Interaction of adsorbates with electric field fluctuations near surfaces: Influence of the STM tip and plasmonic effects

We discuss the interaction of adsorbates with fluctuating electric fields produced by zero-point charge fluctuations of electrons in the substrate. Zero-point motions associated with surface or interface plasmons are a source of such electric field fluctuations. When an adsorbate is on an oxidized surface, direct hybridization of the adsorbate electrons with those in the substrate can be ignored, and the long-ranged Coulomb field provides coupling of the adsorbate's electrons to those in the substrate through the electric field fluctuations. Earlier we developed a formalism that allows one to calculate adsorbate energy-level shifts along with the nonradiative lifetime of excited states with origin in these interactions. The present paper explores the influence of a nearby STM tip on the spectrum of electric field fluctuations. We also describe studies of the energy-level shifts of a Mg porphine molecule on the oxidized NiAl (110) surface, along with the nonradiative lifetime of its LUMO+1 state. The dependence of these effects on the distance between the tip and the oxide layer is studied. We find good agreement with data on the nonradiative lifetime of the LUMO+1 state of this molecule, along with its tip-induced shift in energy.