Electronic structure and bonding properties of K and K $^{+}$ on graphite under external electric field*

The effect of an external electric field on the adsorption of K and K$^{+}$ on the graphite (0001) surface, are studied by means of first- principles total-energy calculations. The results were obtained with the pseudopotentials LCAO method (SIESTA code) and the Generalized Gradient Approximation (GGA) for the exchange-correlation potential. The structural parameters, bonding properties, and electronic structure of the K and K$^{+}$-graphite system are studied in the triangular (2x2) overlayer phase as a function of the external electric field magnitude. We find an important change in the K and K$^{+}$-graphite bonding as a consequence of the charge transfer from the adatom towards the substrate induced by the electric field. However, we find that none of the investigated systems show diffusion of K or K$^{+}$ into graphite even with a strong electric field. The results are discussed in the light of the experimental observed diffusion of K into graphite, presumably induced by external electric fields.