We present a medium-energy ion scattering study of the reconstructed d
iamond (111)-(2 x 1) surface. Measured blocking patterns were compared
with patterns calculated for a variety of structure models. We find t
hat a pi-bonded chain model featuring tilted chains best describes our
data. The tilt explains the observed electronic band gap at the surfa
ce, but is at variance with the results of recent energy minimization
calculations based on the local density approximation. Our results are
in disagreement with a previous ion scattering study of the reconstru
cted diamond (111) surface. (C) 1998 Elsevier Science B.V.