hole state screened by charge transfer from the metal. Absence of shake-up satellite in the deexcitation spectrum following resonant core to bound (e.g., 2Jr* of a CO molecule) level excitation and the mismatch of the binding energy of the screened final state between direct photoemission or normal Auger spectrum and resonantly excited Auger spectrum are explained in terms of the relaxation processes of the excited state before the Auger decay of the core hole. The electronic structures of the adsorbate on a metal surface, which is of eminent interest not only for its own right, but also for gaining deeper insight into the microscopic mechanisms of a wide variety of surface chemical reactions, have been studied by means of various surface electronic spectroscopies such as core (XPS) and valence (UPS) photoemission (PES) for the occupied states, inverse photoemission (IPS) for partially occupied or unoccupied states, and absorption (ABS) for the electronic transitions between the occupied and the unoccupied states. Auger elec­ tron spectroscopy (AES) commonly used to probe the chemical species at or near surfaces is also recognized as a useful tool to determine the energy levels and the bonding configurations of adsorbates, A comparison of these surface electron spectroscopies has been presented in a review article of Plummer et al. 1 ) for a CO molecule on various metal surfaces as a prototype system. In a series of works.. we have studied the ABS,