Figure 1. Illustration of LSPR-mediated charge excitation mechanisms.

Incident photons excite oscillating surface plasmons on an adsorbate-covered Ag nanoparticle surface. These surface plasmon oscillations decay through the formation of energetic electron–hole pairs. In the direct process (a), the electron is excited directly into an unoccupied orbital of matching energy within the adsorbate. In the indirect process (b), the energetic electrons formed by decaying plasmons form a distribution within the metal nanoparticle. Electrons with proper energy can then scatter into available adsorbate orbitals. Because of the nature of the electron distribution formed in the indirect mechanism, more electrons will scatter into lower energy orbitals (II) and chemical transformation will preferentially proceed through that lower energy activated pathway. In the direct mechanism, however, the electrons can potentially excite into higher energy orbitals (III) when that energy matches the incident photon energy. This opens the possibility for selective chemical pathway targeting that impossible in the indirect mechanism.