HEAVIER-THAN-TARGET SURFACE SCATTERING

We have measured the scattering of hyperthermal Cs$^{\mathrm{+}}$ ions (m$=$132.9u) incident at 45$^{\mathrm{o\thinspace }}$and 60$^{\mathrm{o}}$ to the surface of a Cu (0 0 1) (m$=$63.5u) crystal. Energy resolved spectra were measured for a range of angles about the specular angle in the forward direction. Deviation from standard spectra, typically interpreted as binary collisions, is observed due to the projectile-heavier-than-target nature of scattering. The measured spectra can be interpreted as collective events that are more pronounced due to an inverted mass ratio. Linear relationships in perpendicular and parallel ``components'' of energy loss at the two different incident angles help identify scattering events that lead to reflection of heavy atoms from the surface.