Local variations in the reactivity of NO on the Ru(0001)-(I x 1)H surf
ace have been probed with a supersonic molecular beam. The presence of
adsorbed hydrogen atoms creates an elastic scattering channel in the
specular direction, concomitant with a suppression of the initial stic
king probability. Argon is scattered in a qualitatively similar, but i
nstrumentally limited, specular peak from the same surface. The well c
haracterized position of the hydrogen atoms allows the scattering to b
e related to collisions at the threefold hollow sites within the surfa
ce unit cell.