  We highlight a new metal-molecule charge transfer process by tuning the Fermi
energy of plasmonic silver nanoparticles (AgNPs) in-situ. The strong adsorption
of halide ions upshifts the Fermi level of AgNPs by up to ~0.3 eV in the order:
Cl-<Br-<I-, favoring the spontaneous charge transfer to aligned molecular
acceptor orbitals until charge neutrality across the interface is achieved. By
carefully quantifying experimentally and theoretically the Fermi level upshift,
we show for the first time that this effect is comparable in energy to
different plasmonic effects such as the plasmoelectric effect or hot-carriers
production. Moreover, by monitoring in-situ the adsorption dynamic of halide
ions in different AgNP-molecule systems, we show for the first time that the
catalytic role of halide ions in plasmonic nanostructures depends on the
surface affinity of halide ions compared to that of the target molecule.
