Excited states at surfaces: Fano profiles in STM spectroscopy of adsorbates.

The Fano-Anderson model for a discrete state embedded within a continuum is revisited within the context of excitation and decay processes which lead to some manifestations of Fano lineshape profiles. The phenomenon of resonance tunneling between an STM tip and a metal surface upon which there are isolated adsorbed atoms is discussed and the relationship between the spectroscopic signature of such systems and that of the Fano profile is taken up. Recent experimental studies of Kondo systems of magnetic adsorbates such as Co and Ce adsorbed on noble metal (111) surfaces have motivated this work.