First-principles calculation of charge transfer at surfaces: The case of core-excited Ar* (2 p 32 -1 4s) on Ru(0001)

We present an ab initio scheme for the calculation of the resonant charge transfer of electrons at surfaces. The electron initially resides in a bound resonance, i.e., appearing below the vacuum level, associated with a core-excited adsorbate. Our treatment is based on first-principles density-functional calculations of this initial situation using finite slabs. These results are combined with bulk calculations of the substrate material to obtain the Hamiltonian of the semi-infinite system in which the electron evolves. Therefore, we include a realistic description of the electronic structure of both subsystems, substrate and adsorbate, and the interaction between them. The surface Green's function is then computed using the transfer matrix method and projected onto a wave packet localized in the adsorbate. The width and energy of the resonance can be obtained from an analysis of the projected Green's function, and the charge transfer time can be estimated. The calculated width is independent of the wave packet used for the projection, at least as far as there are not several overlapping resonances at neighboring energies. Alternatively, one can directly calculate the time evolution of the population of the initial wave packet. Both alternatives are presented and compared. Our first-principles calculations are based on periodic arrangements of adsorbates on the surface. With an appropriate average of the k resolved results, one can extrapolate to the limit of an isolated adsorbate. We discuss several possibilities to do this. As an application, we focus on the case of the 4s bound resonance of a core-excited Ar* (2 p 32 -1 4s) adsorbate on Ru(0001), for which there are extensive experimental studies. The calculated values and trends are in good agreement with the experimental observations.