Recent experiments of silicon bombardment show that kilo electron volt heavy polyatomic projectiles can increase the non-linear enhancement of the total yield of secondary silicon ions as well as the cluster ones. To understand why the heavy polyatomic projectiles increase the yields, molecular dynamics simulations of the bombardment of a Si(100)-(2x1) surface by Al-n and Au-n, n = 1, 2 with an initial energy of 1.5 keV/atom at the incident angle of 45degrees are carried out. The microscopic analysis shows that upon penetrating into the substrate the Au-2 constituents disintegrate slowly and the collision cascades overlap with a large probability. The process of sharing and depositing energy near the surface is very effective in the Au-2 case under these bombardment parameters. Thus the probability of high yield events for enhancement of cluster yield is increased. (C) 2002 Elsevier Science B.V. All rights reserved.