  We have measured the absolute doubly differential angular sputtering yield
for 20 keV Kr+ impacting a polycrystalline Cu slab at an incidence angle of
{\theta}i = 45{\deg} relative to the surface normal. Sputtered Cu atoms were
captured using collectors mounted on a half dome above the sample, and the
sputtering distribution was measured as a function of the sputtering polar,
{\theta}s, and azimuthal, phi, angles. Absolute results of the sputtering yield
were determined from the mass gain of each collector, the ion dose, and the
solid angle subtended, after irradiation to a total fluence of ~ 1 x 10^18
ions/cm^2. Our approach overcomes shortcomings of commonly used methods that
only provide relative yields as a function of {\theta}s in the incidence plane
(defined by the ion velocity and the surface normal). Our experimental results
display an azimuthal variation that increases with increasing {\theta}s and is
clearly discrepant with simulations using binary collision theory. We attribute
the observed azimuthal anisotropy to ion-induced formation of micro- and
nano-scale surface features that suppress the sputtering yield through
shadowing and redeposition effects, neither of which are accounted for in the
simulations. Our experimental results demonstrate the importance of doubly
differential angular sputtering studies to probe ion sputtering processes at a
fundamental level and to explore the effect of ion-beam-generated surface
roughness.
