  The photon-drag effect, the rectified current in a medium induced by
conservation of momentum of absorbed or redirected light, is a unique probe of
the detailed mechanisms underlying radiation pressure. We revisit this effect
in gold, a canonical Drude metal. We discover that the signal for p-polarized
illumination in ambient air is affected in both sign and magnitude by adsorbed
molecules, opening previous measurements for reinterpretation. Further, we show
that the intrinsic sign of the photon-drag effect is contrary to the prevailing
intuitive model of direct momentum transfer to free electrons.
