A metal target surface was irradiated with a positive helium ion beam, and the scattered ions were counted using magnetic and electric filters that permitted exclusive passage of either positive or negative ions. The target surface's work function was measured by photoemission spectroscopy and tuned by treatment with gaseous water, revealing a linear correlation between the work function and the negative ion yield. Moreover, it was found that a water adlayer on the silver surface could either enhance or suppress charge transfer under appropriate conditions. These experimental results were complemented with quantum chemical calculations that revealed the emergence of electronic resonance between an incident inert gas atom and the metallic surface. Charge transfer between the surface and an incident He center is facilitated by an electronic state of the He-water complex that exists below the Fermi level. This transfer mechanism appears to be the dominant mechanism of negative ion formation.