Effect of molecular adsorbates on the plasmon resonance of metallic nanoparticles

► Broadening of the plasmonic excitation was observed for small couplings to the surface. ► Very large broadenings can occur when increasing the coupling to the surface. ► Effects are maximal when the molecular excitation is in resonance with its plasmonic counterpart. ► Adsorbates with excitations below the plasmonic resonance can produce splitting of the resonance in two different excitations.